[3] arXiv:0905.3554 [ps, pdf, other] Title: The Origin of Extended Disk Galaxies at z=2 Authors: Laura V. Sales, Julio F. Navarro, Joop Schaye, Claudio Dalla Vecchia, Volker Springel, Marcel R. Haas, Amina Helmi Comments: 5 pages, 4 figures. Submitted to MNRAS Letters Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO) Galaxy formation models typically assume that the size and rotation speed of galaxy disks are largely dictated by the mass, concentration, and spin of their surrounding dark matter haloes. Equally important, however, are the fraction of baryons in the halo that collect into the central galaxy, as well as the net angular momentum that they are able to retain during its assembly process. We explore the latter using a set of four large cosmological N-body/gasdynamical simulations drawn from the OWLS (OverWhelmingly Large Simulations) project. These runs differ only in their implementation of feedback from supernovae. We find that, when expressed as fractions of their virial values, galaxy mass and net angular momentum are tightly correlated. Galaxy mass fractions, m_d=M_gal/M_vir, depend strongly on feedback, but only weakly on halo mass or spin over the halo mass range explored here (M_vir>1e11 h^{-1}M_sun). The angular momentum of a galaxy, j_d=J_gal/J_vir, correlates with m_d in a manner that is insensitive to feedback and that deviates strongly from the simple j_d = m_d assumption often adopted in semi-analytic models of galaxy formation. The m_d-j_d correlation implies that, in a given halo, galaxy disk size is maximal when the central galaxy makes up a substantial fraction (~20%-30%) of all baryons within the virial radius. At z=2, such systems may host gaseous disks with radial scale lengths as large as those reported for star-forming disks by the SINS survey, even in moderately massive haloes of average spin. Extended disks at z=2 may thus signal the presence of systems where galaxy formation has been particularly efficient, rather than the existence of haloes with unusually high spin parameter. [4] arXiv:0905.3555 [ps, pdf, other] Title: Disk galaxies at z=2 in OWLS Authors: Julio F. Navarro, Laura V. Sales, Joop Schaye, Claudio Dalla Vecchia, Volker Springel Comments: Conference Proceeding: "Galaxy Evolution: Emerging Insights and Future Challenges", November 11-14, 2008. The University of Texas at Austin. 8 pages, 7 figures Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO) We use the OWLS (OverWhelmingly Large Simulations) set of cosmological Nbody/gasdynamical simulations to study the properties of simulated galaxies at z=2. We focus on the effect of supernova feedback from evolving stars on the baryonic mass and angular momentum content of galaxies that assemble at the center of 1e11-3e12 h^{-1}M_\odot halos. Our main finding is that the mass and angular momentum of such galaxies are strongly coupled, in a way that is approximately independent of feedback: varying the feedback implementation leads, in a given halo, to large variations in galaxy mass but leaves the galaxy mass-angular momentum correlation largely unaltered. In particular, the ratio between the angular momentum of a galaxy and that of its surrounding halo (j_d=J_gal/J_vir) correlates closely with the galaxy mass (expressed in units of the virial mass of the halo; m_d=M_gal/M_vir). This correlation differs substantially from the m_d=j_d assumption commonly adopted in semianalytic models of galaxy formation. We use these results to infer the sizes of disk galaxies at z=2 expected in the LCDM scenario and to interpret recent observations of extended disks at z~2 by the SINS collaboration [20] arXiv:0905.3637 [ps, pdf, other] Title: Low, Milky-Way like, Molecular Gas Excitation of Massive Disk Galaxies at z~1.5 Authors: H. Dannerbauer, E. Daddi, D.A. Riechers, F. Walter, C.L. Carilli, M. Dickinson, D. Elbaz, G.E. Morrison Comments: 5 pages, 4 figures. ApJ Letters in press Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO) We present evidence for Milky-Way-like, low-excitation molecular gas reservoirs in near-IR selected massive galaxies at z~1.5, based on IRAM Plateau de Bure Interferometer CO[3-2] and NRAO Very Large Array CO[1-0] line observations for two galaxies that had been previously detected in CO[2-1] emission. The