We present 3D-Barolo, a new code that derives rotation curves of galaxies from emission-line observations. This software fits 3D tilted-ring models to spectroscopic data-cubes and can be used with a variety of observations: from HI and molecular lines to optical/IR recombination lines. We describe the structure of the main algorithm and show that it performs much better than the standard 2D approach on velocity fields. A number of successful applications, from high to very low spatial resolution data are presented and discussed. 3D-Barolo can recover the true rotation curve and estimate the intrinsic velocity dispersion even in barely resolved galaxies (about 2 resolution elements) provided that the signal to noise of the data is larger that 2-3. It can also be run automatically thanks to its source-detection and first-estimate modules, which make it suitable for the analysis of large 3D datasets. These features make 3D-Barolo a uniquely useful tool to derive reliable kinematics for both local and high-redshift galaxies from a variety of different instruments including the new-generation IFUs, ALMA and the SKA pathfinders.
A growing body of evidence indicates that the star formation rate per unit stellar mass (sSFR) decreases with increasing mass in normal "main-sequence" star forming galaxies. Many processes have been advocated as responsible for such a trend (also known as mass quenching), e.g., feedback from active galactic nuclei (AGNs), and the formation of classical bulges. We determine a refined star formation versus stellar mass relation in the local Universe. To this aim we use the Halpha narrow-band imaging follow-up survey (Halpha3) of field galaxies selected from the HI Arecibo Legacy Fast ALFA Survey (ALFALFA) in the Coma and Local superclusters. By complementing this local determination with high-redshift measurements from the literature, we reconstruct the star formation history of main-sequence galaxies as a function of stellar mass from the present epoch up to z=3. In agreement with previous studies, our analysis shows that quenching mechanisms occur above a threshold stellar mass M_knee that evolves with redshift as propto (1+z)^{2}. Moreover, visual morphological classification of individual objects in our local sample reveals a sharp increase in the fraction of visually-classified strong bars with mass, hinting that strong bars may contribute to the observed downturn in the sSFR above M_knee. We test this hypothesis using a simple but physically-motivated numerical model for bar formation, finding that strong bars can rapidly quench star formation in the central few kpc of field galaxies. We conclude that strong bars contribute significantly to the red colors observed in the inner parts of massive galaxies, although additional mechanisms are likely required to quench the star formation in the outer regions of massive spiral galaxies. Intriguingly, when we extrapolate our model to higher redshifts, we successfully recover the observed redshift evolution for M_knee.
Hydrodynamic simulations of galaxies that include stellar feedback are now able to comfortably reproduce observations of the cool halo gas around star-forming galaxies. Without implementing quasar feedback, however, these simulations under-predict the cool gas content of quasar host halos by more than a factor of two. To better understand the source of this tension, we have compiled an exhaustive sample of 195 quasars at z=1 with constraints on chemically enriched, cool gas traced by MgII absorption in background quasar spectra from the Sloan Digital Sky Survey. We find a strong correlation between quasar luminosity and cool gas covering fraction. In particular, low-luminosity quasars exhibit a mean gas covering fraction comparable to inactive galaxies of similar masses, but more luminous quasars exhibit excess cool gas approaching what is reported previously at z=2.2. Moreover, 40% of the MgII absorption occurs at radial velocities of |v|>300 km/s from the quasar systemic redshift, inconsistent with gas gravitationally bound to the quasar host halo. We discuss possible scenarios to explain the large velocity offsets and observed luminosity dependence of the cool gas near quasars including gas arising in: (1) neighboring halos correlated through large-scale structure at Mpc scales, (2) feedback from luminous quasars, and (3) debris from the mergers thought to trigger luminous quasars. The first of these scenarios can be ruled out by the lack of correlation between quasar luminosity and clustering while the latter two each make distinct predictions that can be tested with additional observations.
Upcoming space-based surveys such as Euclid and WFIRST-AFTA plan to measure Baryonic Acoustic Oscillations (BAOs) in order to study dark energy. These surveys will use IR slitless grism spectroscopy to measure redshifts of a large number of galaxies over a significant redshift range. In this paper, we use the WFC3 Infrared Spectroscopic Parallel Survey (WISP) to estimate the expected number of Halpha (Ha) emitters observable by these future surveys. WISP is an ongoing HST slitless spectroscopic survey, covering the 0.8-1.65micron wavelength range and allowing the detection of Ha emitters up to z~1.5 and [OIII] emitters to z~2.3. We derive the Ha-[OIII] bivariate line luminosity function for WISP galaxies at z~1 using a maximum likelihood estimator that properly accounts for uncertainties in line luminosity measurement, and demonstrate how it can be used to derive the Ha luminosity function from exclusively fitting [OIII] data. Using the z~2 [OIII] line luminosity function, and assuming that the relation between Ha and [OIII] luminosity does not change significantly over the redshift range, we predict the Ha number counts at z~2 - the upper end of the redshift range of interest for the future surveys. For the redshift range 0.7<z<2, we expect ~3000 galaxies/deg^2 for a flux limit of 3x10^{-16} ergs/s/cm^2 (the proposed depth of Euclid galaxy redshift survey) and ~20,000 galaxies/deg^2 for a flux limit of ~10^{-16} ergs/s/cm^2 (the baseline depth of WFIRST galaxy redshift survey).
We present near-infrared observations of the environments around three radio-loud sources (MG1 J0442+0202, 3C 068.2, and MS 1426.9+1052) at redshifts z=1.10,1.57, and 1.83 (respectively), that are surrounded by near-infrared galaxy overdensities. Overdensities with respect to field counts were found to be significant up to 19-sigma, with twelve times the expected number of galaxies within the inner regions of the densest proto-cluster. Color-magnitude relations are constructed in K_s, J-K_s, with each candidate cluster exhibiting a feature consistent with the beginnings of a red sequence. Galaxy models based on the redshift of the radio source are used to compare expected color-magnitude relations for a given formation epoch with the observed red sequence of each candidate, and are found to be consistent with an old (z_f > 5) formation epoch for a few bright, red galaxies on the red sequence.
In the second paper of this series, we present results from cosmological simulations on the demographics of flyby interactions to gauge their potential impact on galaxy evolution. In a previous paper, we demonstrated that flybys -- an interaction where two independent halos inter-penetrate but detach at a later time and do not merge -- occur much more frequently than previously believed. In particular, we found that the frequency of flybys increases at low redshift and is comparable to or even greater than the frequency of mergers for halos $\gtrsim 10^{11} M_\odot/h$. In this paper, we classify flybys according to their orbits and the level of perturbation exacted on both the halos involved. We find that the majority of flybys penetrate deeper than $\sim R_{half}$ of the primary and have an initial relative speed $\sim 1.6\times V_{vir}$ of the primary. The typical flyby mass-ratio is $\sim 0.1$ at high $z$ for all halos, while at low $z$, massive primary halos undergo flybys with small secondary halos. We estimate the perturbation from the flyby on both the primary and the secondary and find that a typical flyby is mostly non-perturbative for the primary halo. However, since a massive primary experiences so many flybys at any given time, they are nearly continually a victim of a perturbative event. In particular, we find flybys that cause $\sim 1\%$ change in the binding energy of a primary halo occurs $\gtrsim 1 $ Gyr$^{-1}$ for halos $> 10^{10} M_\odot/h$ for $z \lesssim 4$. Secondary halos, on the other hand, are highly perturbed by the typical encounter, experiencing a change in binding energy of nearly order unity. Our results imply that flybys can drive a significant part of galaxy transformation at moderate to lower redshifts ($z \lesssim 4$). We touch on implications for observational surveys, mass-to-light ratios, and galaxy assembly bias.
Wrapping around the Milky Way, the Sagittarius stream is the dominant substructure in the halo. Our statistical selection method has allowed us to identify 106 highly likely members of the Sagittarius stream. Spectroscopic analysis of metallicity and kinematics of all members provides us with a new mapping of the Sagittarius stream. We find correspondence between the velocity distribution of stream stars and those computed for a triaxial model of the Milky Way dark matter halo. The Sagittarius trailing arm exhibits a metallicity gradient, ranging from $-0.59$ dex to $-0.97$ dex over 142$^{\circ}$. This is consistent with the scenario of tidal disruption from a progenitor dwarf galaxy that possessed an internal metallicity gradient. We note high metallicity dispersion in the leading arm, causing a lack of detectable gradient and possibly indicating orbital phase mixing. We additionally report on a potential detection of the Sextans dwarf spheroidal in our data.
To study the usefulness of \CFP\ as a tracer of the regions where C\p\ and \HH\ coexist in the interstellar medium. We used the Plateau de Bure Interferometer to synthesize \CFP\ J=1-0 absorption at 102.6 GHz toward the core of the distant HII region W49N at l = 43.2\degr, b=0.0\degr, and we modeled the fluorine chemistry in diffuse/translucent molecular gas. We detected \CFP\ absorption over a broad range of velocity showing that \CFP\ is widespread in the \HH-bearing Galactic disk gas. Originally detected in dense gas in the Orion Bar and Horsehead PDR, \CFP\ was subsequently detected in absorption from diffuse and translucent clouds seen toward \bll\ and 3C111. Here we showed that \CFP\ is distributed throughout the diffuse and translucent molecular disk gas with N(\CFP)/N(\HH) $= 1.5-2.0\times10^{-10}$, increasing to N(\CFP)/N(\HH) $= 3.5\times10^{-10}$ in one cloud at 39 \kms\ having higher N(\HH) $\approx 3\times10^{21}\pcc$. Models of the fluorine chemistry reproduce the observed column densities and relative abundance of HF, from which \CFP\ forms, but generally overpredict the the column density of \CFP\ by factors of 1.4-4. We show that a free space photodissociation rate $\Gamma \ga 10^{-9}\ps$, comparable to that of CH, might account for much of the discrepancy but a recent calculation finds a value about ten times smaller. In the heavily blended and kinematically complex spectra seen toward W49, \CFP\ absorption primarily traces the peaks of the \HH\ distribution.
We perform a discrimination procedure with the spectral index diagram of TiO5 and CaH2+CaH3 to separate M giants from M dwarfs. Using the M giant spectra identified from the LAMOST DR1 with high signal-to-noise ratio (SNR), we have successfully assembled a set of M giant templates, which show more reliable spectral features. Combining with the M dwarf/subdwarf templates in Zhong et al. (2015), we present an extended M-type templates library which includes not only M dwarfs with well-defined temperature and metallicity grid but also M giants with subtype from M0 to M6. Then, the template-fit algorithm were used to automatically identify and classify M giant stars from the LAMOST DR1. The result of M giant stars catalog is cross-matched with 2MASS JHKs and WISE W1/W2 infrared photometry. In addition, we calculated the heliocentric radial velocity of all M giant stars by using the cross-correlation method with the template spectrum in a zero-velocity restframe. Using the relationship between the absolute infrared magnitude MJ and our classified spectroscopic subtype, we derived the spectroscopic distance of M giants with uncertainties of about 40%. A catalog of 8639 M giants is provided. As an additional search result, we also present 101690 M dwarfs/subdwarfs catalog which were classified by our classification pipeline.
The Gaia-ESO Survey (GES) is a large public spectroscopic survey that aims at observing with FLAMES@VLT the main stellar components of our Galaxy. The study of the population of open clusters is one of the main objectives of GES. We present some results from the first 18 months of observations, among them, a preliminary view of the radial metallicity gradient as traced by open cluster data and a comparison of the chemical patterns of clusters located in different parts of the Galactic disk.
We report a detailed investigation of the bulk motions of the nearby Galactic stellar disk, based on three samples selected from the LSS-GAC DR2: a global sample containing 0.57 million FGK dwarfs out to $\sim$ 2 kpc, a local subset of the global sample consisting $\sim$ 5,400 stars within 150 pc, and an anti-center sample containing $\sim$ 4,400 AFGK dwarfs and red clump stars within windows of a few degree wide centered on the Galactic anti-center. The global sample is used to construct a three-dimensional map of bulk motions of the Galactic disk from the solar vicinity out to $\sim$ 2 kpc with a spatial resolution of $\sim$ 250 pc. Typical values of the radial and vertical components of bulk motion range from $-$15 km s$^{-1}$ to 15 km s$^{-1}$, while the lag behind the circular speed dominates the azimuthal component by up to $\sim$ 15 km s$^{-1}$. The map reveals spatially coherent, kpc-scale stellar flows in the disk, with typical velocities of a few tens km s$^{-1}$. Bending- and breathing-mode perturbations are clearly visible, and vary smoothly across the disk plane. Our data also reveal higher-order perturbations, such as breaks and ripples, in the profiles of vertical motion versus height. From the local sample, we find that stars of different populations exhibit very different patterns of bulk motion. Finally, the anti-center sample reveals a number of peaks in stellar number density in the line-of-sight velocity versus distance distribution, with the nearer ones apparently related to the known moving groups. The "velocity bifurcation" reported by Liu et al. (2012) at Galactocentric radii 10--11 kpc is confirmed. However, just beyond this distance, our data also reveal a new triple-peaked structure.
