We explore the connection between galaxies and dark matter halos in the Milky Way (MW) and quantify the implications on properties of the dark matter particle and the phenomenology of low-mass galaxy formation. This is done through a probabilistic comparison of the luminosity function of MW dwarf satellite galaxies to models based on two suites of zoom-in simulations. One suite is dark-matter-only while the other includes a disk component, therefore we can quantify the effect of the MW's baryonic disk on our results. We apply numerous Stellar-Mass-Halo-Mass (SMHM) relations allowing for multiple complexities: scatter, a characteristic break scale, and subhalos which host no galaxy. In contrast to previous works we push the model/data comparison to the faintest dwarfs by modeling observational incompleteness, allowing us to draw three new conclusions. Firstly, we constrain the SMHM relation for $10^2<M_*/M_\odot<10^8$ galaxies, allowing us to bound the peak halo mass of the faintest MW satellite to $M_\mathrm{vir}<2.4\times10^8M_\odot$ ($1\sigma$). Secondly, by translating to a Warm Dark Matter (WDM) cosmology, we bound the thermal relic mass $m_\mathrm{WDM}>2.9$ keV at 95% confidence, on a par with recent constraints from the Lyman-$\alpha$ forest. Lastly, we find that the observed number of ultra-faint MW dwarfs is in tension with the theoretical prediction that reionisation prevents galaxy formation in almost all $10^8M_\odot$ halos. This can be tested with the next generation of deep imaging surveys. To this end, we predict the likely number of detectable satellite galaxies in the Subaru/HSC survey and the LSST. Confronting these predictions with future observations will be amongst our strongest tests of WDM and the effect reionisation on low-mass systems.
Deuterated species are unique and powerful tools in astronomy since they can probe the physical conditions, chemistry, and ionization level of various astrophysical media. Recent observations of several deuterated species along with some of their spin isomeric forms have rekindled the interest for more accurate studies on deuterium fractionation. This paper presents the first publicly available chemical network of multiply deuterated species along with spin chemistry implemented on the latest state-of-the-art gas-grain chemical code `NAUTILUS'. D/H ratios for all deuterated species observed at different positions of TMC-1 are compared with the results of our model, which considers multiply deuterated species along with the spin chemistry of light hydrogen bearing species H2, H2+, H3+ and their isotopologues. We also show the differences in the modeled abundances of non-deuterated species after the inclusion of deuteration and spin chemistry in the model. Finally, we present a list of potentially observable deuterated species in TMC-1 awaiting detection.
Direct collapse black holes (DCBHs) are excellent candidates as seeds of supermassive black holes (SMBHs) observed at $z \gtrsim 6$. The formation of a DCBH requires a strong external radiation field to suppress $\rm H_2$ formation and cooling in a collapsing gas cloud. Such strong field is not easily achieved by first stars or normal star-forming galaxies. Here we investigate a scenario in which the previously-formed DCBH can provide the necessary radiation field for the formation of additional ones. Using one-zone model and the simulated DCBH Spectral Energy Distributions (SEDs) filtered through absorbing gas initially having column density $N_{\rm H}$, we derive the critical field intensity, $J_{\rm LW}^{\rm crit}$, to suppress $\rm H_2$ formation and cooling. For the SED model with $N_{\rm H}=1.3\times10^{25}$ cm$^{-2}$, $8.0\times10^{24}$ cm$^{-2}$ and $5.0\times10^{24}$ cm$^{-2}$, we obtain $J_{\rm LW}^{\rm crit}\approx22$, 35 and 54, all much smaller than for normal star-forming galaxies ($\gtrsim 1000$). X-ray photons from previously-formed DCBHs build up a high-$z$ X-ray background (XRB) that may boost the $J_{\rm LW}^{\rm crit}$. However, we find that in the three SED models $J_{\rm LW}^{\rm crit}$ only increases to $\approx80$, 170 and 390 respectively even when $\dot{\rho}_\bullet$ reaches the maximum value allowed by the present-day XRB level ($0.22, 0.034, 0.006~M_\odot$yr$^{-1}$Mpc$^{-3}$), still much smaller than the galactic value. Although considering the XRB from first galaxies may further increase $J_{\rm LW}^{\rm crit}$, we conclude that our investigation supports a scenario in which DCBH may be more abundant than predicted by models only including galaxies as external radiation sources.
We have detected 13 new runaway-star candidates of spectral type O combining the TGAS (Tycho-Gaia Astrometric Solution) proper motions from Gaia Data Release 1 (DR1) and the sample from GOSSS (Galactic O-Star Spectroscopic Survey). We have also combined TGAS and Hipparcos proper motions to check that our technique recovers many of the previously known O-type runaways in the sample.
Under the $\Lambda$ cold dark matter ($\Lambda$CDM) cosmological models, massive galaxies are expected to be larger in denser environments through frequent hierarchical mergers with other galaxies. Yet, observational studies of low-redshift early-type galaxies have shown no such trend, standing as a puzzle to solve during the past decade. We analyzed 73,116 early-type galaxies at $0.1\leq z < 0.15$, adopting a robust nonparametric size measurement technique and extending the analysis to many massive galaxies. We find for the first time that local early-type galaxies heavier than $10^{11.2}M_{\odot}$ show a clear environmental dependence in mass-size relation, in such a way that galaxies are as much as 20-40% larger in densest environments than in underdense environments. Splitting the sample into the brightest cluster galaxies (BCGs) and non-BCGs does not affect the result. This result agrees with the $\Lambda$CDM cosmological simulations and suggests that mergers played a significant role in the growth of massive galaxies in dense environments as expected in theory.
We study the absorption and scattering of X-ray radiation by interstellar dust particles, which allows us to access the physical and chemical properties of dust. The interstellar dust composition is not well understood, especially on the densest sight lines of the Galactic Plane. X-rays provide a powerful tool in this study. We present newly acquired laboratory measurements of silicate compounds taken at the Soleil synchrotron facility in Paris using the Lucia beamline. The dust absorption profiles resulting from this campaign were used in this pilot study to model the absorption by interstellar dust along the line of sight of the low-mass X-ray binary (LMXB) GX 5-1. The measured laboratory cross-sections were adapted for astrophysical data analysis and the resulting extinction profiles of the Si K-edge were implemented in the SPEX spectral fitting program. We derive the properties of the interstellar dust along the line of sight by fitting the Si K-edge seen in absorption in the spectrum of GX 5-1. We measured the hydrogen column density towards GX 5-1 to be $3.40\pm0.1\times10^{22} \rm cm^{-2}$. The best fit of the silicon edge in the spectrum of GX 5-1 is obtained by a mixture of olivine and pyroxene. In this study, our modeling is limited to Si absorption by silicates with different Mg:Fe ratios. We obtained an abundance of silicon in dust of $4.0\pm0.3\times10^{-5}$ per H atom and a lower limit for total abundance, considering both gas and dust, of $>4.4\times10^{-5}$ per H atom, which leads to a gas to dust ratio of >0.22. Furthermore, an enhanced scattering feature in the Si K-edge may suggest the presence of large particles along the line of sight.
Quasi-simultaneous observations of the Flat Spectrum Radio Quasar PKS 2326-502 were carried out in the gamma-ray, X-ray, UV, optical, near-infrared, and radio bands. Thanks to these observations we are able to characterize the spectral energy distribution of the source during two flaring and one quiescent gamma-ray states. These data were used to constrain one-zone leptonic models of the spectral energy distributions of each flare and investigate the physical conditions giving rise to them. While modeling one flare only required changes to the electron spectrum, the other flare needed changes in both the electron spectrum and the size of the emitting region with respect to the quiescent state. These results are consistent with an emerging pattern of two broad classes of flaring states seen in blazars.