CO[3-2] flux of BzK-21000 at z=1.522 is comparable within the errors to its CO[2-1] flux, implying that the CO[3-2] transition is significantly sub-thermally excited. The combined CO[1-0] observations of the two sources result in a detection at the 3 sigma level that is consistent with a higher CO[1-0] luminosity than that of CO[2-1]. Contrary to what is observed in submillimeter galaxies and QSOs, in which the CO transitions are thermally excited up to J>=3, these galaxies have low-excitation molecular gas, similar to that in the Milky Way and local spirals. This is the first time that such conditions have been observed at high redshift. A Large Velocity Gradient analysis suggests that molecular clouds with density and kinetic temperature comparable to local spirals can reproduce our observations. The similarity in the CO excitation properties suggests that a high, Milky-Way-like, CO to H_2 conversion factor could be appropriate for these systems. If such low-excitation properties are representative of ordinary galaxies at high redshift, centimeter telescopes such as the Expanded Very Large Array and the longest wavelength Atacama Large Millimeter Array bands will be the best tools for studying the molecular gas content in these systems through the observations of CO emission lines. [26] arXiv:0905.3664 [ps, pdf, other] Title: PopStar I: Evolutionary synthesis models description Authors: M. Molla, M.L. Garcia-Vargas, A. Bressan Comments: 21 pages, 29 figures, to be published by MNRAS Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO) We present new evolutionary synthesis models for Simple Stellar Populations for a wide range of ages and metallicities. The models are based on the Padova isochrones. The core of the spectral library is provided by the medium resolution Lejeune et al. atmosphere models. These spectra are complemented by NLTE atmosphere models for hot stars that have an important impact in the stellar cluster's ionizing spectra: O, B and WR stellar spectra at the early ages, and spectra of post-AGB stars and planetary nebulae, at intermediate and old ages. At young ages, our models compare well with other existing models but we find that, the inclusion of the nebular continuum, not considered in several other models, reddens significantly the integrated colours of very young stellar populations. This is consistent with the results of spectral synthesis codes particularly devised for the study of starburst galaxies. At intermediate and old ages, the agreement with literature model is good and, in particular, we reproduce well the observed colours of star clusters in LMC. Given the ability to produce good integrated spectra from the far-UV to the infrared at any age, we consider that our models are particularly suited for the study of high redshift galaxies. These models are available on the web site {this http URL} and also through the Virtual Observatory Tools on the PopStar server. [40] arXiv:0905.3737 [ps, pdf, other] Title: The ACS Nearby Galaxy Survey Treasury Authors: Julianne J. Dalcanton, Benjamin F. Williams, Anil C. Seth, Andrew Dolphin, Jon Holtzman, Keith Rosema, Evan D. Skillman, Andrew Cole, Leo Girardi, Stephanie M. Gogarten, Igor D. Karachentsev, Knut Olsen, Daniel Weisz, Charlotte Christensen, Ken Freeman, Karoline Gilbert, Carme Gallart, Jason Harris, Paul Hodge, Roelof S. de Jong, Valentina Karachentseva, Mario Mateo, Peter B. Stetson, Maritza Tavarez, Dennis Zaritsky, Fabio Governato, Thomas Quinn Comments: 54 pages, including 24 pages of figures and 16 pages of tables. Project website and data available at this http URL Data is also available through MAST. Scheduled to appear in the Astrophysical Journal Supplements Subjects: Galaxy Astrophysics (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR) The ACS Nearby Galaxy Survey Treasury (ANGST) is a systematic survey to establish a legacy of uniform multi-color photometry of resolved stars for a volume-limited sample of nearby galaxies (D<4 Mpc). The survey volume encompasses 69 galaxies in diverse environments, including close pairs, small & large groups, filaments, and truly isolated regions. The galaxies include a nearly complete range of morphological types spanning a factor of ~10^4 in luminosity and star formation rate. The survey data