We present a catalog of 908 objects observed with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) in the vicinity fields of M31 and M33, targeted as globular clusters (GCs) and candidates. The targets include known GCs and candidates selected from the literature, as well as new candidates selected from the Sloan Digital Sky Survey (SDSS). Analysis shows that 356 of them are likely GCs of various degree of confidence, while the remaining ones turn out to be background galaxies and quasars, stars and HII regions in M31 or foreground Galactic stars. The 356 likely GCs include 298 bona fide GCs and 26 candidates known in the literature. Three candidates selected from the Revised Bologna Catalog of M31 GCs and candidates (RBC) and one possible cluster from Johnson et al. are confirmed to be bona fide clusters. We search for new GCs in the halo of the M31 amongst the new candidates selected from the SDSS photometry. Based on radial velocities yielded by LAMSOT spectra and visual examination of the SDSS images, we find 28 objects, 5 bona fide and 23 likely GCs. Amongst the five bona fide GCs, three have been recently discovered independently by others, the remaining 25 are our new identifications, including two bona fide ones. The new identified objects fall at projected distances ranging from 13 to 265 kpc from M31. Of the two newly discovered bona fide GCs, one is located near M33, probably a GC belonging to M33. The other bona fide GC falls on the Giant Stream with a projected distance of 78 kpc from M31. Of the 23 newly identified likely GCs, one has a projected distance of about 265 kpc from M31 and could be an intergalactic cluster.
We use 297 042 main sequence turn-off stars selected from the LSS-GAC to determine the radial and vertical gradients of stellar metallicity of the Galactic disk in the anti-center direction. We determine ages of those turn-off stars by isochrone fitting and measure the temporal variations of metallicity gradients. Our results show that the gradients, both in the radial and vertical directions, exhibit significant spatial and temporal variations. The radial gradients yielded by stars of oldest ages (>11 Gyr) are essentially zero at all heights from the disk midplane, while those given by younger stars are always negative. The vertical gradients deduced from stars of oldest ages (>11Gyr) are negative and show only very weak variations with the Galactocentric distance in the disk plane, $R$, while those yielded by younger stars show strong variations with $R$. After being essentially flat at the earliest epochs of disk formation, the radial gradients steepen as age decreases, reaching a maxima (steepest) at age 7-8 Gyr, and then they flatten again. Similar temporal trends are also found for the vertical gradients. We infer that the assemblage of the Milky Way disk may have experienced at least two distinct phases. The earlier phase is probably related to a slow, pressure-supported collapse of gas, when the gas settles down to the disk mainly in the vertical direction. In the later phase, there are significant radial flows of gas in the disk, and the rate of gas inflow near the solar neighborhood reaches a maximum around a lookback time of 7-8 Gyr. The transition of the two phases occurs around a lookback time between 8 and 11 Gyr. The two phases may be responsible for the formation of the Milky Way thick and thin disks, respectively. And, as a consequence, we recommend that stellar age is a natural, physical criterion to distinguish thin and thick disk stars. ... (abridged)
In this work, we report new quasars discovered in the vicinity fields of the Andromeda (M31) and Triangulum (M33) galaxies with the LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope, also named Guoshoujing Telescope) during the 2013 observational season, the second year of Regular Survey. In total, 1330 new quasars are discovered in an area of $\sim$133 deg$^2$ around M31 and M33. With $i$ magnitudes ranging from 14.79 to 20.0, redshifts from 0.08 to 4.85, the 1330 new quasars represent a significant increase of the number of identified quasars in the vicinity fields of M31 and M33. Up to the moment, there are in total 1870 quasars discovered by LAMOST in this area (see also Huo et al. 2010, 2013). The much enlarged sample of known quasars in this area can potentially be utilized to construct a precise astrometric reference frame for the measurement of the minute proper motions of M31, M33 and the associated substructures, vital for the understanding of the formation and evolution of M31, M33 and the Local Group of galaxies.Meanwhile, amongst the sample, there are in total 45, 98 and 225 quasars with $i$ magnitudes brighter than 17.0, 17.5 and 18.0 respectively. In the aforementioned brightness bins, 15, 35 and 84 quasars are reported here for the first time, 6, 21 and 81 are reported in Huo et al. (2010, 2013), while 0, 1 and 6 are from the Sloan Digital Sky Survey, and 24, 41 and 54 are from the NED database. These bright quasars provide an invaluable sample for the kinematics and chemistry study of the interstellar/intergalactic medium of the Local Group.
Using a sample of over 70, 000 red clump (RC) stars with $5$-$10$% distance accuracy selected from the LAMOST Spectroscopic Survey of the Galactic Anti-center (LSS-GAC), we study the radial and vertical gradients of the Galactic disk(s) mainly in the anti-center direction, covering a significant disk volume of projected Galactocentric radius $7 \leq R_{\rm GC} \leq 14$ kpc and height from the Galactic midplane $0 \leq |Z| \leq 3$ kpc. Our analysis shows that both the radial and vertical metallicity gradients are negative across much of the disk volume probed, and exhibit significant spatial variations. Near the solar circle ($7 \leq R_{\rm GC} \leq 11.5$ kpc), the radial gradient has a moderately steep, negative slope of $-0.08$ dex kpc$^{-1}$ near the midplane ($|Z| < 0.1$ kpc), and the slope flattens with increasing $|Z|$. In the outer disk ($11.5 < R_{\rm GC} \leq 14$ kpc), the radial gradients have an essentially constant, much less steep slope of $-0.01$ dex kpc $^{-1}$ at all heights above the plane, suggesting that the outer disk may have experienced an evolution path different from that of the inner disk. The vertical gradients are found to flatten largely with increasing $R_{\rm GC}$. However, the vertical gradient of the lower disk ($0 \leq |Z| \leq 1$ kpc) is found to flatten with $R_{\rm GC}$ quicker than that of the upper disk ($1 < |Z| \leq 3$ kpc). Our results should provide strong constraints on the theory of disk formation and evolution, as well as the underlying physical processes that shape the disk (e.g. gas flows, radial migration, internal and external perturbations).
A modified gravitational theory is developed in which the gravitational coupling constants $G$ and $Q$ and the effective mass $m_\phi$ of a repulsive vector field run with momentum scale $k$ or length scale $\ell =1/k$, according to a renormalization group flow. The theory can explain cosmological early universe data with a dark hidden photon and late time galaxy and cluster dynamics without dark matter. The theory agrees with solar system and binary pulsar observations.
We study the characteristic size and shape of idealized blazar-induced cascade halos in the $1-100 \, {\rm GeV}$ energy range assuming various non-helical and helical configurations for the intergalactic magnetic field (IGMF). While the magnetic field creates an extended halo, the helicity provides the halo with a twist. Under simplifying assumptions, we assess the parameter regimes for which it is possible to measure the size and shape of the halo from a single source and then to deduce properties of the IGMF. We find that blazar halo measurements with an experiment similar to Fermi-LAT are best suited to probe a helical magnetic field with strength and coherence length today in the ranges $10^{-17} \lesssim B_{0} / {\rm Gauss} \lesssim 10^{-13}$ and $10 \, {\rm Mpc} \lesssim \lambda \lesssim 10 \, {\rm Gpc}$ where $\mathcal{H} \sim B_0^2 / \lambda$ is the magnetic helicity density. Stronger magnetic fields or smaller coherence scales can still potentially be investigated, but the connection between the halo morphology and the magnetic field properties is more involved. Weaker magnetic fields or longer coherence scales require high photon statistics or superior angular resolution.
This thesis assesses the influence of astronomical phenomena on the Earth's biosphere and climate. I examine in particular the relevance of both the path of the Sun through the Galaxy and the evolution of the Earth's orbital parameters in modulating non-terrestrial mechanisms. I build models to predict the extinction rate of species, the temporal variation of the impact cratering rate and ice sheet deglaciations, and then compare these models with other models within a Bayesian framework. I find that the temporal distribution of mass extinction events over the past 550 Myr can be explained just as well by a uniform random distribution as by other models, such as variations in the stellar density local to the Sun arising from the Sun's orbit. Given the uncertainties in the Galaxy model and the Sun's current phase space coordinates, as well as the errors in the geological data, it is not possible to draw a clear connection between terrestrial extinction and the solar motion. In a separate study, I find that the solar motion, which modulates the Galactic tidal forces imposed on Oort cloud comets, does not significantly influence this cratering rate. My dynamical models, together with the solar apex motion, can explain the anisotropic perihelia of long period comets without needing to invoke the existence of a Jupiter-mass solar companion. Finally, I find that variations in the Earth's obliquity play a dominant role in triggering terrestrial deglaciations over the past 2 Myr. The precession of the equinoxes, in contrast, only becomes important in pacing large deglaciations after the transition from the 100-kyr dominant periodicity in the ice coverage to a 41-kyr dominant periodicity, which occurred 0.7 Myr ago.
We present the measurements and modelling of the small-to-intermediate scale (0.1--25 Mpc/h) projected and three-dimensional (3D) redshift-space two-point correlation functions (2PCFs) of local galaxies in the Sloan Digital Sky Survey (SDSS) Data Release 7. We find a clear dependence of galaxy clustering on luminosity in both projected and redshift spaces, generally being stronger for more luminous samples. The measurements are successfully interpreted within the halo occupation distribution (HOD) framework with central and satellite velocity bias parameters to describe galaxy kinematics inside haloes and to model redshift-space distortion (RSD) effects. In agreement with previous studies, we find that more luminous galaxies reside in more massive haloes. Including the redshift-space 2PCFs helps tighten the HOD constraints. Moreover, we find that luminous central galaxies are not at rest at the halo centres, with the velocity dispersion about 30% that of the dark matter. Such a relative motion may reflect the consequence of galaxy and halo mergers, and we find that central galaxies in lower mass haloes tend to be more relaxed with respect to their host haloes. The motion of satellite galaxies in luminous samples is consistent with their following that of the dark matter. For faint samples, satellites tends to have slower motion, with velocity dispersion inside haloes about 85% that of the dark matter. We discuss possible applications of the velocity bias constraints on studying galaxy evolution and cosmology. In the appendix, we characterize the distribution of galaxy redshift measurement errors, which is well described by a Gaussian-convolved double exponential distribution.
The Spitzer Space Telescope mapped the Perseus molecular cloud complex with IRAC and MIPS as part of the c2d Spitzer Legacy project. This paper combines the observations from both instruments giving an overview of low-mass star formation across Perseus from 3.6 to 70 micron. We provide an updated list of young stellar objects with new classifications and source fluxes from previous works, identifying 369 YSOs in Perseus with the Spitzer dataset. By synthesizing the IRAC and MIPS maps of Perseus and building on the work of previous papers in this series (Jorgensen et al. 2006, Rebull et al. 2007), we present a current census of star formation across the cloud and within smaller regions. 67% of the YSOs are associated with the young clusters NGC 1333 and IC 348. The majority of the star formation activity in Perseus occurs in the regions around the clusters, to the eastern and western ends of the cloud complex. The middle of the cloud is nearly empty of YSOs despite containing regions of high visual extinction. The western half of Perseus contains three-quarters of the total number of embedded YSOs (Class 0+I and Flat SED sources) in the cloud and nearly as many embedded YSOs as Class II and III sources. Class II and III greatly outnumber Class 0+I objects in eastern Perseus and IC 348. These results are consistent with previous age estimates for the clusters. Across the cloud, 56% of YSOs and 91% of the Class 0+I and Flat sources are in areas where Av > 5 mag, indicating a possible extinction threshold for star formation.
