arXiv:0801.1670 [ps, pdf, other]
    Title: Metallicities and Physical conditions in Star-forming Galaxies at z~1.0-1.5
    Authors: Xin Liu, Alice E. Shapley, Alison L. Coil, Jarle Brinchmann, Chung-Pei Ma
    Comments: 26 pages, 13 figures; accepted for publication in ApJ
    Subjects: Astrophysics (astro-ph)

    We present a study of the mass-metallicity (M-Z) relation and H II region physical conditions in a sample of 20 star-forming galaxies at 1.0<z<1.5 drawn from the DEEP2 Galaxy Redshift Survey. Our analysis indicates that the zero point of the M-Z relationship evolves with redshift, in the sense that galaxies at fixed stellar mass become more metal-rich at lower redshift. Measurements of [O III]/H-beta and [N II]/H-alpha emission-line ratios show that, on average, objects in the DEEP2 1.0<z<1.5 sample are significantly offset from the excitation sequence observed in nearby H II regions and SDSS emission-line galaxies. In order to fully understand the causes of this offset, which is also observed in z~2 star-forming galaxies, we examine in detail the small fraction of SDSS galaxies that have similar diagnostic ratios to those of the DEEP2 sample. Some of these galaxies indicate evidence for AGN and/or shock activity, which may give rise to their unusual line ratios, and contribute to Balmer emission lines at the level of ~20%. Others indicate no evidence for AGN or shock excitation, yet are characterized by higher electron densities and temperatures, and therefore, interstellar gas pressures, than typical SDSS star-forming galaxies of similar stellar mass. These anomalous objects also have higher concentrations and star-formation rate surface densities, which are directly connected to higher interstellar pressure. Higher star-formation rate surface densities, interstellar pressures, and HII region ionization parameters may also be common at high-redshift. These effects must be taken into account when using strong-line indicators to understand the evolution of heavy elements in galaxies. When such effects are included, the inferred evolution of the M-Z relation out to z~2 is more significant than previous estimates.


arXiv:0801.1673 [ps, pdf, other]
    Title: Downsizing by Shutdown in Red Galaxies
    Authors: A. Cattaneo, A. Dekel, S.M. Faber, B. Guiderdoni
    Comments: 21 pages, 13 figures, submitted to MNRAS
    Subjects: Astrophysics (astro-ph)

    We address the origin of the `downsizing' of elliptical galaxies, according to which the stars in more massive galaxies formed earlier and over a shorter period than those in less massive galaxies. We show that this could be the natural result of a shutdown of star formation in dark matter haloes above a critical mass of 10^12MSun. This is demonstrated using a semianalytic simulation of galaxy formation within the standard hierarchical scenario of structure formation. The assumed threshold mass is motivated by the prediction of stable shock heating above this mass and the finding that such a shutdown reproduces the observed distribution of galaxies in luminosity and colour.The shutdown at a critical halo mass introduces a characteristic stellar mass for the transition of galaxies into the `red sequence' of the galaxy colour-magnitude diagram. Central galaxies of haloes that are more massive today have reached this mass earlier and can therefore grow further along the red sequence by dry mergers, ending up more massive and containing older stars. Small galaxies formed in haloes below the critical mass can shutdown late, when they fall into haloes above the critical mass and become satellites. While our semianalytic simulation that incorporates an explicit shutdown reproduces downsizing as inferred from the stellar ages of ellipticals, we explain why it is much harder to detect downsizing using the mass functions of different galaxy types.



  arXiv:0801.1675 [ps, pdf, other]
    Title: Assessing the Predictive Power of Galaxy Formation Models with the Rest-Frame Optical Luminosity Functions at 2.0<z<3.3
    Authors: Danilo Marchesini, Pieter van Dokkum (Yale University)
    Comments: 6 pages, 2 figures. To appear in the proceedings of `A Century of Cosmology', S. Servolo, August 2007, to be published in Il Nuovo Cimento
    Subjects: Astrophysics (astro-ph)