consists of images taken with ACS on HST, supplemented with archival data and new WFPC2 imaging taken after the failure of ACS. Survey images include wide field tilings covering the full radial extent of each galaxy, and single deep pointings in uncrowded regions of the most massive galaxies in the volume. The new wide field imaging in ANGST reaches median 50% completenesses of m_F475W=28.0 mag, m_F606W=27.3 mag, and m_F814W=27.3 mag, several magnitudes below the tip of the red giant branch (TRGB). The deep fields reach magnitudes sufficient to fully resolve the structure in the red clump. The resulting photometric catalogs are publicly accessible and contain over 34 million photometric measurements of >14 million stars. In this paper we present the details of the sample selection, imaging, data reduction, and the resulting photometric catalogs, along with an analysis of the photometric uncertainties (systematic and random), for both the ACS and WFPC2 imaging. We also present uniformly derived relative distances measured from the apparent magnitude of the TRGB. [38] arXiv:0905.3982 [ps, pdf, other] Title: Environmental Effects on the Star Formation Activity in Galaxies at z=1.2 in the COSMOS Field Authors: Y. Ideue, T. Nagao, Y. Taniguchi, Y. Shioya, T. Saito, T. Murayama, S. Sasaki, J. R. Trump, A. M. Koekemoer, H. Aussel, P. Capak, O. Ilbert, H. McCracken, B. Mobasher, M. Salvato, D. Sanders, N. Scoville Comments: 10pages, 5figures, accepted for publication in ApJ Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO) We investigate the relation between the star-formation activity in galaxies and environment at z=1.2 in the COSMOS field, using the fraction of [OII] emitters and the local galaxy density. The fraction of [OII] emitters appears to be almost constant over the surface density of galaxies between 0.2 and 10 Mpc^-2. This trend is different from that seen in the local universe where the star-formation activity is weaker in higher density regions. To understand this difference between z~1 and z~0, we study the fraction of non-isolated galaxies as a function of local galaxy density. We find that the fraction of non-isolated galaxies increases with increasing density. Our results suggest that the star formation in galaxies at z~1 is triggered by galaxy interaction and/or mergers. [43] arXiv:0905.4020 [ps, pdf, other] Title: The color gradients of spiral disks in the Sloan Digital Sky Survey Authors: C.Z. Liu, S.Y. Shen, Z.Y. Shao, R.X. Chang, J.L. Hou, J. Yin, D.W. Yang Comments: 20 pages, 5 figures, Accepted for publication in RAA (Research in Astronomy and Astrophysics) Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); Galaxy Astrophysics (astro-ph.GA) We investigate the radial color gradients of galactic disks using a sample of about 20,000 face-on spiral galaxies selected from the fourth data release of the Sloan Digital Sky Survey (SDSS-DR4). We combine galaxies with similar concentration, size and luminosity to construct composite galaxies, and then measure their color profiles by stacking the azimuthally averaged radial color profiles of all the member galaxies. Except for the smallest galaxies (R_{50}<3 kpc), almost all galaxies show negative disk color gradients with mean g-r gradient G_{gr}=-0.006 mag kpc^{-1} and r-z gradient G_{rz}=-0.018 mag kpc^{-1}. The disk color gradients are independent of the morphological types of galaxies and strongly dependent on the disk surface brightness \mu_{d}, with lower surface brightness galactic disks having steeper color gradients. We quantify the intrinsic correlation between color gradients and surface brightness as G_{gr}=-0.011\mu_{d}+0.233 and G_{rz}=-0.015\mu_{d}+0.324. These quantified correlations provide tight observational constraints on the formation and evolution models of spiral galaxies. [13] arXiv:0905.4189 [ps, pdf, other] Title: Galactic chemical evolution in hierarchical formation models - I. Early-type galaxies in the local Universe Authors: Matías Arrigoni (1), Scott C. Trager (1), Rachel S. Somerville (2,3), Brad K. Gibson (4) ((1) Kapteyn Astronomical Institute, (2) Space Telescope Science Institute, (3) Johns Hopkins University, (4) University of Central Lancashire) Comments: 18 pages, 14 figures, submitted to