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There has been significant controversy over the mechanisms responsible for forming compact stellar systems like ultra compact dwarfs (UCDs), with suggestions that UCDs are simply the high mass extension of the globular cluster (GC) population, or alternatively, the liberated nuclei of galaxies tidally stripped by larger companions. Definitive examples of UCDs formed by either route have been difficult to find, with only a handful of persuasive examples of stripped-nucleus type UCDs being known. In this paper we present very deep Gemini/GMOS spectroscopic observations of the suspected stripped nucleus UCD NGC 4546-UCD1 taken in good seeing conditions (< 0.7"). With these data we examine the spatially resolved kinematics and star formation history of this unusual object. We find no evidence of a rise in the central velocity dispersion of the UCD, suggesting that this UCD lacks a massive central black hole like those found in some other compact stellar systems, a conclusion confirmed by detailed dynamical modelling. Finally we are able to use our extremely high signal to noise spectrum to detect a temporally extended star formation history for this UCD. We find that the UCD was forming stars since the earliest epochs until at least 1-2 Gyr ago. Taken together these observations confirm that NGC 4546-UCD1 is the remnant nucleus of a nucleated dwarf galaxy that was tidally destroyed by NGC 4546 within the last 1-2 Gyr.
Knots or blobs observed in astrophysical jets are commonly interpreted as shock waves moving along them. Long time observations of the HST-1 knot inside the jet of the galaxy M87 have produced detailed multi-wavelength light curves. In this article, we model these light curves using the semi-analytical approach developed by Mendoza et al. (2009). This model was developed to account for the light curves of working surfaces moving along relativistic jets. These working surfaces are generated by periodic oscillations of the injected flow velocity and mass ejection rates at the base of the jet. Using genetic algorithms to fit the parameters of the model, we are able to explain the outbursts observed in the light curves of the HST-1 knot with an accuracy greater than a 2 sigma statistical confidence level.
We present ultraviolet (UV) follow-up of a sample of potential dwarf galaxy candidates selected for their neutral hydrogen (HI) properties, taking advantage of the low UV background seen by the GALEX satellite and its large and publicly available imaging footprint. The HI clouds, which are drawn from published GALFA-HI and ALFALFA HI survey compact cloud catalogs, are selected to be galaxy candidates based on their spatial compactness and non-association with known high-velocity cloud complexes or Galactic HI emission. Based on a comparison of their UV characteristics to those of known dwarf galaxies, half (48%) of the compact HI clouds have at least one potential stellar counterpart with UV properties similar to those of nearby dwarf galaxies. If galaxies, the star formation rates, HI masses, and star formation efficiencies of these systems follow the trends seen for much larger galaxies. The presence of UV emission is an efficient method to identify the best targets for spectroscopic follow-up, which is necessary to prove that the stars are associated with the compact HI. Further, searches of this nature help to refine the salient HI properties of likely dwarfs (even beyond the Local Group). In particular, HI compact clouds considered to be velocity outliers relative to their neighbor HI clouds have the most significant detection rate of single, appropriate UV counterparts. Correcting for the sky coverage of the two all-Arecibo sky surveys yielding the compact HI clouds, these results may imply the presence of potentially hundreds of new tiny galaxies across the entire sky.
We investigate the multiple stellar populations in one of peculiar globular clusters (GCs) M22 using new ground-based wide-field Ca by and HST/WFC3 photometry with equivalent passbands, confirming our previous result that M22 has a distinctive red-giant branch (RGB) split mainly due to difference in metal abundances. We also make use of radial velocity measurements of the large number of cluster membership stars by other. Our main results are followings. (i) The RGB and the subgiant branch (SGB) number ratios show that the calcium-weak (Ca-w) group is the dominant population of the cluster. However, an irreconcilable difference can be seen in the rather simple two horizontal branch (HB) classification by other. (ii) Each group has its own CN-CH anti-correlation. However, the alleged CN-CH positive correlation is likely illusive. (iii) The location of the RGB bump of the calcium-strong (Ca-s) group is significantly fainter, which may pose a challenge to the helium enhancement scenario in the Ca-s group. (iv) The positions of the center are similar. (v) The Ca-w group is slightly more centrally concentrated, while the the Ca-s is more elongated at larger radii. (vi) The mean radial velocities for both groups are similar, but the Ca-s group has a larger velocity dispersion. (vii) The Ca-s group rotates faster. The plausible scenario for the formation of M22 is that it formed via a merger of two GCs in a dwarf galaxy environment and accreted later to our Galaxy.
The X-ray and mid-IR emission from active galactic nuclei (AGN) are strongly correlated. However, while various published parameterizations of this correlation are consistent with the low-redshift, local Seyfert galaxy population, extrapolations of these relations to high luminosity differ by an order of magnitude at nuL(nu)(6um) = 1e47 erg/s. Using data from the Wide-field Infrared Survey Explorer, we determine the mid-IR luminosities of the most luminous quasars from the Sloan Digital Sky Survey and present a revised formulation of the X-ray to mid-IR relation of AGN which is appropriate from the Seyfert regime to the powerful quasar regime.
The Tully$-$Fisher relation (TFR) expresses the connection between rotating galaxies and the dark matter haloes they inhabit, and therefore contains a wealth of information about galaxy formation. We construct a general framework to investigate whether models based on halo abundance matching are able to reproduce the observed stellar mass TFR and mass$-$size relation (MSR), and use the data to constrain galaxy formation parameters. Our model tests a range of plausible scenarios, differing in the response of haloes to disc formation, the relative angular momentum of baryons and dark matter, the impact of selection effects, and the abundance matching parameters. We show that agreement with the observed TFR puts an upper limit on the scatter between galaxy and halo properties, requires weak or reversed halo contraction, and favours selection effects that preferentially eliminate fast-rotating galaxies. The MSR constrains the ratio of the disc to halo specific angular momentum to be approximately in the range 0.6$-$1.2. We identify and quantify two problems that models of this nature face: 1) They predict too large an intrinsic scatter for the MSR, and 2) they predict too strong an anticorrelation between the TFR and MSR residuals. We argue that resolving these problems requires introducing a correlation between stellar surface density and enclosed dark matter mass, which could be achieved by correlating abundance matching scatter with halo spin, or by performing a second abundance match between galaxy size and halo mass or concentration. Finally, we explore the expected difference between the TFRs of central and satellite galaxies, and find that in the favoured models this difference should be detectable in a sample of $\sim$700 galaxies.
A computed line list for formaldehyde, H$_2{}^{12}$C$^{16}$O, applicable to temperatures up to $T=1500$~K is presented. An empirical potential energy and {\it ab initio} dipole moment surfaces are used as the input to nuclear motion program TROVE. The resulting line list, referred to as \textit{AYTY}, contains 10.3 million rotational-vibrational states and around 10 billion transition frequencies. Each transition includes associated Einstein-$A$ coefficients and absolute transition intensities, for wavenumbers below 10~000 cm\(^{-1}\) and rotational excitations up to \(J=70\). Room-temperature spectra are compared with laboratory measurements and data currently available in the HITRAN database. These spectra show excellent agreement with experimental spectra and highlight the gaps and limitations of the HITRAN data. The full line list is available from the CDS database as well as at \url{www.exomol.com}.
Accurate and complete rotational, rotational-vibrational and rotational-vibrational-electronic line lists are calculated for sodium hydride: both the NaH and NaD isotopologues are considered. These line lists cover all ro-vibrational states of the ground ($X$~$^1\Sigma^+$) and first excited ($A$~$^1\Sigma^+$) electronic states. The calculations use available spectroscopically-determined potential energy curves and new high-quality, \textit{ab initio} dipole moment curves. Partition functions for both isotopologues are calculated and the effect of quasibound states is considered. The resulting line lists are suitable for temperatures up to about 7000~K and are designed for studies of exoplanet atmospheres, brown dwarfs and cool stars. In particular, the NaH $A-X$ band is found to show a broad absorption feature at about 385 nm which should provide a signature for the molecule. All partition functions, lines and transitions are available as Supplementary Information to this article and at \url{www.exomol.com}.
We study orbit of a pressure-confined cloud in the broad-line region (BLR) of active galactic nuclei (AGNs) when the combined effects of the central gravity and anisotropic radiation pressure and the drag force are considered. Physical properties of the intercloud gas such as its pressure and dynamic viscosity are defined as power-law functions of the radial distance. For a drag force proportional to the relative velocity of a cloud and the background gas, a detailed analysis of the orbits is performed for different values of the input parameters. We also present analytical solutions for a situation where the intercloud pressure is uniform and the viscosity is proportional to the inverse square of the radial distance. Our analytical and numerical solutions demonstrate decay of the orbits because of considering the drag force so that a cloud will eventually fall onto the central region after so called time-of-flight. We found that time-of-flight of a BLR cloud is proportional to the inverse of the dimensionless drag coefficient. We discuss if time-of-flight becomes shorter than the life time of the whole system, then existence of mechanisms for continually forming BLR clouds is needed.
We present the photometric properties of a sample of infrared (IR) bright dust obscured galaxies (DOGs). Combining wide and deep optical images obtained with the Hyper Suprime-Cam (HSC) on the Subaru Telescope and all-sky mid-IR (MIR) images taken with Wide-Field Infrared Survey Explorer (WISE), we discovered 48 DOGs with $i - K_\mathrm{s} > 1.2$ and $i - [22] > 7.0$, where $i$, $K_\mathrm{s}$, and [22] represent AB magnitude in the $i$-band, $K_\mathrm{s}$-band, and 22 $\mu$m, respectively, in the GAMA 14hr field ($\sim$ 9 deg$^2$). Among these objects, 31 ($\sim$ 65 %) show power-law spectral energy distributions (SEDs) in the near-IR (NIR) and MIR regime, while the remainder show a NIR bump in their SEDs. Assuming that the redshift distribution for our DOGs sample is Gaussian, with mean and sigma $z$ = 1.99 $\pm$ 0.45, we calculated their total IR luminosity using an empirical relation between 22 $\mu$m luminosity and total IR luminosity. The average value of the total IR luminosity is (3.5 $\pm$ 1.1) $\times$ $10^{13}$ L$_{\odot}$, which classifies them as hyper-luminous infrared galaxies (HyLIRGs). We also derived the total IR luminosity function (LF) and IR luminosity density (LD) for a flux-limited subsample of 18 DOGs with 22 $\mu$m flux greater than 3.0 mJy and with $i$-band magnitude brighter than 24 AB magnitude. The derived space density for this subsample is log $\phi$ = -6.59 $\pm$ 0.11 [Mpc$^{-3}$]. The IR LF for DOGs including data obtained from the literature is well fitted by a double-power law. The derived lower limit for the IR LD for our sample is $\rho_{\mathrm{IR}}$ $\sim$ 3.8 $\times$ 10$^7$ [L$_{\odot}$ Mpc$^{-3}$] and its contributions to the total IR LD, IR LD of all ultra-luminous infrared galaxies (ULIRGs), and that of all DOGs are $>$ 3 %, $>$ 9 %, and $>$ 15 %, respectively.
The analysis of the vertical velocity dispersion of disc stars is the most direct astronomical means of estimating the local dark matter density, $\rho_{DM}$. Current estimates based on the mid-plane dynamic density use a local baryonic correction that ignores the non-local effects of spiral structure and significantly underestimates the amount of dynamically relevant gas; the additional gas plus the remaining uncertainties make it practically impossible to measure $\rho_{DM}$ from mid-plane kinematics alone. The sampling of inhomogeneous tracer populations with different scale-heights and scale-lengths results in a systematic increase in the observed dispersion gradients and changes in the nominal density distributions that, if not properly considered, can be misinterpreted as a sign of more dark matter. If the disc gravity is modelled using an infinite disc, the local variation in the vertical gravity due to the globally exponential disc components results in an underestimation of the baryonic contribution by as much as ~40% Given only the assumptions of stationarity, an axially and vertically symmetric disc, doubly exponential tracer and mass-component density profiles, a phenomenologically justified model for the cross-dispersion component $\sigma_{Rz}$, and a realistic model for $g_z$, it is possible to solve the full vertical Jeans equation analytically for the vertical dispersion $\sigma_{z}(z)$ and hence test the robustness of previous attempts at measuring $\rho_{DM}$. When the model parameters are estimated from SEGUE G dwarf star data, it is still not possible to explain the difference in behaviour seen in the simple thick- and thin-disc datasets reported by Buedenbender et al.. Rather than being a fundamental problem with the kinematical model, this effect appears to be a further sign of the difficulty of defining and handling kinematically homogeneous tracer populations.