    We compare recently measured rest-frame V-band luminosity functions (LFs) of galaxies at redshifts 2.0<z<3.3 to predictions of semianalytic models by De Lucia & Blaizot and Bower et al. and hydrodynamic simulations by Dave et al. The models succeed for some luminosity and redshift ranges and fail for others. A notable success is that the Bower et al. model provides a good match to the observed LF at z~3. However, all models predict an increase with time of the rest-frame V-band luminosity density, whereas the observations show a decrease. The models also have difficulty matching the observed rest-frame colors of galaxies. In all models the luminosity density of red galaxies increases sharply from z~3 to z~2.2, whereas it is approximately constant in the observations. Conversely, in the models the luminosity density of blue galaxies is approximately constant, whereas it decreases in the observations. These discrepancies cannot be entirely remedied by changing the treatment of dust and suggest that current models do not yet provide an adequate description of galaxy formation and evolution since z~3.




arXiv:0801.1677 [ps, pdf, other]
    Title: Precise Measurement of the Cosmological Power Spectrum With a Dedicated 21cm Survey After Reionization
    Authors: Abraham Loeb (Harvard), Stuart Wyithe (Melbourne)
    Comments: 4 pages, 3 figures
    Subjects: Astrophysics (astro-ph); High Energy Physics - Phenomenology (hep-ph)

    Measurements of the 21cm line emission by residual cosmic hydrogen after reionization can be used to trace the power spectrum of density perturbations through a significant fraction of the observable volume of the Universe. We discuss a dedicated 21cm observatory (designed as a significantly upgraded version of the planned MWA) that would probe a number of independent modes that is two orders of magnitude larger than available from existing galaxy surveys (such as SDSS-LRG) or microwave background maps (such as WMAP). We show that the resulting precision in the determination of the matter power spectrum at redshifts z<6 could enable a cosmic-variance limited detection of the signature of a neutrino mass ~0.05eV, in the range suggested by atmospheric neutrino data. The evolution of the linear growth factor with redshift could also constrain exotic theories of gravity or dark energy to an unprecedented precision.



 arXiv:0801.1678 [ps, pdf, other]
    Title: New insights into the stellar content and physical conditions of star-forming galaxies at z = 2-3 from spectral modelling
    Authors: Jarle Brinchmann (1,4), Max Pettini (2), Stephane Charlot (3) ((1) CAUP Porto, (2) IoA Cambridge, (3) IAP, (4) Leiden)
    Comments: 14pages, Accepted for MNRAS
    Subjects: Astrophysics (astro-ph)

    We have used extensive libraries of model and empirical galaxy spectra (assembled respectively from the population synthesis code of Bruzual and Charlot and the fourth data release of the Sloan Digital Sky Survey) to interpret some puzzling features seen in the spectra of high redshift star-forming galaxies. We show that a stellar He II 1640 emission line, produced in the expanding atmospheres of Of and Wolf-Rayet stars, should be detectable with an equivalent width of 0.5-1.5AA in the integrated spectra of star-forming galaxies, provided the metallicity is greater than about half solar. Our models reproduce the strength of the He II 1640 line measured in the spectra of Lyman break galaxies for established values of their metallicities. With better empirical calibrations in local galaxies, this spectral feature has the potential of becoming a useful diagnostic of massive star winds at high, as well as low, redshifts.
    We also uncover a relationship in SDSS galaxies between their location in the [O III]/Hb vs. [N II]/Ha diagnostic diagram (the BPT diagram) and their excess specific star formation rate relative to galaxies of similar mass. We infer that an elevated ionisation parameter U is at the root of this effect, and propose that this is also the cause of the offset of high redshift star-forming galaxies in the BPT diagram compared to local ones. We further speculate that higher electron densities and escape fractions of hydrogen ionising photons may be the factors responsible for the systematically higher values of U in the H II regions of high redshift galaxies. The impact of such differences on abundance determinations from strong nebular lines are considered and found to be relatively minor.


 arXiv:0801.1707 [ps, pdf, other]
    Title: The Formation of Galaxy Disks
    Authors: F.Governato (UW), L.Mayer (U. of Zurich & ETH), C.Brook (UW)
    Comments: To appear in proceedings of "Formation and Evolution of Galaxy Disks", Rome, October 2007, Eds. J.G. Funes, S.J. and E.M. Corsini. Bigger figures than in printed version
    Subjects: Astrophysics (astro-ph)