MNRAS Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO) We study the metallicities and abundance ratios of early-type galaxies in cosmological semi-analytic models (SAMs) within the hierarchical galaxy formation paradigm. To achieve this we implemented a detailed galactic chemical evolution (GCE) model and can now predict abundances of individual elements for the galaxies in the semi-analytic simulations. This is the first time that detailed GCE has been coupled to a SAM that includes feedback from Active Galactic Nuclei (AGN). We find that the new models are able to reproduce the observed mass-metallicity (M*-[Z/H]) relation and, for the first time in a SAM, we reproduce the observed positive slope of the mass-abundance ratio (M*-[$\alpha$/Fe]) relation. Our results indicate that in order to simultaneously match these observations of early-type galaxies, the use of both a very mildly top-heavy IMF (i.e., with a slope of x=1.1 as opposed to a standard x=1.3), and a lower fraction of binaries that explode as Type Ia supernovae appears to be required. We also examine the rate of supernova explosions in the simulated galaxies. In early-type (non-star forming) galaxies, our predictions are also consistent with the observed SNe rates. However, in star-forming galaxies, a higher fraction of SN Ia binaries than in our preferred model is required to match the data, perhaps indicating the need for a population of SN Ia with very short delay times. [18] arXiv:0905.4246 [ps, pdf, other] Title: The global mass - to - light ratio of SLACS lenses Authors: V.F. Cardone, C. Tortora, R. Molinaro, V. Salzano Comments: 22 pages, 3 figures, 5 tables, submitted to Astronomy and Astrophysics Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO) The dark matter content of early,- type galaxies (ETGs) is a hotly debated topic with contrasting results arguing in favour or against the presence of significant dark mass within the effective radius and the change with luminosity and mass. In order to address this question, we investigate here the global mass - to - light ratio $\Upsilon(r) = M(r)/L(r)$ of a sample of 21 lenses observed within the Sloan Lens ACS (SLACS) survey. We follow the usual approach of modeling the galaxy as a two component systems, but we use a phenomenological ansatz for $\Upsilon(r)$, proposed by some of us in Tortora et al. (2007), able to smoothly interpolate between constant $M/L$ models and a wide class of dark matter haloes. The resulting galaxy model is then fitted to the data on the Einstein radius and velocity dispersion. Our phenomenological model turns out to be in well agreement with the data suggesting the presence of massive dark matter haloes in order to explain the lensing and dynamics properties of the SLACS lenses. According to the values of the dark matter mass fraction, we argue that the halo may play a significant role in the inner regions probed by the data, but such a conclusion strongly depends on the adopted initial mass function of the stellar population. Finally, we find that the dark matter mass fraction within $R_{eff}$ scales with both the total luminosity and stellar mass in such a way that more luminous (and hence more massive) galaxies have a larger dark matter content. [23] arXiv:0905.4274 [ps, pdf, other] Title: Strong PAH Emission from z~2 ULIRGs Authors: Vandana Desai, B. T. Soifer, Arjun Dey, Emeric Le Floc'h, Lee Armus, Kate Brand, Michael J. I. Brown, Mark Brodwin, Buell T. Jannuzi, James R. Houck, Daniel W. Weedman, Matthew L. N. Ashby, Anthony Gonzalez, Jiasheng Huang, Howard A. Smith, Harry Teplitz, Steve P. Willner, Jason Melbourne Comments: accepted for publication in ApJ Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO) Using the Infrared Spectrograph on board the Spitzer Space Telescope, we present low-resolution (64 < lambda / dlambda < 124), mid-infrared (20-38 micron) spectra of 23 high-redshift ULIRGs detected in the Bootes field of the NOAO Deep Wide-Field Survey. All of the sources were selected to have 1) fnu(24 micron) > 0.5 mJy; 2) R-[24] > 14 Vega mag; and 3) a prominent rest-frame 1.6 micron stellar photospheric feature redshifted into Spitzer's 3-8 micron IRAC bands. Of these, 20 show emission from polycyclic aromatic hydrocarbons (PAHs), usually interpreted as signatures of star formation. The