We describe observations of the apparently empty ring galaxy ESO 474-G040 obtained with the Southern African Large Telescope (SALT). The observations, consisting of imaging, long-slit spectroscopy and Fabry-Perot mapping of the H-alpha line, allow determining the ring kinematics as well as estimating the metallicity of the ring and the stellar population composition in its various parts. We propose that the object could best be understood as being the result of a past merger of disk galaxies, which formed a gas ring that subsequently disrupted via the bead instability and is presently forming stars.
Due to the renewed interest in dark matter after the upgrade of the large hadron collider and its dedication to dark matter research it is timely to reassess the whole problem. Dark matter is one way to reconcile the discrepancy between the velocity of matter in the outer regions of galaxies and the observed galactic mass. So far, no credible candidate for dark matter has been identified. Here, we develop a model accounting for observations by rotations and interactions between rotating objects analogous to magnetic fields and interactions with moving charges. The magnitude of these fields is described by a fundamental constant of the order 10-41kg-1. The same interactions can be observed in the solar system where they lead to small changes of planetary orbits.
We have carried out multiwavelength observations of the near-by ($z=0.046$) rich, merging galaxy cluster Abell 3376 with the Murchison Widefield Array (MWA). As a part of the GaLactic and Extragalactic All-sky MWA survey (GLEAM), this cluster was observed at 88, 118, 154, 188 and 215 MHz. The known radio relics, towards the eastern and western peripheries of the cluster, were detected at all the frequencies. The relics, with a linear extent of $\sim$ 1 Mpc each, are separated by $\sim$ 2 Mpc. Combining the current observations with those in the literature, we have obtained the spectra of these relics over the frequency range 80 -- 1400 MHz. The spectra follow power laws, with $\alpha$ = $-1.17\pm0.06$ and $-1.37\pm0.08$ for the west and east relics, respectively ($S \propto \nu^{\alpha}$). Assuming the break frequency to be near the lower end of the spectrum we estimate the age of the relics to be $\sim$ 0.4 Gyr. No diffuse radio emission from the central regions of the cluster (halo) was detected. The upper limit on the radio power of any possible halo that might be present in the cluster is a factor of 35 lower than that expected from the radio power and X-ray luminosity correlation for cluster halos. From this we conclude that the cluster halo is very extended ($>$ 500 kpc) and/or most of the radio emission from the halo has decayed. The current limit on the halo radio power is a factor of ten lower than the existing upper limits with possible implications for models of halo formation.
From a large sample of $\approx 170,000$ local SDSS galaxies we find that the Fundamental Metallicity Relation (FMR) has an overabundance of outliers, compared to what would be expected from a Gaussian distribution of residuals, with significantly lower metallicities than predicted from their stellar mass and star formation rate (SFR). This low-metallicity population has lower stellar masses, bimodial specific SFRs with enhanced star formation within the aperture and smaller half-light radii than the general sample, and is hence a physically distinct population. We show that they are consistent with being galaxies that are merging or have recently merged with a satellite galaxy. In this scenario, low-metallicity gas flows in from large radii, diluting the metallicity of star-forming regions and enhancing the specific SFR until the inflowing gas is processed and the metallicity has recovered. We introduce a simple model in which mergers with a mass ratio larger than a minimum dilute the central galaxy's metallicity by an amount that is proportional to the stellar mass ratio for a constant time, and show that it provides an excellent fit to the distribution of FMR residuals. We find the dilution time-scale to be $\tau=1.568_{-0.027}^{+0.029}$ Gyr, the average metallicity depression caused by a 1:1 merger to be $\alpha=0.2480_{-0.0020}^{+0.0017}$ dex and the minimum mass ratio merger that can be discerned from the intrinsic Gaussian scatter in the FMR to be $\xi_\text{min}=0.2030_{-0.0095}^{+0.0127}$ (these are statistical errors only). From this we derive that the average metallicity depression caused by a merger with mass ratio between 1:5 and 1:1 is 0.114 dex.
We monitored BL Lacertae for 13 nights in optical B, V, R, and I bands during October and November 2014 including quasi-simultaneous observations in V and R bands using two optical telescopes in India. We have studied multi-band optical flux variations, colour variation and spectral changes in this blazar. Source was found to be active during the whole monitoring period and showed significant intraday variability on 3 nights in V and R filters while displayed hints of variability on 6 other dates in R passband and on 2 nights in V filter. From the colour-magnitude analysis of the source we found that the spectra of the target gets flatter as it becomes brighter on intra-night timescale. Using discrete correlation technique, we found that intraday light curves in both V and R filters are almost consistent and well correlated with each other. We also generated spectral energy distribution (SED) of the target using the B, V, R, and I data sets for all 13 nights which could help us investigate the physical process responsible for the observed variations in BL Lacertae objects. We also discuss possible physical causes of the observed spectral variability.
We have undertaken a nearly simultaneous optical/UV and X-ray variability study of the flat spectrum radio quasar, 3C 273 using data available from the XMM$-$Newton satellite mission from June 2000 to July 2012. Here we focus on the multi-wavelength flux variability on both intra-day and long time scales of this very well known radio-loud source. We found high flux variability over long time scales in all bands for which observations were made. The optical/UV variability amplitude was more than twice than that in the X-ray bands. There is some frequency dependence of the variability in optical/UV bands in the sense that the variability amplitude increases with increasing frequency; however, the X-ray emissions disagree with this trend as the variability amplitude decreases from soft to hard X-ray bands. On intra-day time scales 3C 273 showed small amplitude variability in X-ray bands. A hardness ratio analysis in the X-ray regime indicates that the particle acceleration mechanism dominates the cooling mechanism during most of the ~12 year span of these observations.
To search for optical variability on a wide range of timescales, we have carried out photometric monitoring of two flat spectrum radio quasars, 3C 454.3 and 3C 279, plus one BL Lac, S5 0716+714, all of which have been exhibiting remarkably high activity and pronounced variability at all wavelengths. CCD magnitudes in B, V, R and I pass-bands were determined for $\sim$ 7000 new optical observations from 114 nights made during 2011 - 2014, with an average length of $\sim$ 4 h each, at seven optical telescopes: four in Bulgaria, one in Greece, and two in India. We measured multiband optical flux and colour variations on diverse timescales. Discrete correlation functions were computed among B, V, R, and I observations, to search for any time delays. We found weak correlations in some cases with no significant time lags. The structure function method was used to estimate any characteristic time-scales of variability. We also investigated the spectral energy distribution of the three blazars using B, V, R, I, J and K pass-band data. We found that the sources almost always follows a bluer-when-brighter trend. We discuss possible physical causes of the observed spectral variability.
Rotation curves of galaxies show a wide range of shapes, which can be paramaterized as scatter in Vrot(1kpc)/Vmax i.e.the ratio of the rotation velocity measured at 1kpc and the maximum measured rotation velocity. We examine whether the observed scatter can be accounted for by combining scatters in disc scale-lengths, the concentration-halo mass relation, and the Mstar-Mhalo relation. We use these scatters to create model galaxy populations; when housed within dark matter halos that have universal, NFW density profiles, the model does not match the lowest observed values of Vrot(1kpc)/Vmax and has too little scatter in Vrot(1kpc)/Vmax compared to observations. By contrast, a model using a mass dependant dark matter profile, where the inner slope is determined by the ratio of Mstar/Mhalo, produces galaxies with low values of Vrot(1kpc)/Vmax and a much larger scatter, both in agreement with observation. We conclude that the large observed scatter in Vrot(1kpc)/Vmax favours density profiles that are significantly affected by baryonic processes. Alternative dark matter core formation models are also disfavoured by the large scatter in Vrot(1kpc)/Vmax, implying that baryonic physics would be required to affect density profiles in such models, which defeats a large part of their motivation without ruling them out.
A two dimensional hydrochemical hybrid code, KM2, is constructed to deal with astrophysical problems that would require coupled hydrodynamical and chemical evolution. The code assumes axisymmetry in cylindrical coordinate system, and consists of two modules: a hydrodynamics module and a chemistry module. The hydrodynamics module solves hydrodynamics using a Godunov-type finite volume scheme and treats included chemical species as passively advected scalars. The chemistry module implicitly solves non-equilibrium chemistry and change of the energy due to thermal processes with transfer of external ultraviolet radiation. Self-shielding effects on photodissociation of CO and H$_2$ are included. In this introductory paper, the adopted numerical method is presented, along with code verifications using the hydrodynamics modules, and a benchmark on the chemistry module with reactions specific to a photon-dominated region (PDR). Finally, as an example of the expected capability, the hydrochemical evolution of a PDR is presented based on the PDR benchmark.
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Detailed studies of stellar populations in the halos of the Milky Way and the Andromeda (M 31) galaxies have shown increasing numbers of tidal streams and dwarf galaxies, attesting to a complicated and on-going process of hierarchical structure formation. The most prominent feature in the halo of M 31 is the Giant Stellar Stream, a structure ~4.5 degrees in extent along the sky, which is close to, but not coincident with the galaxy's minor axis. The stars that make up this stream are kinematically and chemically distinct from the other stars in the halo. Here, we present HST/COS high-resolution ultraviolet absorption spectra of three Active Galactic Nuclei sight lines which probe the M 31 halo, including one that samples gas in the main southwestern portion of the Giant Stream. We see two clear absorption components in many metal species at velocities typical of the M 31 halo and a third, blue-shifted component which arises in the stream. Photoionization modeling of the column density ratios in the different components shows gas in an ionization state typical of that seen in other galaxy halo environments and suggests solar to slightly super-solar metallicity, consistent with previous findings from stellar spectroscopy.
Multiple stellar populations are a widespread phenomenon among Galactic globular clusters. Even though the origin of the enriched material from which new generations of stars are produced remains unclear, it is likely that self-enrichment will be feasible only in clusters massive enough to retain this enriched material. We searched for multiple populations in the low mass (M~1.4 x 10^4 M_sun) globular cluster E 3, analyzing SOAR/Goodman multi-object spectroscopy centered on the blue CN absorption features of 23 red giant branch stars. We find that the CN abundance does not present the typical bimodal behavior seen in clusters hosting multi stellar populations, but rather a unimodal distribution that indicates the presence of a genuine single stellar population, or a level of enrichment much lower than in clusters that show evidence for two populations from high-resolution spectroscopy. E 3 would be the first bona fide Galactic old globular cluster where no sign of self-enrichment is found.
Variations in the stellar initial mass function (IMF) have been invoked to explain the spectroscopic and dynamical properties of early-type galaxies. However, no observations have yet been able to disentangle the physical driver. We analyse here a sample of 24 early-type galaxies drawn from the CALIFA survey, deriving in a homogeneous way their stellar population and kinematic properties. We find that the local IMF is tightly related to the local metallicity, becoming more bottom-heavy towards metal-rich populations. Our result, combined with the galaxy mass-metallicity relation, naturally explains previous claims of a galaxy mass-IMF relation, derived from non-IFU spectra. If we assume that - within the star formation environment of early-type galaxies - metallicity is the main driver of IMF variations, a significant revision of the interpretation of galaxy evolution observables is necessary.
The CALIFA team has recently found that the stellar angular momentum and concentration of late-type spiral galaxies are incompatible with those of lenticular galaxies (S0s), concluding that fading alone cannot satisfactorily explain the evolution from spirals into S0s. Here we explore whether major mergers can provide an alternative way to transform spirals into S0s by analysing the spiral-spiral major mergers from the GalMer database that lead to realistic, relaxed S0-like galaxies. We find that the change in stellar angular momentum and concentration can explain the differences in the $\lambda_\mathrm{Re}$--$R_{90}/R_{50}$ plane found by the CALIFA team. Major mergers thus offer a feasible explanation for the transformation of spirals into S0s.