The galaxy cluster Abell 3376 is a nearby (z=0.046) dissociative merging cluster surrounded by two prominent radio relics and showing an X-ray comet-like morphology. The merger system is comprised of the subclusters A3376W & A3376E. Based on new deep multi-wavelength large-field images and published redshifts, we bring new insights about the history of this merger. Despite the difficulty of applying the weak lensing technique at such low redshift, we successfully recovered the mass distribution in the cluster field. Moreover, with the application of a two-body model, we have addressed the dynamics of these merging system. We have found the individual masses of M_{200}^{W}=3.01_{-1.73}^{+1.27} X 10^14 M_\odot and M_{200}^{E}=0.92_{-0.76}^{+0.45} X 10^14 M_\odot. The cometary shaped X-ray distribution shows only one peak spatially coincident with both Eastern BCG and the A3376E mass peak whereas the gas content of A3376W seems to be stripped out. Our data allowed us to confirm the existence of a third subcluster located at the North, 1147 +- 62 kpc apart from the neighbour subcluster A3376E and having a mass M_{200}^{N}=1.38_{-1.05}^{+0.68} X 10^14 M_\odot. From our dynamical analysis, we found the merging is taking place very close to the plane of the sky, with the merger axis just 10+-11 degrees from it. The application of a two-body analysis code showed that the merging cluster is seen 0.87_{-0.31}^{+0.22} Gyr after the pericentric passage and it is currently going to the point of maximum separation between the subclusters.
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We explore the connection between galaxies and dark matter halos in the Milky Way (MW) and quantify the implications on properties of the dark matter particle and the phenomenology of low-mass galaxy formation. This is done through a probabilistic comparison of the luminosity function of MW dwarf satellite galaxies to models based on two suites of zoom-in simulations. One suite is dark-matter-only while the other includes a disk component, therefore we can quantify the effect of the MW's baryonic disk on our results. We apply numerous Stellar-Mass-Halo-Mass (SMHM) relations allowing for multiple complexities: scatter, a characteristic break scale, and subhalos which host no galaxy. In contrast to previous works we push the model/data comparison to the faintest dwarfs by modeling observational incompleteness, allowing us to draw three new conclusions. Firstly, we constrain the SMHM relation for $10^2<M_*/M_\odot<10^8$ galaxies, allowing us to bound the peak halo mass of the faintest MW satellite to $M_\mathrm{vir}<2.4\times10^8M_\odot$ ($1\sigma$). Secondly, by translating to a Warm Dark Matter (WDM) cosmology, we bound the thermal relic mass $m_\mathrm{WDM}>2.9$ keV at 95% confidence, on a par with recent constraints from the Lyman-$\alpha$ forest. Lastly, we find that the observed number of ultra-faint MW dwarfs is in tension with the theoretical prediction that reionisation prevents galaxy formation in almost all $10^8M_\odot$ halos. This can be tested with the next generation of deep imaging surveys. To this end, we predict the likely number of detectable satellite galaxies in the Subaru/HSC survey and the LSST. Confronting these predictions with future observations will be amongst our strongest tests of WDM and the effect reionisation on low-mass systems.
Deuterated species are unique and powerful tools in astronomy since they can probe the physical conditions, chemistry, and ionization level of various astrophysical media. Recent observations of several deuterated species along with some of their spin isomeric forms have rekindled the interest for more accurate studies on deuterium fractionation. This paper presents the first publicly available chemical network of multiply deuterated species along with spin chemistry implemented on the latest state-of-the-art gas-grain chemical code `NAUTILUS'. D/H ratios for all deuterated species observed at different positions of TMC-1 are compared with the results of our model, which considers multiply deuterated species along with the spin chemistry of light hydrogen bearing species H2, H2+, H3+ and their isotopologues. We also show the differences in the modeled abundances of non-deuterated species after the inclusion of deuteration and spin chemistry in the model. Finally, we present a list of potentially observable deuterated species in TMC-1 awaiting detection.
Direct collapse black holes (DCBHs) are excellent candidates as seeds of supermassive black holes (SMBHs) observed at $z \gtrsim 6$. The formation of a DCBH requires a strong external radiation field to suppress $\rm H_2$ formation and cooling in a collapsing gas cloud. Such strong field is not easily achieved by first stars or normal star-forming galaxies. Here we investigate a scenario in which the previously-formed DCBH can provide the necessary radiation field for the formation of additional ones. Using one-zone model and the simulated DCBH Spectral Energy Distributions (SEDs) filtered through absorbing gas initially having column density $N_{\rm H}$, we derive the critical field intensity, $J_{\rm LW}^{\rm crit}$, to suppress $\rm H_2$ formation and cooling. For the SED model with $N_{\rm H}=1.3\times10^{25}$ cm$^{-2}$, $8.0\times10^{24}$ cm$^{-2}$ and $5.0\times10^{24}$ cm$^{-2}$, we obtain $J_{\rm LW}^{\rm crit}\approx22$, 35 and 54, all much smaller than for normal star-forming galaxies ($\gtrsim 1000$). X-ray photons from previously-formed DCBHs build up a high-$z$ X-ray background (XRB) that may boost the $J_{\rm LW}^{\rm crit}$. However, we find that in the three SED models $J_{\rm LW}^{\rm crit}$ only increases to $\approx80$, 170 and 390 respectively even when $\dot{\rho}_\bullet$ reaches the maximum value allowed by the present-day XRB level ($0.22, 0.034, 0.006~M_\odot$yr$^{-1}$Mpc$^{-3}$), still much smaller than the galactic value. Although considering the XRB from first galaxies may further increase $J_{\rm LW}^{\rm crit}$, we conclude that our investigation supports a scenario in which DCBH may be more abundant than predicted by models only including galaxies as external radiation sources.
We have detected 13 new runaway-star candidates of spectral type O combining the TGAS (Tycho-Gaia Astrometric Solution) proper motions from Gaia Data Release 1 (DR1) and the sample from GOSSS (Galactic O-Star Spectroscopic Survey). We have also combined TGAS and Hipparcos proper motions to check that our technique recovers many of the previously known O-type runaways in the sample.
Under the $\Lambda$ cold dark matter ($\Lambda$CDM) cosmological models, massive galaxies are expected to be larger in denser environments through frequent hierarchical mergers with other galaxies. Yet, observational studies of low-redshift early-type galaxies have shown no such trend, standing as a puzzle to solve during the past decade. We analyzed 73,116 early-type galaxies at $0.1\leq z < 0.15$, adopting a robust nonparametric size measurement technique and extending the analysis to many massive galaxies. We find for the first time that local early-type galaxies heavier than $10^{11.2}M_{\odot}$ show a clear environmental dependence in mass-size relation, in such a way that galaxies are as much as 20-40% larger in densest environments than in underdense environments. Splitting the sample into the brightest cluster galaxies (BCGs) and non-BCGs does not affect the result. This result agrees with the $\Lambda$CDM cosmological simulations and suggests that mergers played a significant role in the growth of massive galaxies in dense environments as expected in theory.