    We present a new set of multi-million particle SPH simulations of the formation of disk dominated galaxies in a cosmological context. Some of these galaxies are higher resolution versions of the models already described in Governato et al (2007). To correctly compare simulations with observations we create artificial images of our simulations and from them measure photometric Bulge to Disk (B/D) ratios and disk scale lengths. We show how feedback and high force and mass resolution are necessary ingredients to form galaxies that have flatter rotation curves, larger I band disk scale lengths and smaller B/D ratios. A new simulated disk galaxy has an I-band disk scale length of 9.2 kpc and a B/D flux ratio of 0.64 (face on, dust reddened).

 arXiv:0801.1795 [ps, pdf, other]
    Title: The mass distribution of RX J1347-1145 from strong lensing
    Authors: A. Halkola, H. Hildebrandt, T. Schrabback, M. Lombardi, M. Bradac, T. Erben, P. Schneider, D. Wuttke
    Comments: Accepted for publication in the A&A
    Subjects: Astrophysics (astro-ph)

    High resolution HST/ACS images of the galaxy cluster RX J1347-1145 have enabled us to identify several new multiple image candidates in the cluster, including a 5 image system with a central image. The multiple images allow us to construct an accurate 2-dimensional mass map of the central part of the cluster. The modelling of the cluster mass includes the most prominent cluster galaxies modelled as truncated isothermal spheres and a smooth halo component that is described with 2 parametric profiles. The mass reconstruction is done using a Markov chain Monte Carlo method that provides us with a total projected mass density as well as estimates for the parameters of interest and their respective errors. The mass profile is in reasonable agreement with previous mass estimates based on the X-ray emission from the hot intra-cluster gas, however the X-ray mass estimates are systematically lower than what we obtain with gravitational lensing.
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 arXiv:0801.1849 [ps, pdf, other]
    Title: Metallicity Calibrations and the Mass-Metallicity Relation for Star-Forming Galaxies
    Authors: Lisa J. Kewley (1), Sara L. Ellison (2) ((1) University of Hawaii, (2) University of Victoria)
    Comments: 26 pages, 5 tables, 11 figures (reduced resolution); accepted for publication in the Astrophysical Journal. . A full resolution version can be downloaded in pdf form (1 Mb) from this http URL
    Subjects: Astrophysics (astro-ph)

    (Abridged) We investigate the effect of metallicity calibrations, AGN classification, and aperture covering fraction on the local mass-metallicity (MZ) relation using 27,730 star-forming galaxies from the Sloan Digital Sky Survey (SDSS) Data Release 4. We analyse the SDSS MZ relation with 10 metallicity calibrations, including theoretical and empirical methods. We show that the choice of metallicity calibration has a significant effect on the shape and y-intercept(12+log(O/H)) of the MZ relation. The absolute metallicity scale (y-int) varies up to 0.7 dex, depending on the calibration used, and the change in shape is substantial. These results indicate that it is critical to use the same metallicity calibration when comparing different luminosity-metallicity or mass-metallicity relations. We present new metallicity conversions that allow metallicities that have been derived using different strong-line calibrations to be converted to the same base calibration. These conversions facilitate comparisons between different samples, particularly comparisons between galaxies at different redshifts for which different suites of emission-lines are available.Our new conversions successfully remove the large 0.7 dex discrepancies between the metallicity calibrations, and we reach agreement in the MZ relation to within 0.03 dex on average. We find that different AGN classification methods have negligible effect on the SDSS MZ relation. We compare the SDSS MZ relation with nuclear and global relations from the Nearby Field Galaxy Survey (NFGS). The turn over of the MZ relation depends on aperture covering fraction. We find that a lower redshift limit of z<0.04 is insufficient for avoiding aperture effects in fiber spectra of the highest stellar mass galaxies.


 arXiv:0801.1851 [pdf, other]
    Title: Dark Matter Halo Profiles in Scale-Free Cosmologies
    Authors: Steffen R. Knollmann, Chris Power, Alexander Knebe
    Comments: 9 pages, 4 figures. Accepted for publication in MNRAS
    Subjects: Astrophysics (astro-ph)