This paper uses dynamical invariants to describe the evolution of collisionless systems subject to time-dependent gravitational forces without resorting to maximum-entropy probabilities. We show that collisionless relaxation can be viewed as a special type of diffusion process in the integral-of-motion space. In time-varying potentials with a fixed spatial symmetry the diffusion coefficients are closely related to virial quantities, such as the specific moment of inertia, the virial factor and the mean kinetic and potential energy of microcanonical particle ensembles. The non-equilibrium distribution function (DF) is found by convolving the initial DF with the Green function that solves Einstein's equation for freely diffusing particles. Such a convolution also yields a natural solution to the Fokker-Planck equations in the energy space. Our mathematical formalism can be generalized to potentials with a time-varying symmetry, where diffusion extends over multiple dimensions of the integral-of-motion space. The new probability theory is in many ways analogous to stochastic calculus, with two significant differences: (i) the equations of motion that govern the trajectories of particles are fully deterministic, and (ii) the diffusion coefficients can be derived self-consistently from microcanonical phase-space averages without relying on ergodicity assumptions. For illustration we follow the cold collapse of $N$-body models in a time-dependent logarithmic potential. Comparison between the analytical and numerical results shows excellent agreement in regions where the potential evolution does not depart too strongly from the adiabatic regime.
We compare the spatial distribution of stars which form in hydrodynamical simulations to the spatial distribution of the gas, using the $\mathcal{Q}$-parameter. The $\mathcal{Q}$-parameter enables a self-consistent comparison between the stars and gas because it uses a pixelated image of the gas as a distribution of points, in the same way that the stars (sink particles in the simulations) are a distribution of points. We find that, whereas the stars have a substructured, or hierarchical spatial distribution ($\mathcal{Q} \sim 0.4 - 0.7$), the gas is dominated by a smooth, concentrated component and typically has $\mathcal{Q} \sim 0.9$. We also find no statistical difference between the structure of the gas in simulations that form with feedback, and those that form without, despite these two processes producing visually different distributions. These results suggest that the link between the spatial distributions of gas, and the stars which form from them, is non-trivial.
We combine SDSS and WISE photometry for the full SDSS spectroscopic galaxy sample, creating SEDs that cover lambda=0.4-22 micron for an unprecedented large and comprehensive sample of 858,365 present-epoch galaxies. Using MAGPHYS we then model simultaneously and consistently both the attenuated stellar SED and the dust emission at 12 micron and 22 micron, producing robust new calibrations for monochromatic mid-IR star formation rate proxies. These modeling results provide the first mid-IR-based view of the bi-modality in star formation activity among galaxies, exhibiting the sequence of star-forming galaxies (main sequence) with a slope of dlogSFR/dlogM*=0.80 and a scatter of 0.39 dex. We find that these new star-formation rates along the SF main sequence are systematically lower by a factor of 1.4 than those derived from optical spectroscopy. We show that for most present-day galaxies the 0.4-22 micron SED fits can exquisitely predict the fluxes measured by Herschel at much longer wavelengths. Our analysis also illustrates that the majorities of stars in the present-day universe is formed in luminous galaxies (~L*) in and around the green valley of the color-luminosity plane. We make the matched photometry catalog and SED modeling results publicly available.
We present the study of a set of N-body+SPH simulations of a Milky Way-like system produced by the radiative cooling of hot gas embedded in a dark matter halo. The galaxy and its gaseous halo evolve for 10 Gyr in isolation, which allows us to study how internal processes affect the evolution of the system. We show how the morphology, the kinematics and the evolution of the galaxy are affected by the input supernova feedback energy E$_{\rm SN}$, and we compare its properties with those of the Milky Way. Different values of E$_{\rm SN}$ do not significantly affect the star formation history of the system, but the disc of cold gas gets thicker and more turbulent as feedback increases. Our main result is that, for the highest value of E$_{\rm SN}$ considered, the galaxy shows a prominent layer of extra-planar cold (log(T)<4.3) gas extended up to a few kpc above the disc at column densities of $10^{19}$ cm$^{-2}$. The kinematics of this material is in agreement with that inferred for the HI halos of our Galaxy and NGC 891, although its mass is lower. Also, the location, the kinematics and the typical column densities of the hot (5.3<log(T)<5.7) gas are in good agreement with those determined from the O$_{\rm VI}$ absorption systems in the halo of the Milky Way and external galaxies. In contrast with the observations, however, gas at log(T)<5.3 is lacking in the circumgalactic region of our systems.
We complement a recent ApJ Letter by Luo et al. by comparing the fraction of starburst galaxies which are interacting with the overall fraction of interacting galaxies in the nearby galaxy population (within 40 Mpc). We confirm that in starburst galaxies the fraction of interacting galaxies is enhanced, by a factor of around 2, but crucially we do so by studying a sample of almost 1500 of the nearest galaxies, including many dwarfs and irregulars. We discuss how adjusting the starburst definition influences the final result and conclude that our result is stable. We find significantly lower fractions of interacting galaxies than Luo et al. did from their larger but more distant sample of galaxies, and argue that the difference is most likely due to various biases in the sample selection, with a representative sample of the nearest galaxies, such as the one used here, being the best possible representation of a general picture. Our overall conclusion is that interactions can and do increase the number of starburst galaxies, and that this result is extremely robust. By far most starburst galaxies, however, show no evidence of a present interaction.
We present a new survey of HCN(1-0) emission, a tracer of dense molecular gas, focused on the little-explored regime of normal star-forming galaxy disks. Combining HCN, CO, and infrared (IR) emission, we investigate the role of dense gas in Star Formation (SF), finding systematic variations in both the apparent dense gas fraction and the apparent SF efficiency (SFE) of dense gas. The latter may be unexpected, given the popularity of gas density threshold models to explain SF scaling relations. We used the IRAM 30-m telescope to observe HCN(1-0) across 29 nearby disk galaxies whose CO(2-1) emission has previously been mapped by the HERACLES survey. Because our observations span a range of galactocentric radii, we are able to investigate the properties of the dense gas as a function of local conditions. We focus on how the IR/CO, HCN/CO, and IR/HCN ratios (observational cognates of the SFE, dense gas fraction, and dense gas SFE) depend on the stellar surface density and the molecular/atomic ratio. The HCN/CO ratio correlates tightly with these two parameters across a range of 2.1 dex and increases in the high surface density parts of galaxies. Simultaneously, the IR/HCN ratio decreases systematically with these same parameters and is ~6-8 times lower near galaxy centers than in the outer regions. For fixed line-mass conversion factors, these results are incompatible with a simple model in which SF depends only on the gas mass above some density threshold. Only a specific set of environment-dependent conversion factors can render our observations compatible with such a model. Whole cloud models, such as the theory of turbulence regulated SF, do a better job of matching our data. We explore one such model in which variations in the Mach number and in the mean density would respectively drive the trends within galaxy disks and the differences between disk and merging galaxies (abridged).
A sample of 70 E+A galaxies are selected from 37, 206 galaxies in the second data release (DR2) of Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) according to the criteria for E+A galaxies defined by Goto, and each of these objects is further visually identified. In this sample, most objects are low redshift E+A galaxies with z < 0.25, and locate in the high latitude sky area with magnitude among 14 to 18 mag in g, r and i bands. A stellar population analysis for the whole sample indicates that the E+A galaxies are characterized by both young and old stellar populations (SPs), and the metal-rich SPs have relatively higher contributions than the metal-poor ones. Additionally, a morphological classification for these objects is performed based on the images taken from the Sloan Digital Sky Survey (SDSS).
Astrophysical dynamo theories provide various mechanisms for magnetic field amplification inside galaxies, where weak initial fields grow exponentially on various timescales. We investigate the particular role played by stellar feedback mechanisms in creating strong fluid turbulence, allowing for a magnetic dynamo to emerge. Performing magnetohydrodynamic simulations of isolated cooling halos, for both dwarf and Milky Way sized objects, we compare the magnetic field evolution for various initial field topologies and various stellar feedback mechanisms. We find that feedback can indeed drive strong gas turbulence which gives rise to a fast exponential magnetic field growth. Our simulations feature typical properties of Kolmogorov turbulence with a $k ^{-5/3}$ kinetic energy spectrum, as well as the characteristic properties of a small-scale dynamo, with a $k^{3/2}$ magnetic energy spectrum as predicted by Kazantsev dynamo theory. In these feedback-dominated galaxies, stellar feedback provides forcing on large scales close to the halo scale radius, providing thus exponential field growth on all scales within the galaxy. We also investigate simulations with a final quiescent phase by manually turning off the feedback. As turbulence decreases, the galactic fountain settles into a thin, rotationally supported disk. The magnetic field develops a large-scale, well-ordered structure with quadrupole symmetry, irrespective of the initial field topology, which is in good agreement with magnetic field observations of nearby spirals. Our findings suggest that weak initial seed fields were first amplified by a small-scale dynamo during a violent, feedback-dominated early phase in the galaxy formation history, followed by a more quiescent evolution, where the fields have slowly decayed or were maintained via large-scale dynamo action.
We identify 4 unusually bright (H<~25.5) galaxies from HST and Spitzer CANDELS data with probable redshifts z~7-9. These identifications constitute the brightest-known galaxies to date at z>~7.5. As Y-band observations are not available over the full CANDELS program to perform a standard Lyman-break selection of z>7 galaxies, here we employ an alternate strategy using the deep Spitzer/IRAC data. We identify z~7.1-9.1 galaxies by selecting z>~6 galaxies from the HST CANDELS data that show quite red IRAC [3.6]-[4.5] colors, indicating a strong [OIII] line in the 4.5mu band. This selection strategy was validated using a modest sample for which we have deep Y-band coverage. Here we focus on using this criterion to select the brightest z>~7 sources. Applying the IRAC criteria to all HST-selected optical-dropout galaxies over the full ~900 arcmin**2 of the 5 CANDELS fields revealed four unusually bright z~7.1, 7.6, 7.9 and 8.6 candidates. The median [3.6]-[4.5] color of our selected z~7.1-9.1 sample is consistent with rest-frame [OIII]+Hbeta EWs of ~1600A in the [4.5] band. Keck/MOSFIRE spectroscopy has already been reported for one of our selected sources EGS-zs8-1, showing Lyalpha at a redshift of 7.7302$\pm$0.0006. We present similar Keck/MOSFIRE spectroscopy for a second selected galaxy with a probable 4.7sigma Lyalpha-line at a redshift of 7.4770+/-0.0008. Both have H-band magnitudes of ~25 mag and are ~0.5 mag more luminous (M(UV)~-22.0) than any previously discovered z~8 galaxy, with important implications for the UV LF. Our 3 brightest, highest redshift z>~7 galaxies all lie within the CANDELS EGS field, providing a dramatic illustration of the potential impact of field-to-field variance.
We present a theory of large-scale dynamo action in a turbulent flow that has stochastic, zero-mean fluctuations of the $\alpha$ parameter. We extend the Kraichnan-Moffatt model to explore effects of finite memory of $\alpha$ fluctuations, in a spirit similar to that of Sridhar & Singh (2014), hereafter SS14. Using the first-order smoothing approximation, we derive a linear integro-differential equation governing the dynamics of the large-scale magnetic field, which is non-perturbative in the $\alpha$-correlation time $\tau_{\alpha}$. We recover earlier results in the exactly solvable white-noise (WN) limit where the Moffatt drift does not contribute to the dynamo growth/decay. To study finite memory effects, we reduce the integro-differential equation to a partial differential equation by assuming that the $\tau_{\alpha}$ be small but nonzero and the large-scale magnetic field is slowly varying. We derive the dispersion relation and provide explicit expression for the growth rate as a function of four independent parameters. When $\tau_{\alpha}\neq 0$, we find that: (i) in the absence of the Moffatt drift, but with finite Kraichnan diffusivity, only strong $\alpha$-fluctuations can enable a mean-field dynamo (this is qualitatively similar to the WN case); (ii) in the general case when also the Moffatt drift is nonzero, both, weak or strong $\alpha$ fluctuations, can lead to a large-scale dynamo; and (iii) there always exists a wavenumber ($k$) cutoff at some large $k$ beyond which the growth rate turns negative, irrespective of weak or strong $\alpha$ fluctuations. Thus we show that a finite Moffatt drift can always facilitate large-scale dynamo action if sufficiently strong, even in case of weak $\alpha$ fluctuations, and the maximum growth occurs at intermediate wavenumbers ($\sim$ few times $k_{\alpha}$).