We study the absorption and scattering of X-ray radiation by interstellar dust particles, which allows us to access the physical and chemical properties of dust. The interstellar dust composition is not well understood, especially on the densest sight lines of the Galactic Plane. X-rays provide a powerful tool in this study. We present newly acquired laboratory measurements of silicate compounds taken at the Soleil synchrotron facility in Paris using the Lucia beamline. The dust absorption profiles resulting from this campaign were used in this pilot study to model the absorption by interstellar dust along the line of sight of the low-mass X-ray binary (LMXB) GX 5-1. The measured laboratory cross-sections were adapted for astrophysical data analysis and the resulting extinction profiles of the Si K-edge were implemented in the SPEX spectral fitting program. We derive the properties of the interstellar dust along the line of sight by fitting the Si K-edge seen in absorption in the spectrum of GX 5-1. We measured the hydrogen column density towards GX 5-1 to be $3.40\pm0.1\times10^{22} \rm cm^{-2}$. The best fit of the silicon edge in the spectrum of GX 5-1 is obtained by a mixture of olivine and pyroxene. In this study, our modeling is limited to Si absorption by silicates with different Mg:Fe ratios. We obtained an abundance of silicon in dust of $4.0\pm0.3\times10^{-5}$ per H atom and a lower limit for total abundance, considering both gas and dust, of $>4.4\times10^{-5}$ per H atom, which leads to a gas to dust ratio of >0.22. Furthermore, an enhanced scattering feature in the Si K-edge may suggest the presence of large particles along the line of sight.
Quasi-simultaneous observations of the Flat Spectrum Radio Quasar PKS 2326-502 were carried out in the gamma-ray, X-ray, UV, optical, near-infrared, and radio bands. Thanks to these observations we are able to characterize the spectral energy distribution of the source during two flaring and one quiescent gamma-ray states. These data were used to constrain one-zone leptonic models of the spectral energy distributions of each flare and investigate the physical conditions giving rise to them. While modeling one flare only required changes to the electron spectrum, the other flare needed changes in both the electron spectrum and the size of the emitting region with respect to the quiescent state. These results are consistent with an emerging pattern of two broad classes of flaring states seen in blazars.
The galaxy cluster Abell 3376 is a nearby (z=0.046) dissociative merging cluster surrounded by two prominent radio relics and showing an X-ray comet-like morphology. The merger system is comprised of the subclusters A3376W & A3376E. Based on new deep multi-wavelength large-field images and published redshifts, we bring new insights about the history of this merger. Despite the difficulty of applying the weak lensing technique at such low redshift, we successfully recovered the mass distribution in the cluster field. Moreover, with the application of a two-body model, we have addressed the dynamics of these merging system. We have found the individual masses of M_{200}^{W}=3.01_{-1.73}^{+1.27} X 10^14 M_\odot and M_{200}^{E}=0.92_{-0.76}^{+0.45} X 10^14 M_\odot. The cometary shaped X-ray distribution shows only one peak spatially coincident with both Eastern BCG and the A3376E mass peak whereas the gas content of A3376W seems to be stripped out. Our data allowed us to confirm the existence of a third subcluster located at the North, 1147 +- 62 kpc apart from the neighbour subcluster A3376E and having a mass M_{200}^{N}=1.38_{-1.05}^{+0.68} X 10^14 M_\odot. From our dynamical analysis, we found the merging is taking place very close to the plane of the sky, with the merger axis just 10+-11 degrees from it. The application of a two-body analysis code showed that the merging cluster is seen 0.87_{-0.31}^{+0.22} Gyr after the pericentric passage and it is currently going to the point of maximum separation between the subclusters.
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We present new deep photometry of the rich globular cluster (GC) systems around the Brightest Cluster Galaxies UGC 9799 (Abell 2052) and UGC 10143 (Abell 2147), obtained with the HST ACS and WFC3 cameras. For comparison, we also present new reductions of similar HST/ACS data for the Coma supergiants NGC 4874 and 4889. All four of these galaxies have huge cluster populations (to the radial limits of our data, comprising from 12000 to 23000 clusters per galaxy). The metallicity distribution functions (MDFs) of the GCs can still be matched by a bimodal-Gaussian form where the metal-rich and metal-poor modes are separated by ~0.8 dex, but the internal dispersions of each mode are so large that the total MDF becomes very broad and nearly continuous from [Fe/H] = -2.4 to Solar. There are, however, significant differences between galaxies in the relative numbers of \emph{metal-rich} clusters, suggesting that they underwent significantly different histories of mergers with massive, gas-rich halos. Lastly, the proportion of metal-poor GCs rises especially rapidly outside projected radii R > 4 R_eff, suggesting the importance of accreted dwarf satellites in the outer halo. Comprehensive models for the formation of GCs as part of the hierarchical formation of their parent galaxies will be needed to trace the systematic change in structure of the MDF with galaxy mass, from the distinctly bimodal form in smaller galaxies up to the broad continuum that we see in the very largest systems.
We report a new analysis protocol for HCN hyperfine data, based on the PYSPECKIT package, and results of using this new protocol to analyse a sample area of seven massive molecular clumps from the Census of High- and Medium-mass Protostars (CHaMP) survey, in order to derive maps of column density for this species. There is a strong correlation between the HCN integrated intensity, IHCN, and previously reported IHCO+ in the clumps, but IN2H+ is not well correlated with either of these other two "dense gas tracers". The four fitted parameters from PYSPECKIT in this region fall in the range of VLSR = 8-10 km/s, {\sigma} V = 1.2-2.2 km/s, Tex = 4-15 K, and {\tau} = 0.2-2.5. These parameters allow us to derive a column density map of these clouds, without limiting assumptions about the excitation or opacity. A more traditional (linear) method of converting IHCN to total mass column gives much lower clump masses than our results based on the hyperfine analysis. This is primarily due to areas in the sample region of low I, low Tex, and high {\tau} . We conclude that there may be more dense gas in these massive clumps not engaged in massive star formation than previously recognized. If this result holds for other clouds in the CHaMP sample, it would have dramatic consequences for the calibration of the Kennicutt-Schmidt star formation laws, including a large increase in the gas depletion time-scale in such regions.
We report on the energetics of molecular outflows in 14 local Ultraluminous Infrared Galaxies (ULIRGs) that show unambiguous outflow signatures (P-Cygni profiles or high-velocity absorption wings) in the far-infrared lines of OH measured with the Herschel/PACS spectrometer. Detection of both ground-state (at 119 and 79 um) and one or more radiatively-excited (at 65 and 84 um) lines allows us to model the nuclear gas (<~300 pc) as well as the more extended components using spherically symmetric radiative transfer models. The highest molecular outflow velocities are found in buried sources, in which slower but massive expansion of the nuclear gas is also observed. With the exception of a few outliers, the outflows have momentum fluxes of (2-5)xL_IR/c and mechanical luminosities of (0.1-0.3)% of L_IR. The moderate momentum boosts in these sources (<~3) suggest that the outflows are mostly momentum-driven by the combined effects of AGN and nuclear starbursts, as a result of radiation pressure, winds, and supernovae remnants. In some sources (~20%), however, powerful (10^{10.5-11} Lsun) AGN feedback and (partially) energy-conserving phases are required, with momentum boosts in the range 3-20. These outflows appear to be stochastic strong-AGN feedback events that occur throughout the merging process. In a few sources, the outflow activity in the innermost regions has subsided in the last ~1 Myr. While OH traces the molecular outflows at sub-kpc scales, comparison of the masses traced by OH with those previously inferred from tracers of more extended outflowing gas suggests that most mass is loaded (with loading factors of Mdot/SFR=1-10) from the central galactic cores (a few x 100 pc). Outflow depletion timescales are <10^8 yr, shorter than the gas consumption timescales by factors of 1.1-15, and are anti-correlated with the AGN luminosity.
