Evolution of Hot Gas and Dark Halos in Group-Dominant Elliptical Galaxies: Influence of Cosmic Inflow

We study the complete dynamical evolution of hot interstellar gas in massive elliptical galaxies born into a simple flat universe beginning with an overdense perturbation. Within the turn-around radius dark matter flows in a self-similar fashion into a stationary Navarro-Frenk-White halo and the baryonic gas shocks. After a few gigayears, when enough gas accumulates within the accretion shock, the de Vaucouleurs stellar system is constructed and the energy from Type II supernovae is released. The stars and dark halo are matched to NGC 4472. Gas continues to enter the galaxy by secondary infall and by stellar mass loss based on a Salpeter IMF. After about 13 Gyrs the temperature and density distribution in the hot gas agree quite well with the hot interstellar gas observed in NGC 4472. As a result of supernova-driven outflow, the present day baryonic fraction has a deep minimum in the outer galactic halo. When relatively gas-rich, X-ray luminous models are spatially truncated at early times, simulating tidal events that may have occurred during galaxy group dynamics, the current locus of truncated models lies just along the $L_x$, X-ray size correlation among well-observed ellipticals, providing another striking confirmation of our simple model of elliptical evolution.Comment: 16 pages in AASTEX LaTeX with 14 figures; accepted by Astrophysical Journa

Photometry and the Metallicity Distribution of the Outer Halo of M31

We have conducted a wide-field CCD-mosaic study of the resolved red-giant branch (RGB) stars of M31, in a field located 20 kpc from the nucleus along the SE minor axis. In our (I, V-I) color-magnitude diagram, RGB stars in the top three magnitudes of the M31 halo are strongly present. Photometry of a more distant control field to subtract field contamination is used to derive the `cleaned' luminosity function and metallicity distribution function (MDF) of the M31 halo field. From the color distribution of the foreground Milky Way halo stars, we find a reddening E(V-I)= 0.10 +/- 0.02 for this field, and from the luminosity of the RGB tip, we determine a distance modulus (m-M)_o = 24.47 +/- 0.12 (= 783 +/- 43 kpc). The MDF is derived from interpolation within an extensive new grid of RGB models (Vandenberg et al. 2000). The MDF is dominated by a moderately high-metallicity population ([m/H]~ -0.5) found previously in more interior M31 halo/bulge fields, and is much more metal-rich than the [m/H]~ -1.5 level in the Milky Way halo. A significant (~30% - 40%, depending on AGB star contribution) metal-poor population is also present. To first order, the shape of the MDF resembles that predicted by a simple, single-component model of chemical evolution starting from primordial gas with an effective yield y=0.0055. It strongly resembles the MDF recently found for the outer halo of the giant elliptical NGC 5128 (Harris et al. 2000), though NGC 5128 has an even lower fraction of low-metallicity stars. Intriguingly, in both NGC 5128 and M31, the metallicity distribution of the globular clusters in M31 does not match the halo stars; the clusters are far more heavily weighted to metal-poor objects. We suggest similarities in the formation and early evolution of massive, spheroidal stellar systems.Comment: to appear in the Astronomical Journal; 43 pages, including 15 figure

A Slow Merger History of Field Galaxies Since z~1

Using deep infrared observations conducted with the CISCO imager on the Subaru Telescope, we investigate the field-corrected pair fraction and the implied merger rate of galaxies in redshift survey fields with Hubble Space Telescope imaging. In the redshift interval, 0.5 < z < 1.5, the fraction of infrared-selected pairs increases only modestly with redshift to 7% +- 6% at z~1. This is nearly a factor of three less than the fraction, 22% +- 8%, determined using the same technique on HST optical images and as measured in a previous similar study. Tests support the hypothesis that optical pair fractions at z~1 are inflated by bright star-forming regions that are unlikely to be representative of the underlying mass distribution. By determining stellar masses for the companions, we estimate the mass accretion rate associated with merging galaxies. At z~1, we estimate this to be 2x10^{9 +- 0.2} solar masses per galaxy per Gyr. Although uncertainties remain, our results suggest that the growth of galaxies via the accretion of pre-existing fragments remains as significant a phenomenon in the redshift range studied as that estimated from ongoing star formation in independent surveys.Comment: 5 pages, accepted for publication in ApJ Letter

Questions on pure luminosity evolution for ellipticals

The explanation for the existence of an excess population of faint blue galaxies (FBGs) has been a mystery for nearly two decades, and remains one of the grand astronomical issues to date. Existing models cannot explain all of the observational data such as galaxy number counts in the optical and infrared passbands and the redshift distributions of galaxies. Here, by modelling the morphological number counts derived from the Hubble Space Telescope, as well as the number counts in optical and infrared passbands, and the redshift and color distributions of galaxies obtained from ground-based observations, we show that the `FBG problem' cannot be resolved if elliptical galaxies are assumed to have formed in an instantaneous burst of star formation at high redshift with no subsequent star formation events, which is just the conventional scenario for formation and evolution of ellipticals. There exist great discrepancies between the observed $B-K$ color distribution and the predicted distribution for ellipticals by such a pure luminosity evolution (PLE) model in the context of the conventional scenario. Neither can the mild evolution (i.e., the star formation events have lasted for a longer time than those of the instantaneous burst and passive evolution since the formation of galaxies) for ellipticals be accepted in the context of PLE assumption. The introduction of dust extinction also cannot save the PLE models. This conclusion holds for each of the three cosmological models under consideration: flat, open and $\Lambda$-dominated. Hence, our investigation suggests that PLE assumption for elliptical galaxies is questionable, and number evolution may be essential for ellipticals.Comment: 20pages (AASTeX), 10 eps figures, 4 tables, to appear in ApJ, 1999, Vol. 51