We propose a novel method to constrain the spatial extent of dust around galaxies through the measurement of dust temperature. Our method combines the dust emission of galaxies from far-infrared (FIR) image stacking analysis and the quasar reddening due to the dust absorption around galaxies. As a specific application of our method, we use the stacked FIR emission profiles of SDSS photometric galaxies over the IRAS 100$\mu$m map, and the recent measurement of the SDSS galaxy-quasar cross-correlation. If we adopt a single-temperature dust model, the resulting temperature is around 18K, which is consistent with a typical dust temperature for a central part of galaxies. If we assume an additional dust component with much lower temperature, the current data imply the temperature of the galactic dust needs to be higher, 20K to 30K. Since the model of the density and temperature distribution of dust adopted in the current paper is very simple, we cannot draw any strong conclusion at this point. Nevertheless our novel method with the elaborated theoretical model and multi-band measurement of dust will offer an interesting constraint on the statistical nature of galactic dust.
In the framework of metric $f(R)$ gravity, we find the dispersion relation for the propagation of tightly wound spiral density waves in the surface of rotating, self-gravitating disks. Also, new Toomre-like stability criteria for differentially rotating disks has been derived for both fluid and stellar disks.
We present a comprehensive analysis of the globular cluster (GC) system of the Local Group dwarf irregular galaxy NGC 6822. Our study is based on homogeneous optical and near-IR photometry, as well as long-slit spectroscopic observations which are used to determine new radial velocities for 6 GCs, two of which had no previous spectroscopic information. We construct optical-near IR colour-colour diagrams and through comparison to simple stellar population models infer that the GCs have old ages consistent with being 9 Gyr or older, while their metallicities are in the range between -1.6 < [Fe/H] < -0.4. We conduct a kinematic analysis of the GC population and find tentative evidence for weak net rotation of the GC system, in the same sense as that exhibited by the underlying spheroid. The most likely amplitude of rotation is ~10 km/s, approximately half the magnitude of the observed velocity dispersion. Finally, we use the GCs to estimate the dynamical mass of NGC 6822 within 11 kpc and we formally find it to be in the range between (3-4)10^9 Msun. This implies an overall mass-to-light ratio in the range of ~ 30-40 and indicates that NGC 6822 is highly dark matter dominated. The mass and the corresponding mass-to-light ratio estimates are affected by various additional systematic effects due to limitations of the data and the model that are not necessary reflected in the formal uncertainties.
The velocity, position, and action variable evolution of a tidal stream drawn out of a star cluster in a triaxial isochrone potential containing a sub-halo population reproduces many of the orbital effects of more general cosmological halos but allows easy calculation of orbital actions. We employ a spherical shell code which we show accurately reproduces the results of a tree gravity code for a collisionless star cluster. Streams from clusters on high eccentricity orbits, $e\gtrsim 0.6$, can spread out so much that the amount of material at high enough surface density to stand out on the sky may be only a few percent of the stream's total mass. Low eccentricity streams remain more spatially coherent, but sub-halos both broaden the stream and displace the centerline with details depending on the orbits allowed within the potential. Overall, the majority of stream particles have changes in their total actions of only 1-2\%, leaving the mean stream relatively undisturbed. A halo with 1\% of the mass in sub-halos typically spreads the velocity distribution about a factor of two wider than would be expected for a smooth halo. Strong density variations, "gaps", along with mean velocity offsets, are clearly detected in low eccentricity streams for even a 0.2\% sub-halo mass fraction. Around one hundred velocity measurements per kiloparsec of stream will enable tests for the presence of a local sub-halo density as small as 0.2-0.5\% of the local mass density, with about 1\% predicted for 30 kiloparsec orbital radii streams.
Galactic HI (neutral hydrogen) shells are central to our understanding of the interstellar medium (ISM), which plays a key role in the development and evolution of galaxies, including our own. Several models involving supernovae and stellar winds have contributed to our broad understanding, but a complete, detailed picture remains elusive. To extend existing Galactic shell catalogs, we visually examined the SETHI (Search for Extraterrestrial HI) database to identify shell-like structures. This high-sensitivity 21-cm radio survey covering the Arecibo sky uniquely provides high-resolution data on shells at a wide range of Galactic latitudes. We present basic information (location, radial velocity, angular size, shape) for 74 previously unidentified HI shells. Due to limitations of coverage and data quality, and the biases inherent in search techniques, our catalog is not a complete sample of Galactic shells. We discuss the catalog completeness, and comment on the new shells' relationship with known interstellar structure as warranted. Unlike many previous catalogs, this sample is not biased towards expanding shells. Where possible we also estimate the kinematic distances, physical sizes, expansion velocities, and energies of these shells. Overall, they are relatively large and old, each the result of multiple supernovae. Unlike previous surveys, we do not find that the shells in our sample are preferentially aligned relative to the Galactic plane.
The velocity function derived from large scale surveys can be compared with the predictions of LCDM cosmology, by matching the measured rotation velocities Vrot of galaxies to the maximum circular velocity of dark matter (DM) halos Vmax. For Vrot<50km/s, a major discrepancy arises between the observed and LCDM velocity functions. However, the manner in which different observational measures of Vrot are associated with Vmax is not straight forward in dwarf galaxies. We instead relate galaxies to DM halos using the empirical baryon- mass to halo-mass relation, and show that different observational measures of Vrot result in very different velocity functions. We show how the W50 velocity function, i.e. using the HI profile line width at 50% of peak HI flux to measure Vrot, can be reconciled with a LCDM cosmology. Our semi-empirical methodology allows us to determine the region of rotation curves that are probed by HI measurements (RHI), and shows that the Vrot of dwarfs are generally measured at a fraction of Rmax, explaining their tendency to have rising rotation curves. We provide fitting formulae for relating RHI and Reff (the effective radius) to the virial radius of DM halos. To continue to use velocity functions as a probe of LCDM cosmology, it is necessary to be precise about how the different measures of rotation velocity are probing the mass of the DM halos, dropping the assumption that any measure of rotational velocity can be equally used as a proxy for Vmax.
We know that magnetic fields are pervasive across all scales in the Universe and over all of cosmic time and yet our understanding of many of the properties of magnetic fields is still limited. We do not yet know when, where or how the first magnetic fields in the Universe were formed, nor do we fully understand their role in fundamental processes such as galaxy formation or cosmic ray acceleration or how they influence the evolution of astrophysical objects. The greatest challenge to addressing these issues has been a lack of deep, broad bandwidth polarimetric data over large areas of the sky. The Square Kilometre Array will radically improve this situation via an all-sky polarisation survey that delivers both high quality polarisation imaging in combination with observations of 7-14 million extragalactic rotation measures. Here we summarise how this survey will improve our understanding of a range of astrophysical phenomena on scales from individual Galactic objects to the cosmic web.
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Dark matter cores within galaxy haloes can be formed by energy feedback from star forming regions: an energy balance suggests that the maximum core formation efficiency arises in galaxies with M$_{\star}\sim10^{8.5}$M$_{\odot}$. We show that a model population of galaxies, in which the density profile has been modified by such baryonic feedback, is able to explain the observed galaxy velocity function and Tully-Fisher relations significantly better than a model in which a universal cuspy density profile is assumed. Alternative models, namely warm or self-interacting dark matter, also provide a better match to these observed relations than a universal profile model does, but make different predictions for how halo density profiles vary with mass compared to the baryonic feedback case. We propose that different core formation mechanisms may be distinguished based on the imprint they leave on galaxy populations over a wide range of mass. Within the current observational data we find evidence of the expected signatures of the mass dependence of core formation generated by baryonic feedback.
We present Keck/DEIMOS spectroscopy of stars in the recently discovered Milky Way satellites Hydra II, Pisces II, and Laevens 1. We measured a velocity dispersion of 5.4 (+3.6 -2.4) km/s for Pisces II, but we did not resolve the velocity dispersions of Hydra II or Laevens 1. We marginally resolved the metallicity dispersions of Hydra II and Pisces II but not Laevens 1. Furthermore, Hydra II and Pisces II obey the luminosity-metallicity relation for Milky Way dwarf galaxies (<[Fe/H]> = -2.02 +/- 0.08 and -2.45 +/- 0.07, respectively), whereas Laevens 1 does not (<[Fe/H]> = -1.68 +/- 0.05). The kinematic and chemical properties suggest that Hydra II and Pisces II are dwarf galaxies, and Laevens 1 is a globular cluster. We determined that two of the previously observed blue stars near the center of Laevens 1 are not members of the cluster. A third blue star has ambiguous membership. If it is a member, we suggest that it could be a Type II Cepheid variable. Hydra II has a radial velocity <v_helio> = 303.1 +/- 1.4 km/s, similar to the leading arm of the Magellanic stream. The mass-to-light ratio for Pisces II is 370 (+310 -240) M_sun/L_sun. It is not among the most dark matter-dominated dwarf galaxies, but it is still worthy of inclusion in the search for gamma rays from dark matter self-annihilation.
We present a detailed analysis of a large-scale galactic outflow in the CGM of a massive (M_h ~ 10^12.5 Msun), star-forming (6.9 Msun/yr), sub-L* (0.5 L_B*) galaxy at z=0.39853 that exhibits a wealth of metal-line absorption in the spectra of the background quasar Q 0122-003 at an impact parameter of 163 kpc. The galaxy inclination angle (i=63 degree) and the azimuthal angle (Phi=73 degree) imply that the QSO sightline is passing through the projected minor-axis of the galaxy. The absorption system shows a multiphase, multicomponent structure with ultra-strong, wide velocity spread OVI (logN = 15.16\pm0.04, V_{90} = 419 km/s) and NV (logN = 14.69\pm0.07, V_{90} = 285 km/s) lines that are extremely rare in the literature. The highly ionized absorption components are well explained as arising in a low density (10^{-4.2} cm^{-3}), diffuse (10 kpc), cool (10^4 K) photoionized gas with a super-solar metallicity ([X/H] > 0.3). From the observed narrowness of the Lyb profile, the non-detection of SIV absorption, and the presence of strong CIV absorption we rule out equilibrium/non-equilibrium collisional ionization models. The low-ionization photoionized gas with a density of 10^{-2.5} cm^{-3} and a metallicity of [X/H] > -1.4 is possibly tracing recycled halo gas. We estimate an outflow mass of ~2x10^{10} Msun, a mass-flow rate of ~54 Msun/yr, a kinetic luminosity of ~9x10^{41} erg/s, and a mass loading factor of ~8 for the outflowing high-ionization gas. These are consistent with the properties of "down-the-barrel" outflows from infrared-luminous starbursts as studied by Rupke et al. Such powerful, large-scale, metal-rich outflows are the primary means of sufficient mechanical and chemical feedback as invoked in theoretical models of galaxy formation and evolution.
The deep, wide-area (900 arcmin**2) near-IR/WFC3/IR + Spitzer/IRAC observations over the CANDELS program represents a significant resource for constraining the bright end of the z~9 and z~10 luminosity functions (LFs) in the UV. We recently reported the discovery of 6 luminous z~9-10 candidates over the GOODS-North+South fields, but extending this search to the full CANDELS program was limited due to the lack of HST-depth 1.05-micron observations in the other 3 CANDELS fields. Here we attempt to significantly realize the potential of CANDELS for z=9-10 science by combining a search over all 5 fields with results from a new HST program (B9-CANDELS) designed to follow up the highest-probability z~9-10 galaxy candidates with observations at 1.05 microns. The targeted z~9-10 candidates are preselected by taking advantage of the full HST, Spitzer/IRAC S-CANDELS observations, and the deepest-available ground-based optical+near-IR observations. With our follow-up program now 91% complete, we identify 4 new high-probability z~9-10 galaxies. This brings our total sample of bright z~9-10 galaxies to 14, including several other new sources from the CANDELS GOODS + ERS fields.Through extensive simulations, we replicate the selection process for our sample (both the preselection and follow-up) and obtain an accurate estimate of the volume density of bright galaxies (M_{UV,AB}<-20) at both z~9 and z~10. The volume density of bright z~9 and z~10 galaxies that we find is 4.5(-1.3)(+2.5)x and 8(-3)(+7)x lower than found at z~8. When compared with the best-fit evolution in the UV luminosities densities from z~8 to z~4 (>0.4L*), the luminosity densities we find at z~9 and z~10 are ~2x lower than the extrapolated trends. We would expect significant additional gains in these results from the on-going 500-orbit BoRG[z910] program and by obtaining additional follow-up observations over the CANDELS-WIDE fields.