    We explore the dependence of the central logarithmic slope of dark matter halo density profiles $\alpha$ on the spectral index $n$ of the linear matter power spectrum $P(k)$ using cosmological $N$-body simulations of scale-free models (i.e. $P(k) \propto k^n$). For each of our simulations we identify samples of well resolved haloes in dynamical equilibrium and we analyse their mass profiles. By parameterising the mass profile using a ``generalised'' Navarro, Frenk & White profile in which the central logarithmic slope $\alpha$ is allowed to vary while preserving the $r^{-3}$ asymptotic form at large radii, we obtain preferred central slopes for haloes in each of our models. There is a strong correlation between $\alpha$ and $n$, such that $\alpha$ becomes shallower as $n$ becomes steeper. However, if we normalise our mass profiles by $r_{-2}$, the radius at which the logarithmic slope of the density profile is -2, we find that these differences are no longer present. We conclude that there is no evidence for convergence to a unique central asymptotic slope, at least on the scales that we can resolve.


arXiv:0801.1860 [ps, pdf, other]
    Title: A Significant Population of Very Luminous Dust-Obscured Galaxies at Redshift z ~ 2
    Authors: Arjun Dey (1), B. T. Soifer (2), Vandana Desai (2), Kate Brand (3), Emeric LeFloc'h (4), Michael J. Brown (5), Buell T. Jannuzi (1), Lee Armus (2), Shane Bussmann (6), Mark Brodwin (1), Chao Bian (2), Peter Eisenhardt (7), Sarah Higdon (8), Daniel Weedman (9), Steve Willner (10) ((1) NOAO, (2) Caltech, (3) STScI, (4) U of Hawaii, (5) Monash, (6) U of Arizona, (7) JPL, (8) Georgia Southern U, (9) Cornell, (10) Harvard/CfA)
    Comments: Accepted for publication in the Astrophysical Journal
    Subjects: Astrophysics (astro-ph)

    Observations with Spitzer Space Telescope have recently revealed a significant population of high-redshift z~2 dust-obscured galaxies (DOGs) with large mid-IR to UV luminosity ratios. These galaxies have been missed in traditional optical studies of the distant universe. We present a simple method for selecting this high-z population based solely on the ratio of the observed mid-IR 24um to optical R-band flux density. In the 8.6 sq.deg Bootes NDWFS Field, we uncover ~2,600 DOG candidates (= 0.089/sq.arcmin) with 24um flux densities F24>0.3mJy and (R-[24])>14 (i.e., F[24]/F[R] > 1000). These galaxies have no counterparts in the local universe, and become a larger fraction of the population at fainter F24, representing 13% of the sources at 0.3~mJy. DOGs exhibit evidence of both star-formation and AGN activity, with the brighter 24um sources being more AGN- dominated. We have measured spectroscopic redshifts for 86 DOGs, and find a broad z distribution centered at <z>~2.0. Their space density is 2.82E-5 per cubic Mpc, similar to that of bright sub-mm-selected galaxies at z~2. These redshifts imply very large luminosities LIR>~1E12-14 Lsun. DOGs contribute ~45-100% of the IR luminosity density contributed by all z~2 ULIRGs, suggesting that our simple selection criterion identifies the bulk of z~2 ULIRGs. DOGs may be the progenitors of ~4L* present-day galaxies seen undergoing a luminous,short- lived phase of bulge and black hole growth. They may represent a brief evolution phase between SMGs and less obscured quasars or galaxies. [Abridged]


 arXiv:0801.1995 [ps, pdf, other]
    Title: The Dependence of Galaxy Morphology and Structure on Environment and Stellar Mass
    Authors: Arjen van der Wel
    Comments: Accepted for publication in ApJ Letters, 5 pages, 5 figures
    Subjects: Astrophysics (astro-ph)

    From the Sloan Digital Sky Survey (SDSS), Data Release 5 (DR5), we extract a sample of 4594 galaxies at redshifts 0.02<z<0.03, complete down to a stellar mass of M=10^10 Msol. We quantify their structure (Sersic index), morphology (Sersic index + `Bumpiness'), and local environment. We show that morphology and structure are intrinsically different galaxy properties, and we demonstrate that this is a physically relevant distinction by showing that these properties depend differently on galaxy mass and environment. Structure mainly depends on galaxy mass whereas morphology mainly depends on environment. This is driven by variations in star-formation activity, as traced by color, which only weakly affects the structure of a galaxy, but strongly affects its morphological appearance. The implication of our results is that the existence of the morphology-density relation is intrinsic and not just due to a combination of more fundamental, underlying relations. Our findings have consequences for high-redshift studies, which often use some measure of structure as a proxy for morphology. A direct comparison with local samples selected through visually classified morphologies may lead to biases in the inferred evolution of the morphological mix of the galaxy population, and misinterpretations in terms of how galaxy evolution depends on mass and environment.