We report new HST COS and STIS spectroscopy of a star-forming region (~100 solar masses/year) in the center of the X-ray cluster RXJ1532.9+3021 (z=0.362), to follow-up the CLASH team discovery of luminous UV filaments and knots in the central massive galaxy. We detect broad (~500 km/s) Lyman alpha emission lines with extraordinarily high equivalent width (EQW~200 Angstroms) and somewhat less broadened H-alpha (~220 km/s). Forbidden lines of [N V] and [O VI] are not detected, which constrains the rate at which gas cools through temperatures of 10^6 K to be less than about 10 solar masses/year. The COS spectra also show a flat rest-frame UV continuum with weak stellar photospheric features, consistent with the presence of recently-formed hot stars forming at a rate of ~10 solar masses/year, uncorrected for dust extinction. The slope and absorption lines in these UV spectra are similar to those of Lyman Break Galaxies at z approximately 3, albeit those with the highest Lyman-alpha equivalent widths and star-formation rates. This high-EQW Lyman-alpha source is a high-metallicity galaxy rapidly forming stars in structures that look nothing like disks. This mode of star formation could significantly contribute to the spheroidal population of galaxies. The constraint on the luminosity of any [O VI] forbidden line emission is stringent enough to rule out steady and simultaneous gas cooling and star formation, unlike similar systems in the Phoenix Cluster and Abell 1795. The fact that the current star formation rate differs from the local mass cooling rate is consistent with recent simulations of episodic AGN feedback and star formation in a cluster atmosphere.
Context. X-ray spectral variability analyses of Active Galactic Nuclei (AGN) with moderate luminosities and redshifts typically show a softer when brighter behaviour. Such trend has been rarely investigated for high-luminosity AGNs ($ L_{bol}\gtrsim 10^{44}$ erg/s), nor for a wider redshift range (e.g., $0\lesssim z\lesssim 5$). Aims. We present an analysis of the spectral variability based on a large sample of 2,700 quasars, measured at several different epochs, extracted from the fifth release of the XMM-Newton Serendipitous Source Catalogue. Methods. We quantify the spectral variability through the parameter $\beta$ defined as the ratio between the change in the photon index $\Gamma$ and the corresponding logarithmic flux variation, $\beta=-\Delta\Gamma/\Delta\log F_X$. Results. Our analysis confirms a softer when brighter behaviour also for our sample, extending to high luminosity and redshift the general trend previously found. We estimate an ensemble value of the spectral variability parameter $\beta=-0.69\pm0.03$. We do not find dependence of $\beta$ on redshift, X-ray luminosity, black hole mass, Eddington ratio. A subsample of radio-loud sources shows a smaller spectral variability parameter. There is also some change with the X-ray flux, with smaller $\beta$ (in absolute value) for brighter sources. We also find significant correlations for a few individual sources, indicating more negative values for some sources.
We searched in the public archive of the Chandra X-ray Observatory as of March 2016 and assembled a sample of 719 galaxies within 50 Mpc with ACIS observations available. By cross-correlation with the optical or near-infrared nuclei of these galaxies, 314 of them are identified to have an X-ray active galactic nucleus (AGN). The majority of them are low-luminosity AGNs and are unlikely X-ray binaries based upon their spatial distribution and luminosity functions. The AGN fraction is around 60% for elliptical galaxies and early-type spirals, but drops to roughly 20% for Sc and later types, consistent with previous findings in the optical. However, the X-ray survey is more powerful in finding weak AGNs, especially from regions with active star formation that may mask the optical AGN signature. For example, 31% of the H II nuclei are found to harbor an X-ray AGN. For most objects, a single power-law model subject to interstellar absorption is adequate to fit the spectrum, and the typical photon index is found to be around 1.8. For galaxies with a non-detection, their stacked Chandra image shows an X-ray excess with a luminosity of a few times 10^37 erg/s on average around the nuclear region, possibly composed of faint X-ray binaries. This paper reports on the technique and results of the survey; in-depth analysis and discussion of the results will be reported in forthcoming papers.
A previous article argued that the antagonism between sight-lines with and without a bump at 2200 \AA\ disappears and the observed properties of interstellar extinction can be globally understood if it is accepted that scattered starlight contaminates the observed spectrum of reddened stars. The present paper provides a better understanding of the characteristics of this scattered light, and examines the consequences of this revision of interstellar extinction theory for the 2200 \AA\ bump and the diffuse interstellar bands (DIBs) questions. Two implications are worth noting: (1) the effect of interstellar extinction on cosmic distance estimations needs to be reconsidered in light of the new paradigm of interstellar extinction; (2) hypothetic particles, such as the Poly Aromatic Hydrogenated molecules, the existence of which has never been proven, are unnecessary to explain the peculiarities of interstellar extinction. Hydrogen, by far the most abundant constituent of interstellar clouds, should alone account for the three major features of interstellar extinction: the departure of the continuum from linearity, the bump, and the DIBs.
UV and optically selected candidates for stellar tidal disruption events (TDE) often exhibit broad spectral features (HeII emission, H$\alpha$ emission, or absorption lines) on a blackbody-like continuum (1e4K<T<1e5K). The lines presumably emit from TDE debris or circumnuclear clouds photoionized by the flare. Line velocities however are much lower than expected from a stellar disruption by supermassive black hole (SMBH), and are somewhat faster than expected for the broad line region (BLR) clouds of a persistently active galactic nucleus (AGN). The distinctive spectral states are not strongly related to observed luminosity and velocity, nor to SMBH mass estimates. We use exhaustive photoionization modelling to map the domain of fluxes and cloud properties that yield (e.g.) a He-overbright state where a large HeII(4686A)/H$\alpha$ line-ratio creates an illusion of helium enrichment. Although observed line ratios occur in a plausible minority of cases, AGN-like illumination can not reproduce the observed equivalent widths. We therefore propose to explain these properties by a light-echo photoionization model: the initial flash of a hot blackbody (detonation) excites BLR clouds, which are then seen superimposed on continuum from a later, expanded, cooled stage of the central luminous source. The implied cloud mass is substellar, which may be inconsistent with a TDE. Given these and other inconsistencies with TDE models (e.g. host-galaxies distribution) we suggest to also consider alternative origins for these nuclear flares, which we briefly discuss (e.g. nuclear supernovae and starved/subluminous AGNs).
Whereas many measurements in cosmology depend on the use of integrated distances or time, galaxies evolving passively on a time scale much longer than their age difference allow us to determine the expansion rate H(z) solely as a function of the redshift-time derivative dz/dt. These model-independent `cosmic chronometers' can therefore be powerful discriminators for testing different cosmologies. In previous applications, the available sources strongly disfavoured models (such as LambdaCDM) predicting a variable acceleration, preferring instead a steady expansion rate over the redshift range 0 < z < 2. A more recent catalog of 30 objects appears to suggest non-steady expansion. In this paper, we show that such a result is entirely due to the inclusion of a high, locally-inferred value of the Hubble constant H_0 as an additional datum in a set of otherwise pure cosmic-chronometer measurements. This H_0, however, is not the same as the background Hubble constant if the local expansion rate is influenced by a Hubble Bubble. Used on their own, the cosmic chronometers completely reverse this conclusion, favouring instead a constant expansion rate out to z ~ 2.
Variability in the X-rays is a key ingredient in understanding and unveiling active galactic nuclei (AGN) properties. In this band flux variations occur on short time scales (hours) as well as on larger times scales. While short time scale variability is often investigated in single source studies, only few works are able to explore flux variation on very long time scales.This work provides a statistical analysis of the AGN long term X-ray variability. We study variability on the largest time interval ever investigated for the 0.2-2 keV band, up to $\sim 20$ years rest-frame for a sample of 220 sources. Moreover, we study variability for 2,700 quasars up to $\sim 8$ years rest-frame in the same (soft) band.We build our source sample using the 3XMM serendipitous source catalogue data release 5, and data from ROSAT All Sky Survey Bright and Faint source catalogues. In order to select only AGN we use the Sloan Digital Sky Survey quasar catalogues data releases 7 and 12. Combining ROSAT and XMM-Newton observations, we investigate variability using the structure function analysis which describes the amount of variability as a function of the lag between the observations.Our work shows an increase of the structure function up to 20 years. We do not find evidence of a plateau in the structure function on these long time scales.The increase of the structure function at long time lags suggests that variability in the soft X-rays can be influenced by flux variations originated in the accretion disk or that they take place in a region large enough to justify variation on such long time scales.