We present an integral field study of the internal structure, kinematics and stellar population of the almost edge-on, intermediate luminosity ($L_ {*}$) elliptical galaxy NGC~4697. We build extended 2-dimensional (2D) maps of the stellar kinematics and line-strengths of the galaxy up to $\sim 0.7 $ effective radii (R$_{eff}$) using a mosaic of 8 VIMOS (VIsible Multi-Objects Spectrograph on the VLT) integral-field unit pointings. We find clear evidence for a rotation-supported structure along the major axis from the 2D kinematical maps, confirming the previous classification of this system as a `fast-rotator'. We study the correlations between the third and fourth Gauss-Hermite moments of the line-of-sight velocity distribution (LOSVD) $h_3$ and $h_4$ with the rotation parameter ($V/\sigma$), and compare our findings to hydrodynamical simulations. We find remarkable similarities to predictions from gas-rich mergers. Based on photometry, we perform a bulge/disk decomposition and study the stellar population properties of the two components. The bulge and the disk show different stellar populations, with the stars in the bulge being older (age$_{\rm bulge}=13.5^{+1.4}_{-1.4}$ Gyr, age$_{\rm disk}=10.5^{+1.6}_{-2.0}$Gyr) and more metal-poor ($\mathrm{[M/H]_{bulge}} = -0.17^{+0.12}_{-0.1}$, $\mathrm{[M/H]_{disk}}=-0.03^{+0.02}_{-0.1}$). The evidence of a later-formed, more metal-rich disk embedded in an older, more metal-poor bulge, together with the LOSVD structure, supports a mass assembly scenario dominated by gas-rich minor mergers and possibly with a late gas-rich major merger that left a previously rapidly rotating system unchanged. The bulge and the disk do not show signs of different stellar Initial Mass Function slopes, and both match well with a Milky Way-like IMF.
The density wave theory for the grand-design two-armed spiral pattern in galaxies is successful in explaining several observed features. However, the long-term persistence of this spiral structure is a serious problem since the group transport would destroy it within about a billion years as shown in a classic paper by Toomre. In this paper we include the low velocity dispersion component, namely gas, on an equal footing with stars in the formulation of the density wave theory, and obtain the dispersion relation for this coupled system. We show that the inclusion of gas makes the group transport slower by a factor of few, thus allowing the pattern to persist longer - for several billion years. Though still less than the Hubble time, this helps in making the spiral structure more long-lived. Further we show that addition of gas is essential to get a stable wave for the observed pattern speed for the Galaxy, which otherwise is not possible for a one-component stellar disc.
We explore the assembly history of the M31 bulge within a projected major-axis radius of 180" (~680 pc) by studying its stellar populations in Hubble Space Telescope WFC3 and ACS observations. Colors formed by comparing near-ultraviolet vs. optical bands are found to become bluer with increasing major-axis radius, which is opposite to that predicted if the sole sources of near-ultraviolet light were old extreme horizontal branch stars with a negative radial gradient in metallicity. Spectral energy distribution fits require a metal-rich intermediate-age stellar population (300 Myr to 1 Gyr old, ~solar metallicity) in addition to the dominant old population. The radial gradients in age and metallicity of the old stellar population are consistent with those in previous works. For the intermediate-age population, we find an increase in age with radius and a mass fraction that increases up to 2% at 680 pc away from the center. We exclude contamination from the M31 disk and/or halo as the main origin for this population. Our results thus suggest that intermediate-age stars exist beyond the central 5" (19 pc) of M31 and contribute ~1% of the total stellar mass in the bulge. These stars could be related to the secular growth of the M31 bulge.
We present an unprecedented measurement of the disc stability and local instability scales in the luminous infrared Seyfert 1 host, NGC7469, based on ALMA observations of dense gas tracers and with a synthesized beam of 165 x 132 pc. While we confirm that non-circular motions are not significant in redistributing the dense interstellar gas in this galaxy, we find compelling evidence that the dense gas is a suitable tracer for studying the origin of its intensely high-mass star forming ring-like structure. Our derived disc stability parameter accounts for a thick disc structure and its value falls below unity at the radii in which intense star formation is found. Furthermore, we derive the characteristic instability scale and find a striking agreement between our measured scale of ~ 180 pc, and the typical sizes of individual complexes of young and massive star clusters seen in high-resolution images.
Using V band photometry of the WINGS survey, we derive galaxy luminosity functions (LF) in nearby clusters. This sample is complete down to Mv=-15.15, and it is homogeneous, thus allowing the study of an unbiased sample of clusters with different characteristics. We constructed the photometric LF for 72 out of the original 76 WINGS clusters, excluding only those without a velocity dispersion estimate. For each cluster we obtained the LF for galaxies in a region of radius=0.5 x r200, and fitted them with single and double Schechter's functions. We also derive the composite LF for the entire sample, and those pertaining to different morphological classes. Finally we derive the spectroscopic cumulative LF for 2009 galaxies that are cluster members. The double Schechter fit parameters are neither correlated with the cluster velocity dispersion, nor with the X-ray luminosity. Our median values of the Schechter's fit slope are, on average, in agreement with measurements of nearby clusters, but are less steep that those derived from large surveys, such as the SDSS. Early--type galaxies outnumber late-types at all magnitudes, but both early and late types contribute equally to the faint end of the LF. Finally, the spectroscopic LF is in excellent agreement with the ones derived for A2199, A85 and Virgo, and with the photometric one at the bright magnitudes (where both are available). There is a large spread in the LF of different clusters. However, this spread is not caused by correlation of the LF shape with cluster characteristics such as X--ray luminosity or velocity dispersions. The faint end is flatter than what previously derived (alpha_f=-1.7) at odds with what predicted from numerical simulations.
Recently some pessimism has been expressed about our lack of progress in understanding quasars over the 50+ year since their discovery. It is worthwhile to look back at some of the progress that has been made - but still lies under the radar - perhaps because few people are working on optical/UV spectroscopy in this field. Great advances in understanding quasar phenomenology have emerged using eigenvector techniques. The 4D eigenvector 1 context provides a surrogate H-R Diagram for quasars with a source main sequence driven by Eddington ratio convolved with line-of-sight orientation. Appreciating the striking differences between quasars at opposite ends of the main sequence (so-called population A and B sources) opens the door towards a unified model of quasar physics, geometry and kinematics. We present a review of some of the progress that has been made over the past 15 years, and point out unsolved issues.
The Spitzer-Cosmic Assembly Deep Near-Infrared Extragalactic Legacy Survey (S-CANDELS; PI G. Fazio) is a Cycle 8 Exploration Program designed to detect galaxies at very high redshifts (z > 5). To mitigate the effects of cosmic variance and also to take advantage of deep coextensive coverage in multiple bands by the Hubble Space Telescope Multi-Cycle Treasury Program CANDELS, S-CANDELS was carried out within five widely separated extragalactic fields: the UKIDSS Ultra-Deep Survey, the Extended Chandra Deep Field South, COSMOS, the HST Deep Field North, and the Extended Groth Strip. S-CANDELS builds upon the existing coverage of these fields from the Spitzer Extended Deep Survey (SEDS) by increasing the integration time from 12 hours to a total of 50 hours but within a smaller area, 0.16 square degrees. The additional depth significantly increases the survey completeness at faint magnitudes. This paper describes the S-CANDELS survey design, processing, and publicly-available data products. We present IRAC dual-band 3.6+4.5 micron catalogs reaching to a depth of 26.5 AB mag. Deep IRAC counts for the roughly 135,000 galaxies detected by S-CANDELS are consistent with models based on known galaxy populations. The increase in depth beyond earlier Spitzer/IRAC surveys does not reveal a significant additional contribution from discrete sources to the diffuse Cosmic Infrared Background (CIB). Thus it remains true that only roughly half of the estimated CIB flux from COBE/DIRBE is resolved.
The results of the searching on the bases of GPS UKIDSS survey's data of dense compact stellar clusters in the vicinity of 19 YSOs with high and middle masses are presented. Totally we have revealed clusters in 12 areas. Around 5 objects (IRAS 18151-1208, IRAS 18316-0602, IRAS 19110+1045, IRAS 19213+1723, IRAS 20056+3350) they are newly detected. The clusters associated with IRAS 05168+3634, IRAS 20188+3928, IRAS 19374+2352 and IRAS 19388+2357 sources have been already revealed on the less depth data than GPS UKIDSS survey images. The compact groups of stars located in the vicinity of IRAS 05358+3543, IRAS 18507+0121 and IRAS 20198+3716 sources belong to the more extensive clusters. The radii and stellar density have significant gradient: from 0.2 to 2.7 pc and from 3 to 1000 stars/arcmin^2 respectively. In the vicinity of 7 IRAS sources (IRAS 18174-1612, IRAS 18360-0537, IRAS 18385-0512, IRAS 18517+0437, IRAS 19092+0841, IRAS 19410+2336, IRAS 20126+4104) the stellar clusters were not revealed.
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The current hierarchical merging paradigm and $\Lambda$CDM predict that the $z \sim 4-8$ universe should be a time in which the most massive galaxies are transitioning from their initial halo assembly to the later baryonic evolution seen in star-forming galaxies and quasars. However, no evidence of this transition has been found in many high redshift galaxy surveys including CFHTLS, CANDELS and SPLASH, the first studies to probe the high-mass end at these redshifts. Indeed, if halo mass to stellar mass ratios estimated at lower-redshift continue to $z \sim 6-8$, CANDELS and SPLASH report several orders of magnitude more $M \sim 10^{12-13} M_\odot$ halos than are possible to have formed by those redshifts, implying these massive galaxies formed impossibly early. We consider various systematics in the stellar synthesis models used to estimate physical parameters and possible galaxy formation scenarios in an effort to reconcile observation with theory. Although known uncertainties can greatly reduce the disparity between recent observations and cold dark matter merger simulations, even taking the most conservative view of the observations, there remains considerable tension with current theory.
We carry out a direct search for bar-like non-circular flows in intermediate-inclination, gas-rich disk galaxies with a range of morphological types and photometric bar classifications from the first data release (DR1) of the CALIFA survey. We use the DiskFit algorithm to apply rotation only and bisymmetric flow models to H$\alpha$ velocity fields for 49/100 CALIFA DR1 systems that meet our selection criteria. We find satisfactory fits for a final sample of 37 systems. DiskFit is sensitive to the radial or tangential components of a bar-like flow with amplitudes greater than $15\,$km$\,$s$^{-1}$ across at least two independent radial bins in the fit, or ~2.25 kpc at the characteristic final sample distance of ~75 Mpc. The velocity fields of 25/37 $(67.6^{+6.6}_{-8.5}\%)$ galaxies are best characterized by pure rotation, although only 17/25 $(68.0^{+7.7}_{-10.4}\%)$ of them have sufficient H$\alpha$ emission near the galaxy centre to afford a search for non-circular flows. We detect non-circular flows in the remaining 12/37 $(32.4^{+8.5}_{-6.6}\%)$ galaxies. We conclude that the non-circular flows detected in 11/12 $(91.7^{+2.8}_{-14.9}\%)$ systems stem from bars. Galaxies with intermediate (AB) bars are largely undetected, and our detection thresholds therefore represent upper limits to the amplitude of the non-circular flows therein. We find 2/23 $(8.7^{+9.6}_{-2.9}\%)$ galaxies that show non-circular motions consistent with a bar-like flow, yet no photometric bar is evident. This suggests that in ~10% of galaxies either the existence of a bar may be missed completely in photometry or other processes may drive bar-like flows and thus secular galaxy evolution.