 arXiv:0801.2160 [ps, pdf, other]
    Title: The role of AGN in the colour transformation of galaxies at redshifts z~1
    Authors: A. Georgakakis, K. Nandra, R. Yan, S. P. Willner, J. M. Lotz, C. M. Pierce, M. C. Cooper, E. S. Laird, D. C. Koo, P. Barmby, J. A. Newman, J. R. Primack, A. L. Coil
    Comments: Accepted for publication in MNRAS
    Subjects: Astrophysics (astro-ph)

    We explore the role of AGN in establishing and/or maintaining the bimodal colour distribution of galaxies by quenching their star-formation and hence, causing their transition from the blue to the red cloud. Important tests for this scenario include (i) the X-ray properties of galaxies in the transition zone between the two clouds and (ii) the incidence of AGN in post-starbursts, i.e. systems observed shortly after (<1Gyr) the termination of their star-formation. We perform these tests by combining deep Chandra observations with multiwavelength data from the AEGIS survey. Stacking the X-ray photons at the positions of galaxies (0.4<z<0.9) not individually detected at X-ray wavelengths suggests a population of obscured AGN among sources in the transition zone and in the red cloud. Their mean X-ray and mid-IR properties are consistent with moderately obscured low-luminosity AGN, Compton thick sources or a mix of both. Morphologies show that major mergers are unlikely to drive the evolution of this population but minor interactions may play a role. The incidence of obscured AGN in the red cloud (both direct detections and stacking results) suggests that BH accretion outlives the termination of the star-formation. This is also supported by our finding that post-starburst galaxies at z~0.8 and AGN are associated, in agreement with recent results at low-z. A large fraction of post-starbursts and red cloud galaxies show evidence for at least moderate levels of AGN obscuration. This implies that if AGN outflows cause the colour transformation of galaxies, then some nuclear gas and dust clouds either remain unaffected or relax to the central galaxy regions after the quenching their star-formation.




 arXiv:0801.2269 [ps, pdf, other]
    Title: The evolution of the morphological scale of early-type galaxies since z=2
    Authors: P. Saracco, M. Longhetti, S. Andreon, A. Mignano (INAF - Osservatorio Astronomico di Brera)
    Comments: 4 pages, 2 figures. To appear in the proceedings of `A Century of Cosmology', S. Servolo, August 2007, to be published in Il Nuovo Cimento
    Subjects: Astrophysics (astro-ph)

    We present the morphological analysis based on HST-NICMOS observations in the F160W filter of a sample of 30 early-type galaxies spectroscopically confirmed at 1.2<z<2. We derive the effective radius R_e and the mean surface brightness mu_e of galaxies in the rest-frame R-band. We find that early-types at z~1.5 are characterized by a surface brightness (SB) much higher then their local counterparts with comparable R_e. In particular, we find that the mean SB of these early-types should get fainter by ~2.5 mag from z~1.5 to z=0 to match the SB of the local early-types with comparable R_e. This evolution exceeds by a factor two the luminosity evolution expected for early-types in this redshift range and more than a factor three the one derived from the observed luminosity function of galaxies. Consequently, an evolution of the effective radius R_e from the epoch of their formation towards z=0 has to be invoked and the hypothesis of fixed size rejected.



arXiv:0801.2281 [ps, pdf, other]
    Title: Testing different stellar mass estimators at 1<z<2
    Authors: Marcella Longhetti, Paolo Saracco, Arturo Mignano (INAF - Osservatorio di Brera)
    Comments: 4 pages, 2 figures. To appear in the proceedings of `A Century of Cosmology', S. Servolo, August 2007, to be published in Il Nuovo Cimento
    Subjects: Astrophysics (astro-ph)