We present evidence for halo assembly bias as a function of geometric environment. By classifying GAMA galaxy groups as residing in voids, sheets, filaments or knots using a tidal tensor method, we find that low-mass haloes that reside in knots are older than haloes of the same mass that reside in voids. This result provides direct support to theories that link strong halo tidal interactions with halo assembly times. The trend with geometric environment is reversed at large halo mass, with haloes in knots being younger than haloes of the same mass in voids. We find a clear signal of halo downsizing - more massive haloes host galaxies that assembled their stars earlier. This overall trend holds independently of geometric environment. We support our analysis with an in-depth exploration of the L-Galaxies semi-analytic model, used here to correlate several galaxy properties with three different definitions of halo formation time. We find a complex relationship between halo formation time and galaxy properties, with significant scatter. We confirm that stellar mass to halo mass ratio, specific star-formation rate and mass-weighed age are reasonable proxies of halo formation time, especially at low halo masses. Instantaneous star-formation rate is a poor indicator at all halo masses. Using the same semi-analytic model, we create mock spectral observations using complex star-formation and chemical enrichment histories, that approximately mimic GAMA's typical signal-to-noise and wavelength range. We use these mocks to assert how well potential proxies of halo formation time may be recovered from GAMA-like spectroscopic data.
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We present new deep photometry of the rich globular cluster (GC) systems around the Brightest Cluster Galaxies UGC 9799 (Abell 2052) and UGC 10143 (Abell 2147), obtained with the HST ACS and WFC3 cameras. For comparison, we also present new reductions of similar HST/ACS data for the Coma supergiants NGC 4874 and 4889. All four of these galaxies have huge cluster populations (to the radial limits of our data, comprising from 12000 to 23000 clusters per galaxy). The metallicity distribution functions (MDFs) of the GCs can still be matched by a bimodal-Gaussian form where the metal-rich and metal-poor modes are separated by ~0.8 dex, but the internal dispersions of each mode are so large that the total MDF becomes very broad and nearly continuous from [Fe/H] = -2.4 to Solar. There are, however, significant differences between galaxies in the relative numbers of \emph{metal-rich} clusters, suggesting that they underwent significantly different histories of mergers with massive, gas-rich halos. Lastly, the proportion of metal-poor GCs rises especially rapidly outside projected radii R > 4 R_eff, suggesting the importance of accreted dwarf satellites in the outer halo. Comprehensive models for the formation of GCs as part of the hierarchical formation of their parent galaxies will be needed to trace the systematic change in structure of the MDF with galaxy mass, from the distinctly bimodal form in smaller galaxies up to the broad continuum that we see in the very largest systems.
We report a new analysis protocol for HCN hyperfine data, based on the PYSPECKIT package, and results of using this new protocol to analyse a sample area of seven massive molecular clumps from the Census of High- and Medium-mass Protostars (CHaMP) survey, in order to derive maps of column density for this species. There is a strong correlation between the HCN integrated intensity, IHCN, and previously reported IHCO+ in the clumps, but IN2H+ is not well correlated with either of these other two "dense gas tracers". The four fitted parameters from PYSPECKIT in this region fall in the range of VLSR = 8-10 km/s, {\sigma} V = 1.2-2.2 km/s, Tex = 4-15 K, and {\tau} = 0.2-2.5. These parameters allow us to derive a column density map of these clouds, without limiting assumptions about the excitation or opacity. A more traditional (linear) method of converting IHCN to total mass column gives much lower clump masses than our results based on the hyperfine analysis. This is primarily due to areas in the sample region of low I, low Tex, and high {\tau} . We conclude that there may be more dense gas in these massive clumps not engaged in massive star formation than previously recognized. If this result holds for other clouds in the CHaMP sample, it would have dramatic consequences for the calibration of the Kennicutt-Schmidt star formation laws, including a large increase in the gas depletion time-scale in such regions.
We report on the energetics of molecular outflows in 14 local Ultraluminous Infrared Galaxies (ULIRGs) that show unambiguous outflow signatures (P-Cygni profiles or high-velocity absorption wings) in the far-infrared lines of OH measured with the Herschel/PACS spectrometer. Detection of both ground-state (at 119 and 79 um) and one or more radiatively-excited (at 65 and 84 um) lines allows us to model the nuclear gas (<~300 pc) as well as the more extended components using spherically symmetric radiative transfer models. The highest molecular outflow velocities are found in buried sources, in which slower but massive expansion of the nuclear gas is also observed. With the exception of a few outliers, the outflows have momentum fluxes of (2-5)xL_IR/c and mechanical luminosities of (0.1-0.3)% of L_IR. The moderate momentum boosts in these sources (<~3) suggest that the outflows are mostly momentum-driven by the combined effects of AGN and nuclear starbursts, as a result of radiation pressure, winds, and supernovae remnants. In some sources (~20%), however, powerful (10^{10.5-11} Lsun) AGN feedback and (partially) energy-conserving phases are required, with momentum boosts in the range 3-20. These outflows appear to be stochastic strong-AGN feedback events that occur throughout the merging process. In a few sources, the outflow activity in the innermost regions has subsided in the last ~1 Myr. While OH traces the molecular outflows at sub-kpc scales, comparison of the masses traced by OH with those previously inferred from tracers of more extended outflowing gas suggests that most mass is loaded (with loading factors of Mdot/SFR=1-10) from the central galactic cores (a few x 100 pc). Outflow depletion timescales are <10^8 yr, shorter than the gas consumption timescales by factors of 1.1-15, and are anti-correlated with the AGN luminosity.
We report new HST COS and STIS spectroscopy of a star-forming region (~100 solar masses/year) in the center of the X-ray cluster RXJ1532.9+3021 (z=0.362), to follow-up the CLASH team discovery of luminous UV filaments and knots in the central massive galaxy. We detect broad (~500 km/s) Lyman alpha emission lines with extraordinarily high equivalent width (EQW~200 Angstroms) and somewhat less broadened H-alpha (~220 km/s). Forbidden lines of [N V] and [O VI] are not detected, which constrains the rate at which gas cools through temperatures of 10^6 K to be less than about 10 solar masses/year. The COS spectra also show a flat rest-frame UV continuum with weak stellar photospheric features, consistent with the presence of recently-formed hot stars forming at a rate of ~10 solar masses/year, uncorrected for dust extinction. The slope and absorption lines in these UV spectra are similar to those of Lyman Break Galaxies at z approximately 3, albeit those with the highest Lyman-alpha equivalent widths and star-formation rates. This high-EQW Lyman-alpha source is a high-metallicity galaxy rapidly forming stars in structures that look nothing like disks. This mode of star formation could significantly contribute to the spheroidal population of galaxies. The constraint on the luminosity of any [O VI] forbidden line emission is stringent enough to rule out steady and simultaneous gas cooling and star formation, unlike similar systems in the Phoenix Cluster and Abell 1795. The fact that the current star formation rate differs from the local mass cooling rate is consistent with recent simulations of episodic AGN feedback and star formation in a cluster atmosphere.