PAH features indicate redshifts in the range 1.5 < z < 3.0, with a mean of =1.96 and a dispersion of 0.30. Based on local templates, these sources have extremely large infrared luminosities, comparable to that of submillimeter galaxies. Our results confirm previous indications that the rest-frame 1.6 micron stellar bump can be efficiently used to select highly obscured starforming galaxies at z~2, and that the fraction of starburst-dominated ULIRGs increases to faint 24 micron flux densities. Using local templates, we find that the observed narrow redshift distribution is due to the fact that the 24 micron detectability of PAH-rich sources peaks sharply at z = 1.9. We can analogously explain the broader redshift distribution of Spitzer-detected AGN-dominated ULIRGs based on the shapes of their SEDs. Finally, we conclude that z~2 sources with a detectable 1.6 micron stellar opacity feature lack sufficient AGN emission to veil the 7.7 micron PAH band. [16] arXiv:0905.4361 [ps, pdf, other] Title: Clustering of K-band selected local galaxies Authors: Bin Ma, Kelai Meng, Jun Pan, Jiasheng Huang, Longlong Feng Comments: 13 pages, 6 figures, accepted by RAA Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO) We present detailed clustering analysis of a large K-band selected local galaxy sample, which is constructed from the 2MASS and the SDSS and consists of $82,486$ galaxies with $10 < K < 13.5$ and $0.01 < z < 0.1$. The two-point correlation function of the magnitude-limited sample in real space at small scales is well described by a power law $\xi(r)=(r/6.44\pm0.23)^{-1.81\pm0.02}$. The pairwise velocity dispersion is derived from the anisotropic two-point correlation function and find the dispersion $\sigma_{12}=685\pm 17\kms$ if its scale invariance is assumed, which is larger than values measured in optical bands selected galaxy samples. We further investigate the dependence of the two-point correlation function and the $\sigma_{12}$ on the $g-r$ color and the $K$-band luminosity, obtain similar results to previous works in optical bands. Comparing a mock galaxy sample with our real data indicates that the semi-analytical model can not mimic the $\sigma_{12}$ in observation albeit it can approximate the two-point correlation function within measurement uncertainties. [26] arXiv:0905.4424 [ps, pdf, other] Title: Probing the 2-D kinematic structure of early-type galaxies out to 3 effective radii Authors: Robert N. Proctor, Duncan A. Forbes, Aaron J. Romanowsky, Jean P. Brodie, Jay Strader, Max Spolaor, J. Trevor Mendel, Lee Spitler Comments: 20 pages, 22 figures, Accepted for publication in MNRAS Subjects: Galaxy Astrophysics (astro-ph.GA) We detail an innovative new technique for measuring the 2-D velocity moments (rotation velocity, velocity dispersion and Gauss-Hermite coefficients h$_3$ and h$_4$) of the stellar populations of galaxy halos using spectra from Keck DEIMOS multi-object spectroscopic observations. The data are used to reconstruct 2-D rotation velocity maps. Here we present data for five nearby early-type galaxies to ~3 effective radii. We provide significant insights into the global kinematic structure of these galaxies, and challenge the accepted morphological classification in several cases. We show that between 1-3 effective radii the velocity dispersion declines very slowly, if at all, in all five galaxies. For the two galaxies with velocity dispersion profiles available from planetary nebulae data we find very good agreement with our stellar profiles. We find a variety of rotation profiles beyond 1 effective radius, i.e rotation speed remaining constant, decreasing \emph{and} increasing with radius. These results are of particular importance to studies which attempt to classify galaxies by their kinematic structure within one effective radius, such as the recent definition of fast- and slow- rotator classes by the SAURON project. Our data suggests that the rotator class may change when larger galacto-centric radii are probed. This has important implications for dynamical modeling of early-type galaxies. The data from this study are available on-line. [1] arXiv:0905.4499 [ps, pdf, other] Title: Cosmic evolution of submillimeter galaxies and their contribution to stellar mass assembly Authors: Michał J. Michałowski, Jens Hjorth, Darach Watson Comments: Submitted to MNRAS. Comments welcome. 