    Physical parameters of galaxies (as luminosity, stellar mass, age) are often derived by means of the model templates which best fit their spectro-photometric data. We have performed a quantitative test aimed at exploring the ability of this procedure in recovering the physical parameters of early-type galaxies at 1<z<2. A wide range of simulated SEDs, reproducing those of early-type galaxies at 1<z<2 with assigned age and mass, are used to build mock photometric catalogs with wavelength coverage and photometric uncertainties similar to those of two topical surveys (i.e. VVDS and GOODS). The best fitting analysis of the simulated photometric data allows to study the differences among the recovered parameters and the input ones. Results indicate that the stellar masses measured by means of optical bands are affected by larger uncertainties with respect to those obtained from near-IR bands, and they frequently underestimate the real values. The M/L ratio in the V band results strongly underestimated, even when derived from the recently proposed recipe based on rest-frame optical colours (e.g. (B-V)).




 arXiv:0801.2388 [ps, pdf, other]
    Title: On the Stellar Populations in Faint Red Galaxies in the Hubble Ultra Deep Field
    Authors: Amelia M. Stutz, Casey Papovich, Daniel J. Eisenstein
    Comments: 18 pages, 10 figures, and 4 tables. Accepted for publication in ApJ
    Subjects: Astrophysics (astro-ph)

    We study the nature of faint, red-selected galaxies at z ~ 2-3 using the Hubble Ultra Deep Field (HUDF) and Spitzer IRAC photometry. We detect candidate galaxies to H < 26 mag, probing lower-luminosity (lower mass) galaxies at these redshifts. We identify 32 galaxies satisfying the (J - H) > 1.0 mag color selection, 16 of which have unblended [3.6um] and [4.5um] IRAC photometry. We derive photometric redshifts, masses, and stellar population parameters for these objects. We find that the selected objects span a diverse range of properties over a large range of redshifts, 1 < z < 3.5. A substantial fraction (11/32) appear to be lower-redshift (z < 2.5), heavily obscured dusty galaxies or edge-on spiral galaxies, while others (12/32) appear to be galaxies at 2 < z < 3.5 whose light at rest-frame optical wavelengths is dominated by evolved stellar populations. Interestingly, by including Spitzer data many candidates for galaxies dominated by evolved stellar populations are rejected, and for only a subset of the sample (6/16) do the data favor this interpretation. We place an upper limit on the space and stellar mass density of candidate massive evolved galaxies. The z > 2.5 objects that are dominated by evolved stellar populations have a space density at most one-third that of z ~ 0 red, early-type galaxies. Therefore, at least two-thirds of present-day early-type galaxies assemble or evolve into their current configuration at redshifts below 2.5. We find a dearth of candidates for low-mass galaxies at 1.5 < z < 3 that are dominated by passively evolving stellar populations even though the data should be sensitive to them; thus, at these redshifts, galaxies whose light is dominated by evolved stellar populations are restricted to only those galaxies that have assembled high stellar mass.[Abridged]




arXiv:0801.2410 [ps, pdf, other]
    Title: Finding Galaxy Groups In Photometric Redshift Space: the Probability Friends-of-Friends (pFoF) Algorithm
    Authors: I-hui Li, Howard K.C. Yee
    Comments: AJ accepted
    Subjects: Astrophysics (astro-ph)

    We present a structure finding algorithm designed to identify galaxy groups in photometric redshift data sets: the probability friends-of-friends (pFoF) algorithm. This algorithm is derived by combining the friends-of-friends algorithm in the transverse direction and the photometric redshift probability densities in the radial dimension. The innovative characteristic of our group-finding algorithm is the improvement of redshift estimation via the constraints given by the transversely connected galaxies in a group, based on the assumption that all galaxies in a group have the same redshift. Tests using the Virgo Consortium Millennium Simulation mock catalogs allow us to show that the recovery rate of the pFoF algorithm is larger than 80% for mock groups of at least $2\times10^{13}M_{\sun}$, while the false detection rate is about 10% for pFoF groups containing at least $\sim8$ net members. Applying the algorithm to the CNOC2 group catalogs gives results which are consistent with the mock catalog tests. From all these results, we conclude that our group-finding algorithm offers an effective yet simple way to identify galaxy groups in photometric redshift catalogs.