Context. X-ray spectral variability analyses of Active Galactic Nuclei (AGN) with moderate luminosities and redshifts typically show a softer when brighter behaviour. Such trend has been rarely investigated for high-luminosity AGNs ($ L_{bol}\gtrsim 10^{44}$ erg/s), nor for a wider redshift range (e.g., $0\lesssim z\lesssim 5$). Aims. We present an analysis of the spectral variability based on a large sample of 2,700 quasars, measured at several different epochs, extracted from the fifth release of the XMM-Newton Serendipitous Source Catalogue. Methods. We quantify the spectral variability through the parameter $\beta$ defined as the ratio between the change in the photon index $\Gamma$ and the corresponding logarithmic flux variation, $\beta=-\Delta\Gamma/\Delta\log F_X$. Results. Our analysis confirms a softer when brighter behaviour also for our sample, extending to high luminosity and redshift the general trend previously found. We estimate an ensemble value of the spectral variability parameter $\beta=-0.69\pm0.03$. We do not find dependence of $\beta$ on redshift, X-ray luminosity, black hole mass, Eddington ratio. A subsample of radio-loud sources shows a smaller spectral variability parameter. There is also some change with the X-ray flux, with smaller $\beta$ (in absolute value) for brighter sources. We also find significant correlations for a few individual sources, indicating more negative values for some sources.
We searched in the public archive of the Chandra X-ray Observatory as of March 2016 and assembled a sample of 719 galaxies within 50 Mpc with ACIS observations available. By cross-correlation with the optical or near-infrared nuclei of these galaxies, 314 of them are identified to have an X-ray active galactic nucleus (AGN). The majority of them are low-luminosity AGNs and are unlikely X-ray binaries based upon their spatial distribution and luminosity functions. The AGN fraction is around 60% for elliptical galaxies and early-type spirals, but drops to roughly 20% for Sc and later types, consistent with previous findings in the optical. However, the X-ray survey is more powerful in finding weak AGNs, especially from regions with active star formation that may mask the optical AGN signature. For example, 31% of the H II nuclei are found to harbor an X-ray AGN. For most objects, a single power-law model subject to interstellar absorption is adequate to fit the spectrum, and the typical photon index is found to be around 1.8. For galaxies with a non-detection, their stacked Chandra image shows an X-ray excess with a luminosity of a few times 10^37 erg/s on average around the nuclear region, possibly composed of faint X-ray binaries. This paper reports on the technique and results of the survey; in-depth analysis and discussion of the results will be reported in forthcoming papers.
A previous article argued that the antagonism between sight-lines with and without a bump at 2200 \AA\ disappears and the observed properties of interstellar extinction can be globally understood if it is accepted that scattered starlight contaminates the observed spectrum of reddened stars. The present paper provides a better understanding of the characteristics of this scattered light, and examines the consequences of this revision of interstellar extinction theory for the 2200 \AA\ bump and the diffuse interstellar bands (DIBs) questions. Two implications are worth noting: (1) the effect of interstellar extinction on cosmic distance estimations needs to be reconsidered in light of the new paradigm of interstellar extinction; (2) hypothetic particles, such as the Poly Aromatic Hydrogenated molecules, the existence of which has never been proven, are unnecessary to explain the peculiarities of interstellar extinction. Hydrogen, by far the most abundant constituent of interstellar clouds, should alone account for the three major features of interstellar extinction: the departure of the continuum from linearity, the bump, and the DIBs.
UV and optically selected candidates for stellar tidal disruption events (TDE) often exhibit broad spectral features (HeII emission, H$\alpha$ emission, or absorption lines) on a blackbody-like continuum (1e4K<T<1e5K). The lines presumably emit from TDE debris or circumnuclear clouds photoionized by the flare. Line velocities however are much lower than expected from a stellar disruption by supermassive black hole (SMBH), and are somewhat faster than expected for the broad line region (BLR) clouds of a persistently active galactic nucleus (AGN). The distinctive spectral states are not strongly related to observed luminosity and velocity, nor to SMBH mass estimates. We use exhaustive photoionization modelling to map the domain of fluxes and cloud properties that yield (e.g.) a He-overbright state where a large HeII(4686A)/H$\alpha$ line-ratio creates an illusion of helium enrichment. Although observed line ratios occur in a plausible minority of cases, AGN-like illumination can not reproduce the observed equivalent widths. We therefore propose to explain these properties by a light-echo photoionization model: the initial flash of a hot blackbody (detonation) excites BLR clouds, which are then seen superimposed on continuum from a later, expanded, cooled stage of the central luminous source. The implied cloud mass is substellar, which may be inconsistent with a TDE. Given these and other inconsistencies with TDE models (e.g. host-galaxies distribution) we suggest to also consider alternative origins for these nuclear flares, which we briefly discuss (e.g. nuclear supernovae and starved/subluminous AGNs).
Whereas many measurements in cosmology depend on the use of integrated distances or time, galaxies evolving passively on a time scale much longer than their age difference allow us to determine the expansion rate H(z) solely as a function of the redshift-time derivative dz/dt. These model-independent `cosmic chronometers' can therefore be powerful discriminators for testing different cosmologies. In previous applications, the available sources strongly disfavoured models (such as LambdaCDM) predicting a variable acceleration, preferring instead a steady expansion rate over the redshift range 0 < z < 2. A more recent catalog of 30 objects appears to suggest non-steady expansion. In this paper, we show that such a result is entirely due to the inclusion of a high, locally-inferred value of the Hubble constant H_0 as an additional datum in a set of otherwise pure cosmic-chronometer measurements. This H_0, however, is not the same as the background Hubble constant if the local expansion rate is influenced by a Hubble Bubble. Used on their own, the cosmic chronometers completely reverse this conclusion, favouring instead a constant expansion rate out to z ~ 2.
Variability in the X-rays is a key ingredient in understanding and unveiling active galactic nuclei (AGN) properties. In this band flux variations occur on short time scales (hours) as well as on larger times scales. While short time scale variability is often investigated in single source studies, only few works are able to explore flux variation on very long time scales.This work provides a statistical analysis of the AGN long term X-ray variability. We study variability on the largest time interval ever investigated for the 0.2-2 keV band, up to $\sim 20$ years rest-frame for a sample of 220 sources. Moreover, we study variability for 2,700 quasars up to $\sim 8$ years rest-frame in the same (soft) band.We build our source sample using the 3XMM serendipitous source catalogue data release 5, and data from ROSAT All Sky Survey Bright and Faint source catalogues. In order to select only AGN we use the Sloan Digital Sky Survey quasar catalogues data releases 7 and 12. Combining ROSAT and XMM-Newton observations, we investigate variability using the structure function analysis which describes the amount of variability as a function of the lag between the observations.Our work shows an increase of the structure function up to 20 years. We do not find evidence of a plateau in the structure function on these long time scales.The increase of the structure function at long time lags suggests that variability in the soft X-rays can be influenced by flux variations originated in the accretion disk or that they take place in a region large enough to justify variation on such long time scales.
We present evidence for halo assembly bias as a function of geometric environment. By classifying GAMA galaxy groups as residing in voids, sheets, filaments or knots using a tidal tensor method, we find that low-mass haloes that reside in knots are older than haloes of the same mass that reside in voids. This result provides direct support to theories that link strong halo tidal interactions with halo assembly times. The trend with geometric environment is reversed at large halo mass, with haloes in knots being younger than haloes of the same mass in voids. We find a clear signal of halo downsizing - more massive haloes host galaxies that assembled their stars earlier. This overall trend holds independently of geometric environment. We support our analysis with an in-depth exploration of the L-Galaxies semi-analytic model, used here to correlate several galaxy properties with three different definitions of halo formation time. We find a complex relationship between halo formation time and galaxy properties, with significant scatter. We confirm that stellar mass to halo mass ratio, specific star-formation rate and mass-weighed age are reasonable proxies of halo formation time, especially at low halo masses. Instantaneous star-formation rate is a poor indicator at all halo masses. Using the same semi-analytic model, we create mock spectral observations using complex star-formation and chemical enrichment histories, that approximately mimic GAMA's typical signal-to-noise and wavelength range. We use these mocks to assert how well potential proxies of halo formation time may be recovered from GAMA-like spectroscopic data.