We study the rest-frame ultra-violet sizes of massive (~0.8 x 10^11 M_Sun) galaxies at 3.4<z<4.2, selected from the FourStar Galaxy Evolution Survey (ZFOURGE), by fitting single Sersic profiles to HST/WFC3/F160W images from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS). Massive quiescent galaxies are very compact, with a median circularized half-light radius r_e = 0.63 +/- 0.18 kpc. Removing 5/16 (31%) sources with signs of AGN activity does not change the result. Star-forming galaxies have r_e = 2.0 +/- 0.60 kpc, 3.2 +/- 1.3 x larger than quiescent galaxies. Quiescent galaxies at z~4 are on average 6.0 +\- 0.17 x smaller than at z~0 and 1.9 +/- 0.7 x smaller than at z~2. Star-forming galaxies of the same stellar mass are 2.4 +/- 0.7 x smaller than at z~0. Overall, the size evolution at 0<z<4 is well described by a powerlaw, with r_e = 5.08 +/- 0.28 (1+z)^(-1.44+/-0.08) kpc for quiescent and r_e = 6.02 +/- 0.28 (1+z)^(-0.72+/-0.05) kpc for star-forming galaxies. Compact star-forming galaxies are rare in our sample: we find only 1/14 (7%) with r_e / (M / 10^11 M_Sun)^0.75 < 1.5, whereas 13/16 (81%) of the quiescent galaxies is compact. The number density of compact quiescent galaxies at z~4 is 1.8 +/- 0.8 x 10^-5 Mpc^-3 and increases rapidly, by >5 x, between 2<z<4. The paucity of compact star-forming galaxies at z~4 and their large rest-frame ultra-violet median sizes suggest that the formation phase of compact cores is very short and/or highly dust obscured.
We present a sub-100 pc-scale analysis of the CO molecular gas emission and kinematics of the gravitational lens system SDP.81 at redshift 3.042 using Atacama Large Millimetre/submillimetre Array (ALMA) science verification data and a visibility-plane lens reconstruction technique. We find clear evidence for an excitation dependent structure in the unlensed molecular gas distribution, with emission in CO (5-4) being significantly more diffuse and structured than in CO (8-7). The intrinsic line luminosities are 1.04+/-0.13x10^10 and 0.32+/-0.07x10^10 K km/s pc^2 for CO (5-4) and CO (8-7), respectively, after correcting for the differential magnification. A kinemetry analysis of the velocity-fields shows evidence for a perturbed disk with multiple velocity components. Source reconstructions from ALMA and the Hubble Space Telescope are combined to investigate the morphological structure of the stellar, molecular gas and dust components of SDP.81. Together with Karl G. Jansky Very Large Array CO (1-0) data, they provide corroborative evidence for a complex ~2 kpc-scale starburst that is embedded within a larger structure that is ~15 kpc in size.
There is ample evidence from rotation curves that dark matter halo's around disk galaxies have nontrivial dynamics. Of particular significance are: a) the cored dark matter profile of disk galaxies, b) correlations of the shape of rotation curves with baryonic properties, and c) the Tully-Fisher relation. Dark matter halo's around disk galaxies may have nontrivial dynamics if dark matter is strongly self interacting and dissipative. Multicomponent hidden sector dark matter featuring a massless `dark photon' (from an unbroken dark $U(1)$ gauge interaction) which kinetically mixes with the ordinary photon provides a concrete example of such dark matter. The kinetic mixing interaction facilitates halo heating by enabling ordinary supernovae to be a source of these `dark photons'. Dark matter halo's can expand and contract in response to the heating and cooling processes, but for a sufficiently isolated halo should have evolved to a steady state or `equilibrium' configuration where heating and cooling rates locally balance. This dynamics allows the dark matter density profile to be related to the distribution of ordinary supernovae in the disk of a given galaxy. In a previous paper a simple and predictive formula was derived encoding this relation. Here we improve on previous work by modelling the supernovae distribution via the measured UV and $H\alpha$ fluxes. The resulting dark matter halo profile is then tested against the rotation curve data of all 26 dwarf galaxies in the LITTLE THINGS sample. The dissipative dark matter concept is further developed and some conclusions drawn.
The regular or chaotic character of orbits of stars moving in the meridional plane (R,z) of an axially symmetric elliptical galaxy with a dense, massive spherical nucleus and a dark matter halo component is under investigation. In particular, we explore how the flattening of an elliptical galaxy influences the overall orbital structure of the system, by computing in each case the percentage of chaotic orbits, as well as the percentages of orbits composing the main regular families. In an attempt to discriminate safely and with certainty between regular and chaotic motion, we use the Smaller ALingment Index (SALI) method to extensive samples of orbits obtained by integrating numerically the basic equations of motion as well as the variational equations. In addition, a technique which is based mainly on the field of spectral dynamics that utilizes the Fourier transform of the time series of each coordinate is used for classifying the regular orbits into different families and also to recognize the secondary resonances that usually bifurcate from them. Three cases are considered in our work: (i) the case where the elliptical galaxy is prolate (ii) the case where a spherically symmetric elliptical galaxy is present and (iii) the case where the elliptical galaxy has an oblate shape. Comparison between the current results and early related work is also made.
We present a direct comparison between the observed star formation rate functions (SFRF) and the state-of-the-art predictions of semi-analytic models (SAM) of galaxy formation and evolution. We use the PACS Evolutionary Probe Survey (PEP) and Herschel Multi-tiered Extragalactic Survey (HerMES) data-sets in the COSMOS and GOODS-South fields, combined with broad-band photometry from UV to sub-mm, to obtain total (IR+UV) instantaneous star formation rates (SFRs) for individual Herschel galaxies up to z~4, subtracted of possible active galactic nucleus (AGN) contamination. The comparison with model predictions shows that SAMs broadly reproduce the observed SFRFs up to z~2, when the observational errors on the SFR are taken into account. However, all the models seem to under-predict the bright-end of the SFRF at z>2. The cause of this underprediction could lie in an improper modelling of several model ingredients, like too strong (AGN or stellar) feedback in the brighter objects or too low fall-back of gas, caused by weak feedback and outflows at earlier epochs.
Variations in the observed dust emission and extinction indicate a systematic evolution of grain properties in the transition from the diffuse interstellar medium (ISM) to denser molecular clouds. The differences in the dust spectral energy distribution (SED) observed from the diffuse ISM to denser regions, namely an increase in the spectral index at long wavelengths, an increase in the FIR opacity, and a decrease in temperature, are usually assumed to be the result of changes in dust properties. We investigate if evolutionary processes, such as coagulation and accretion, are able to change the dust properties of grains in a way that is consistent with observations. We use a core-mantle grain model to describe diffuse ISM-type grains, and using DDA we calculate how the accretion of mantles and coagulation into aggregates vary the grain optical properties. We calculate the dust SED and extinction using DustEM and the radiative transfer code CRT. We show that the accretion of an aliphatic carbon mantle on diffuse ISM-type dust leads to an increase in the FIR opacity by a factor of about 2 and in the FIR/submm spectral index from 1.5 to 1.8, and to a decrease in the temperature by about 2 K. We also show that the coagulation of these grains into aggregates further decreases the temperature by 3 K and increases the spectral index up to a value of $\sim$2. The FIR opacity is increased by a factor of 3 (7) for these aggregates (with an additional ice-mantle) compared to the diffuse ISM-dust. Dust evolution in the ISM resulting from coagulation and accretion, leads to significant changes in the optical properties of the grains that can explain the observed variations in the dust SED in the transition from the diffuse ISM to denser regions.
We present the detection of a giant radio halo (GRH) in the Sunyaev-Zel'dovich (SZ)-selected merging galaxy cluster ACT-CL J0256.5+0006 ($z = 0.363$), observed with the Giant Metrewave Radio Telescope at 325 MHz and 610 MHz. We find this cluster to host a faint ($S_{610} = 5.6 \pm 1.4$ mJy) radio halo with an angular extent of 2.6 arcmin, corresponding to 0.8 Mpc at the cluster redshift, qualifying it as a GRH. J0256 is one of the lowest-mass systems, $M_{\rm 500,SZ} = (5.0 \pm 1.2) \times 10^{14} M_\odot$, found to host a GRH. We measure the GRH at lower significance at 325 MHz ($S_{325} = 10.3 \pm 5.3$ mJy), obtaining a spectral index measurement of $\alpha^{610}_{325} = 1.0^{+0.7}_{-0.9}$. This result is consistent with the mean spectral index of the population of typical radio halos, $\alpha = 1.2 \pm 0.2$. Adopting the latter value, we determine a 1.4 GHz radio power of $P_{1.4\text{GHz}} = (1.0 \pm 0.3) \times 10^{24}$ W Hz$^{-1}$, placing this cluster within the scatter of known scaling relations. Various lines of evidence, including the ICM morphology, suggest that ACT-CL J0256.5+0006 is composed of two subclusters. We determine a merger mass ratio of 7:4, and a line-of-sight velocity difference of $v_\perp = 1880 \pm 280$ km s$^{-1}$. We construct a simple merger model to infer relevant time-scales in the merger. From its location on the $P_{\rm 1.4GHz}{-}L_{\rm X}$ scaling relation, we infer that we observe ACT-CL J0256.5+0006 approximately 500 Myr before first core crossing.
Zinc in stars is an important reference element because it is a proxy to Fe in studies of damped Lyman-alpha systems, permitting a comparison of chemical evolution histories of bulge stellar populations and DLAs. In terms of nucleosynthesis, it behaves as an alpha element because it is enhanced in metal-poor stars. The aim of this work is to derive the iron-peak element Zn abundances in 56 bulge giants from high resolution spectra. These results are compared with data from other bulge samples, as well as from disk and halo stars, and damped Lyman-alpha systems, in order to better understand the chemical evolution in these environments. High-resolution spectra were obtained using FLAMES+UVES on the Very Large Telescope. We find [Zn/Fe]=+0.24+-0.02 in the range -1.3 < [Fe/H] < -0.5 and [Zn/Fe]=+0.06+-0.02 in the range -0.5 < [Fe/H] < -0.1, whereas for [Fe/H] > -0.1, it shows a spread of -0.60 < [Zn/Fe] < +0.15, with most of these stars having low [Zn/Fe]<0.0. These low zinc abundances at the high metallicity end of the bulge define a decreasing trend in [Zn/Fe] with increasing metallicities. A comparison with Zn abundances in DLA systems is presented, where a dust-depletion correction was applied for both Zn and Fe. Finally, we present a chemical evolution model of Zn enrichment in massive spheroids, representing a typical classical bulge.
We fit model spectral energy distributions to each pixel in 67 nearby (<z>=0.0057) galaxies using broadband photometry from the Sloan Digital Sky Survey and GALEX. For each galaxy, we compare the stellar mass derived by summing the mass of each pixel to that found from fitting the entire galaxy treated as an unresolved point source. We find that, while the pixel-by-pixel and unresolved masses of galaxies with low specific star formation rates (such as ellipticals and lenticulars) are in rough agreement, the unresolved mass estimate for star-forming galaxies is systematically lower then the measurement from spatially-resolved photometry. The discrepancy is strongly correlated with sSFR, with the highest sSFRs in our sample having masses underestimated by 25% (0.12 dex) when treated as point sources. We found a simple relation to statistically correct mass estimates derived from unresolved broad-band SED fitting to the resolved mass estimates: m_{resolved} = m_{unresolved}/(-0.057log(sSFR) + 0.34) where sSFR is in units of yr^{-1}. We study the effect of varying spatial resolution by degrading the image resolution of the largest images and find a sharp decrease in the pixel-by-pixel mass estimate at a physical scale of approximately 3 kpc, which is comparable to spiral arm widths. The effects we observe are consistent with the "outshining" idea which posits that the youngest stellar populations mask more massive, older -- and thus fainter -- stellar populations. Although the presence of strong dust lanes can also lead to a drastic difference between resolved and unresolved mass estimates (up to 45% or 0.3 dex) for any individual galaxy, we found that resolving dust does not affect mass estimates on average. The strong correlation between mass discrepancy and sSFR is thus most likely due to the outshining systematic bias.
Merging haloes with similar masses (i.e., major mergers) pose significant challenges for halo finders. We compare five halo finding algorithms' (AHF, HBT, Rockstar, SubFind, and VELOCIraptor) recovery of halo properties for both isolated and cosmological major mergers. We find that halo positions and velocities are often robust, but mass biases exist for every technique. The algorithms also show strong disagreement in the prevalence and duration of major mergers, especially at high redshifts (z>1). This raises significant uncertainties for theoretical models that require major mergers for, e.g., galaxy morphology changes, size changes, or black hole growth, as well as for finding Bullet Cluster analogues. All finders not using temporal information also show host halo and subhalo relationship swaps over successive timesteps, requiring careful merger tree construction to avoid problematic mass accretion histories. We suggest that future algorithms should combine phase-space and temporal information to avoid the issues presented.
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