11 pages (+21 pages as appendix), 6 figures, 6 tables. Tables A1-A5 can be found in the source file in the machine-readable form. For SED templates, see this http URL Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); Galaxy Astrophysics (astro-ph.GA) The nature of galaxies selected at submillimeter wavelengths (SMGs, S_850 > 3 mJy), some of the bolometrically most luminous objects at high redshifts, is still elusive. In particular their star formation histories and source of emission are not accurately constrained. In this paper we present the first spectral energy distribution fits of 76 SMGs with spectroscopic redshifts using all photometric datapoints from ultraviolet to radio simultaneously. We find that they are highly star-forming (median star formation rate 659 Msol yr^{-1}), hosting significant stellar populations (median stellar mass 3.5x10^{11} Msol) of which only a minor part (7%) has been formed in the ongoing starburst episode. This implies that in the past, SMGs experienced either another starburst episode or merger with several galaxies. The properties of SMGs suggest that they are progenitors of present-day elliptical galaxies. We find that these bright SMGs contribute significantly to the cosmic star formation rate density (8%) and stellar mass density (9-17%) at redshifts 4-2. Using number counts at low fluxes we find that as much as one third of the cosmic star formation at these redshifts took place in SMGs brighter than 0.1 mJy. We find evidence that the local linear far-infrared-radio correlation holds up to redshift of 3.6. This also suggests that the major source of infrared emission in SMGs is star formation not active galactic nuclei (AGNs). [5] arXiv:0905.4503 [ps, pdf, other] Title: The Rest-Frame Optical Luminosity Function of Cluster Galaxies at z<0.8 and the Assembly of the Cluster Red Sequence Authors: Gregory Rudnick, Anja von der Linden, Roser Pello, Alfonso Aragon-Salamanca, Danilo Marchesini, Douglas Clowe, Gabriella De Lucia, Claire Halliday, Pascale Jablonka, Bo Milvang-Jensen, Bianca Poggianti, Roberto Saglia, Luc Simard, Simon White, Dennis Zaritsky Comments: Accepted for publication in the Astrophysical Journal. 36 pages, 16 figures, 10 tables Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO) We present the rest-frame optical luminosity function (LF) of red sequence galaxies in 16 clusters at 0.4 10^{-9} Mpc^{-3}yr^{-1} is expected at z=6 for halos with virial temperature T_{vir}~10^4 K (mass ~10^8 Msun), sufficient to initiate cooling even with strong negative radiative feedback. Under the assumption of a single Population III supernova formed per metal-free halo, we expect an observed supernova rate of 2.6x10^{-3} deg^{-2}yr^{-1} in the same redshift range. These metal-free stars and their supernovae will be isolated and outside galaxies (at distances >150 h^{-1} kpc) and thus significantly less biased than the general population of ~10^8 Msun halos at z~6. Supernova searches for metal-free explosions must thus rely on large area surveys. If metal-free stars produce very luminous supernovae, like SN2006gy, then a multi-epoch survey reaching m_AB =27 at 1 micron is sufficient for detecting them at z=6. While the Large Synoptic Survey Telescope will not reach this depth in the z band, it will be able to detect several tens of Population III supernovae in the i and r bands at z <5.5, when their observed rate is down to 3-8x10^{-4} deg^{-2} yr^{-1}. [7] arXiv:0905.4505 [ps, pdf, other] Title: Extremely-Metal Poor Stars in the Milky Way: A Second Generation Formed after Reionization Authors: Michele Trenti (1), Michael Shull (1), ((1) U Colorado) Comments: 15 pages, 3 figures, submitted to ApJ Subjects: Galaxy Astrophysics (astro-ph.GA); Cosmology and Extragalactic Astrophysics (astro-ph.CO) Cosmological simulations of Population III star formation suggest an initial mass function (IMF) biased toward very massive stars (M>100Msun) formed in minihalos at redshift z>20, when the cooling is driven by molecular hydrogen. However, this result conflicts with observations of extremely-metal poor (EMP) stars in the Milky Way halo, whose r-process elemental abundances appear to be incompatible with