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We present a study of the kinematics of 101 stars observed with VIMOS around Whiting1, a globular cluster embedded in the Sagittarius tidal stream. The obtained velocity distribution shows the presence of two wraps of that halo substructure at the same heliocentric distance as that of the cluster and with well differentiated mean radial velocities. The most prominent velocity component seems to be associated with the trailing arm of Sagittarius with $<v_{\rm r}>$ ~ -130 km/s, which is consistent with the velocity of Whiting1. This result supports that this globular cluster was formed in Sagittarius and recently accreted by the Milky Way. The second component with $<v_{\rm r}>$ ~ 120 km/s might correspond to the leading arm of Sagittarius, which has been predicted by numerical simulations but with no conclusive observational evidence of its existence presented so far. This detection of the old leading wrap of Sagittarius in the southern hemisphere may be used to confirm and further constrain the models for its orbit and evolution.
A three degrees of freedom (3-dof) barred galaxy model composed of a spherically symmetric nucleus, a bar, a flat disc and a spherically symmetric dark matter halo is used for investigating the dynamics of the system. We use colour-coded plots to demonstrate how the value of the semi-major axis of the bar influences the regular or chaotic dynamics of the 3-dof system. For distinguishing between ordered and chaotic motion we use the Smaller ALingment Index (SALI) method, a fast yet very accurate tool. Undoubtedly, the most important elements of the dynamics are the normally hyperbolic invariant manifolds (NHIMs) located in the vicinity of the index 1 Lagrange points $L_2$ and $L_3$. These manifolds direct the flow of stars over the saddle points, while they also trigger the formation of rings and spirals. The dynamics in the neighbourhood of the saddle points is visualized by bifurcation diagrams of the Lyapunov orbits as well as by the restriction of the Poincar\'e map to the NHIMs. In addition, we reveal how the semi-major axis of the bar influences the structure of these manifolds which determine the final stellar structure (rings or spirals). Our numerical simulations suggest that in galaxies with weak bars the formation of $R_1$ rings or $R_1'$ pseudo-rings is favoured. In the case of galaxies with intermediate and strong bars the invariant manifolds seem to give rise to $R_1R_2$ rings and twin spiral formations, respectively. We also compare our numerical outcomes with earlier related work and with observational data.
Mc Gaugh et al. (2016) have found, by investigating a large sample of Spirals, a tight non linear relationship between the total radial acceleration, connected with the Dark Matter phenomenon, and its component which comes from the distribution of baryonic matter, as the stellar and HI disks. The strong link between these two quantities is considered by them and by other researchers, as challenging the scenario featuring the presence of DM halos in galaxies. Or, at least, to indicate the peculiar nature of the underlying dark matter particles. We have explored this issue by investigating a larger number of galaxies by means of several techniques of analysis. Our results support and even increase, both qualitatively and quantitatively, the validity of McGaugh et al. (2016) 's relationship. However, we prove that such relationship exists also in the scenario featuring dark matter halos + ordinary baryonic matter and that it arises by the fact the DM is less concentrated than the luminous matter and it is progressively more abundant in lower luminosity objects. These properties are due to well known astrophysical effects: the implications of this relationship for the properties of dark matter halos are nothing of new or of unexpected. The relationship, definitively, is not a portal to go beyond the standard picture of $\Lambda$CDM galaxy formation.
The expansion of HII regions can trigger the formation of stars. An overdensity of young stellar objects (YSOs) is observed at the edges of HII regions but the mechanisms that give rise to this phenomenon are not clearly identified. Moreover, it is difficult to establish a causal link between HII-region expansion and the star formation observed at the edges of these regions. A clear age gradient observed in the spatial distribution of young sources in the surrounding might be a strong argument in favor of triggering. We have observed the Galactic HII region RCW120 with herschel PACS and SPIRE photometers at 70, 100, 160, 250, 350 and 500$\mu$m. We produced temperature and H$_2$ column density maps and use the getsources algorithm to detect compact sources and measure their fluxes at herschel wavelengths. We have complemented these fluxes with existing infrared data. Fitting their spectral energy distributions (SEDs) with a modified blackbody model, we derived their envelope dust temperature and envelope mass. We computed their bolometric luminosities and discuss their evolutionary stages. The herschel data, with their unique sampling of the far infrared domain, have allowed us to characterize the properties of compact sources observed towards RCW120 for the first time. We have also been able to determine the envelope temperature, envelope mass and evolutionary stage of these sources. Using these properties we have shown that the density of the condensations that host star formation is a key parameter of the star-formation history, irrespective of their projected distance to the ionizing stars.
The laboratory gas phase spectrum recently published by Campbell et al. has reinvigorated attempts to confirm the presence of the C$_{60}^+$ cation in the interstellar medium, thorough an analysis of the spectra of hot, reddened stars. This search is hindered by at least two issues that need to be addressed: (i) the wavelength range of interest is severely polluted by strong water- vapour lines coming from the Earth's atmosphere; (ii) one of the major bands attributed to C$_{60}^+$, at 9633 \AA, is blended with the stellar Mg{\sc ii} line, which is susceptible to non-local-thermodynamic equilibrium effects in hot stellar atmospheres. Both these issues are here carefully considered here for the first time, based on high-resolution and high signal-to-noise ratio echell\'e spectra for 19 lines of sight. The result is that the presence of C$_{60}^+$ in interstellar clouds is brought into question.
A doubly-peaked quasar microlensing event in the lensed Twin Quasar Q0957+561 A,B (Colley and Schild 2003) is analysed within several lensing models. In the most realistic model a lens resolves in image B the ellipse shaped, bright inner rim of the quasar's accretion disk, intersecting it twice. This lens weighs 0.5 Earth mass and is located inside the Galaxy, at 3 kpc distance. During the passing, it partially occults the source, which allows to describe it as a primordial gas cloud of 1.4 Solar radius and 17 K temperature, in accordance with the theory of Gravitational Hydrodynamics. Lensing by such objects against the Magellanic Clouds and Galactic centre will also lead to occultation dips.
We address the presently exciting issue of the presence of stellar clusters in the periphery of the Magellanic Clouds (MCs) and beyond by making use of a wealth of wide-field high-quality images released in advance from the Magellanic Stellar Hystory (SMASH) survey. We conducted a sound search for new stellar cluster candidates from suitable kernel density estimators running for appropriate ranges of radii and stellar densities. In addition, we used a functional relationship to account for the completeness of the SMASH field sample analyzed that takes into account not only the number of fields used but also their particular spatial distribution; the present sample statistically represents ~ 50$% of the whole SMASH survey. The relative small number of new stellar cluster candidates identified, most of them distributed in the outer regions of the Magellanic Clouds, might suggest that the lack of detection of a larger number of new cluster candidates beyond the main bodies of the Magellanic Clouds could likely be the outcome once the survey be completed.
We use the Wide Field Camera 3 onboard the Hubble Space Telescope to obtain deep, high-resolution photometry of the young (~ 100 Myr) star cluster NGC1850 in the Large Magellanic Cloud. We analyze the cluster colour-magnitude diagram (CMD) and find that it hosts an extended main sequence turn-off (MSTO) and a double MS. We demonstrate that these features cannot be due to photometric errors, field star contamination, or differential reddening. From a comparison with theoretical models and Monte Carlo simulations, we show that a coeval stellar population featuring a distribution of stellar rotation rates can reproduce the MS split quite well. However, it cannot reproduce the observed MSTO region, which is significantly wider than the simulated ones. Exploiting narrow-band Halpha imaging, we find that the MSTO hosts a population of Halpha-emitting stars which are interpreted as rapidly rotating Be-type stars. We explore the possibility that the discrepancy between the observed MSTO morphology and that of the simulated simple stellar population (SSP) is caused by the fraction of these objects that are highly reddened, but we rule out this hypothesis. We demonstrate that the global CMD morphology is well-reproduced by a combination of SSPs that cover an age range of ~ 35 Myr as well as a wide variety of rotation rates. We derive the cluster mass and escape velocity and use dynamical evolution models to predict their evolution starting at an age of 10 Myr. We discuss these results and their implications in the context of the extended MSTO phenomenon.