those expected from very massive Population III progenitors. We propose that the IMF of second-generation stars formed at z>10, before reionization, is deficient in sub-solar mass stars, owing to the high cosmic microwave background temperature floor. The observed EMP stars are formed preferentially at z<10 in pockets of gas enriched to metallicity Z>10^{-3.5} Zsun by winds from Population II stars. Our cosmological simulations of structure formation show that current samples of EMP stars can only constrain the IMF of late-time Population III stars, formed at z<13 in halos with virial temperature Tvir 10^4 K. This suggests that pair instability supernovae were not produced primarily by this population. To begin probing the IMF of Population III stars formed at higher redshift will require a large survey, with at least 500 and probably several thousand EMP stars of metallicities Z~10^{-3.5} Zsun. [11] arXiv:0905.4525 [ps, pdf, other] Title: The effect of dry mergers on the color-magnitude relation of early-type galaxies Authors: Rosalind E. Skelton, Eric F. Bell, Rachel S. Somerville Comments: 5 pages, 1 figure, accepted for publication in ApJL Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO) We investigate the effect of dry merging on the color-magnitude relation (CMR) of galaxies and find that the amount of merging predicted by a hierarchical model results in a red sequence that compares well with the observed low-redshift relation. A sample of 29,000 early-type galaxies selected from the Sloan Digital Sky Survey Data Release 6 shows that the bright end of the CMR has a shallower slope and smaller scatter than the faint end. This magnitude dependence is predicted by a simple toy model in which gas-rich mergers move galaxies onto a "creation red sequence" (CRS) by quenching their star formation, and subsequent mergers between red, gas-poor galaxies (so-called "dry" mergers) move galaxies along the relation. We use galaxy merger trees from a semianalytic model of galaxy formation to test the amplitude of this effect and find a change in slope at the bright end that brackets the observations, using gas fraction thresholds of 10-30% to separate wet and dry mergers. A more realistic model that includes scatter in the CRS shows that dry merging decreases the scatter at the bright end. Contrary to previous claims, the small scatter in the observed CMR thus cannot be used to constrain the amount of dry merging. [31] arXiv:0905.4669 [ps, pdf, other] Title: Morphological quenching of star formation: making early-type galaxies red Authors: Marie Martig, Frederic Bournaud, Romain Teyssier, Avishai Dekel Comments: Submitted to ApJ. Movies available at this http URL Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO) We point out a natural mechanism for quenching of star formation in early-type galaxies. It automatically links the color of a galaxy with its morphology and does not require gas consumption, removal or termination of gas supply. Given that star formation takes place in gravitationally unstable gas disks, it can be quenched when a disk becomes stable against fragmentation to bound clumps. This can result from the growth of a stellar spheroid, for instance by mergers. We present the concept of morphological quenching (MQ) using standard disk instability analysis, and demonstrate its natural occurrence in a cosmological simulation using an efficient zoom-in technique. We show that the transition from a stellar disk to a spheroid can be sufficient to stabilize the gas disk, quench star formation, and turn an early-type galaxy red and dead while gas accretion continues. The turbulence necessary for disk stability can be stirred up by sheared perturbations within the disk in the absence of bound star-forming clumps. While gas stripping processes are limited to dense groups and clusters, and other quenching mechanisms like AGN feedback, virial shock heating and gravitational heating, are limited to halos more massive than 10^12 Mo, the MQ can explain the appearance of red ellipticals even in less massive halos and in the field. The dense gas disks observed in some of today's red ellipticals may be the relics of this mechanism, whereas red galaxies with quenched gas disks are expected to be more frequent at high redshift.