We present Atacama Large Millimeter/sub-millimeter Array (ALMA) Cycle-2 observations of the HBC 494 molecular outflow and envelope. HBC 494 is an FU Ori-like object embedded in the Orion A cloud and is associated with the reflection nebulae Re50 and Re50N. We use $^{12}$CO, $^{13}$CO and C$^{18}$O spectral line data to independently describe the outflow and envelope structures associated with HBC 494. The moment-1 map of the $^{12}$CO emission shows the widest outflow cavities in a Class I object known to date (opening angle $\sim$ 150$^{^{\circ}}$). The morphology of the wide outflow is likely to be due to the interaction between winds originating in the inner disc and the surrounding envelope. The low-velocity blue- and red-shifted $^{13}$CO and C$^{18}$O emission trace the rotation and infall motion of the circumstellar envelope. Using molecular line data and adopting standard methods for correcting optical depth effects, we estimate its kinematic properties, including an outflow mass on the order of 10$^{-1}$ M$_{\odot}$. Considering the large estimated outflow mass for HBC 494, our results support recent theoretical work suggesting that wind-driven processes might dominate the evolution of protoplanetary discs via energetic outflows.
Nearly all Galactic globular clusters host stars that display characteristic abundance anti-correlations, like the O-rich/Na-poor pattern typical of field halo stars, together with O-poor/Na-rich additional components. A recent spectroscopic investigation questioned the presence of O-poor/Na-rich stars amongst a sample of asymptotic giant branch stars in the cluster M 4, at variance with the spectroscopic detection of a O-poor/Na-rich component along both the cluster red giant branch and horizontal branch. This is contrary to what is expected from the cluster horizontal branch morphology and horizontal branch stellar evolution models. Here we have investigated this issue by employing the CUBI= (U-B)-(B-I) index, that previous studies have demonstrated to be very effective in separating multiple populations along both the red giant and asymptotic giant branch sequences. We confirm previous results that the RGB is intrinsically broad in the V-CUBI diagram, with the presence of two components which nicely correspond to the two populations identified by high-resolution spectroscopy. We find that AGB stars are distributed over a wide range of CUBI values, in close analogy with what is observed for the RGB, demonstrating that the AGB of M4 also hosts multiple stellar populations.
We present an investigation of the globular cluster population synthesis method of McWilliam & Bernstein (2008), focusing on the impact of NLTE modeling effects and CMD discretization. Johnson-Cousins-Bessel U-B, B-V, V-I, and J-K colors are produced for 96 synthetic integrated light spectra with two different discretization prescriptions and three degrees of NLTE treatment. These color values are used to compare NLTE and LTE derived population ages. Relative contributions of different spectral types to the integrated light spectra for different wavebands are measured. Integrated light NLTE spectra are shown to be more luminous in the UV and optical than LTE spectra, but show stronger absorption features in the IR. The main features showing discrepancies between NLTE and LTE integrated light spectra may be attributed to light metals, primarily Fe I, Ca I, and Ti I, as well as TiO molecular bands. Main Sequence stars are shown to have negligible NLTE effects at IR wavelengths compared to more evolved stars. Photometric color values are shown to vary at the millimagnitude level as a function of CMD discretization. Finer CMD sampling for the upper main sequence and turnoff, base of the red giant branch, and the horizontal branch minimizes this variation. Differences in ages derived from LTE and NLTE IL spectra are found to range from 0.55 to 2.54 Gyr, comparable to the uncertainty in GC ages derived from color indices with observational uncertainties of 0.01 magnitudes, the limiting precision of the Harris catalog (Harris 1996).
We model dust formation in a core collapse supernova explosion like that of SN 1987A. In treating the gas-phase formation of dust grain nuclei as a chemical process, our model borrows from and improves upon the recent progress toward modeling dust formation. We compute the synthesis of fourteen species of dust grains in supernova ejecta generated with a stellar evolutionary and explosive nucleosynthesis calculation designed to approximate the parameters of SN 1987A. We explicitly integrate a non-equilibrium network of the chemical reactions contributing to the production of each species' condensation nuclei and follow the growth of condensation nuclei into grains via accretion and coagulation. We include the effects of radioactive decay of Co56, Co57, Ti44, and Na22 on the chemistry and thermodynamics of the ejecta, and of grain electric charge on grain growth. The grain temperature is used to calculate the evaporation rate. The van der Waals force between grains is included in the coagulation model. In addition to evaporation, we include He+, Ne+, Ar+, and O weathering as grain destruction processes. We use our dust synthesis model in combination with a crude model for anisotropic Ni56 dredge-up into the core ejecta, which produces the so-called "nickel bubbles", to compute the total dust mass and molecular-species-specific grain size distribution. We find a total dust mass between 0.4 and 0.73 M_sun, depending on the density in the perturbations caused by the nickel bubbles. The dominant grain species produced, from highest mass to lowest mass, are: magnesia, silicon, forsterite, iron sulfide, carbon, silicon dioxide, alumina, and iron. The combined grain size distribution is a power law dN/da~a^{-4.39}, steeper than the observationally inferred dN/da~a^{-3.5}. Early ejecta compaction by expanding radioactive Ni56 bubbles strongly enhances dust synthesis.
Aims. We report the discovery of a strong outburst of the embedded young stellar object (YSO), namely UKIDSS-J185318.36+012454.5, located in the star-forming region associated with IRAS 18507+0121 source and GAL 034.4+00.23 HII region. Methods. Using the archival photometric data and images we determined the amplitude and the epoch of the outburst, as well as the evolution stage and the basic parameters of the object. Results. According to the near and mid-infrared colors and spectral energy distribution, we classify the object as an intermediate-mass young stellar object (YSO) with Class 0/I evolution stage. The outburst occurred in the period of 2004-2006. The amplitude of brightness is as least Ks = 5.0 mag. The summation of the photometric and spectral data does not allow to classify UKIDSS-J185318.36+012454.5 as FUor or EXor. We can consider it as an eruptive variable with mixed characteristics or MNor type object.
Estimates of galaxy distances based on indicators that are independent of cosmological redshift are fundamental to astrophysics. Researchers use them to establish the extragalactic distance scale, to underpin estimates of the Hubble constant, and to study peculiar velocities induced by gravitational attractions that perturb the motions of galaxies with respect to the Hubble flow of universal expansion. In 2006 the NASA/IPAC Extragalactic Database (NED) began making available a comprehensive compilation of redshift-independent extragalactic distance estimates. A decade later, this compendium of distances (NED-D) now contains more than 100,000 individual estimates based on primary and secondary indicators, available for more than 28,000 galaxies, and compiled from over 2,000 references in the refereed astronomical literature. This article describes the methodology, content, and use of NED-D, and addresses challenges to be overcome in compiling such distances. Currently, 75 different distance indicators are in use. We include a figure that facilitates comparison of the indicators with significant numbers of estimates in terms of the minimum, 25th percentile, median, 75th percentile, and maximum distances spanned. Brief descriptions of the indicators, including examples of their use in the database, are given in an Appendix.
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