The DEEP2 Galaxy Redshift Survey: The Evolution of Void Statistics from z~1 to z~0

We present measurements of the void probability function (VPF) at z~1 using data from the DEEP2 Redshift Survey and its evolution to z~0 using data from the Sloan Digital Sky Survey (SDSS). We measure the VPF as a function of galaxy color and luminosity in both surveys and find that it mimics trends displayed in the two-point correlation function, $\xi$; namely that samples of brighter, red galaxies have larger voids (i.e. are more strongly clustered) than fainter, blue galaxies. We also clearly detect evolution in the VPF with cosmic time, with voids being larger in comoving units at z~0. We find that the reduced VPF matches the predictions of a `negative binomial' model for galaxies of all colors, luminosities, and redshifts studied. This model lacks a physical motivation, but produces a simple analytic prediction for sources of any number density and integrated two-point correlation function, \bar{\xi}. This implies that differences in the VPF across different galaxy populations are consistent with being due entirely to differences in the population number density and \bar{\xi}. The robust result that all galaxy populations follow the negative binomial model appears to be due to primarily to the clustering of dark matter halos. The reduced VPF is insensitive to changes in the parameters of the halo occupation distribution, in the sense that halo models with the same \bar{\xi} will produce the same VPF. For the wide range of galaxies studied, the VPF therefore does not appear to provide useful constraints on galaxy evolution models that cannot be gleaned from studies of \bar{\xi} alone. (abridged)Comment: 17 pages, 15 figures, ApJ accepte

Stellar and Gas properties of High HI Mass-to-Light Ratio Galaxies in the Local Universe

We present a multi-wavelength study (BVRI band photometry and HI line interferometry) of nine late-type galaxies selected from the HIPASS Bright Galaxy Catalog on the basis of apparently high HI mass-to-light ratios (3 M_sun/L_sun < M_HI/L_B < 27 M_sun/L_sun). We found that most of the original estimates for M_HI/L_B based on available photographic magnitudes in the literature were too high, and conclude that genuine high HI mass-to-light ratio (>5 M_sun/L_sun) galaxies are rare in the Local Universe. Extreme high M_HI/L_B galaxies like ESO215-G?009 appear to have formed only the minimum number of stars necessary to maintain the stability of their HI disks, and could possibly be used to constrain galaxy formation models. They may to have been forming stars at a low, constant rate over their lifetimes. The best examples all have highly extended HI disks, are spatially isolated, and have normal baryonic content for their total masses but are deficent in stars. This suggests that high M_HI/L_B galaxies are not lacking the baryons to create stars, but are underluminous as they lack either the internal or external stimulation for more extensive star formation.Comment: 29 Pages, 59 Figures. Accepted for publication in AJ (to be published ~April 2006

The Search for Intergalactic Hydrogen Clouds in Voids

I present the results of a search for intergalactic hydrogen clouds in voids. Clouds are detected by their HI LyA absorption lines in the HST spectra of low-redshift AGN. The parameter with which the environments of clouds are characterized is the tidal field, which places a lower limit on the cloud mass-density which is dynamically stable against disruption. Galaxy redshift catalogs are used to sum the tidal fields along the lines of sight, sorting clouds according to tidal field upper, or lower limits. The analytical methodology employed is designed to detect gas clouds whose expansion following reionization is restrained by dark matter perturbations. End-products are the cloud equivalent width distribution functions (EWDF) of catalogs formed by sorting clouds according to various tidal field upper, or lower limits. Cumulative EWDFs are steep in voids (S ~ -1.5 \pm 0.2), but flatter in high tidal field zones (S ~ -0.5 \pm 0.1). Most probable cloud Doppler parameters are ~30 km/s in voids and ~60 km/s in proximity to galaxies. In voids, the cumulative line density at low EW (~ 15 mA) is ~ 500 per unit redshift. The void filling factor is found to be 0.87 <= f_v <= 0.94. The void EWDF is remarkably uniform over this volume, with a possible tendency for more massive clouds to be in void centers. The size and nature of the void cloud population suggested by this study is completely unanticipated by the results of published 3-D simulations, which predict that most clouds are in filamentary structures around galaxy concentrations, and that very few observable absorbers would lie in voids. Strategies for modeling this population are briefly discussed.Comment: 21 pages, 19 figures, apjemulate style, to appear in ApJ vol. 57

CARMA Large Area Star Formation Survey: Observational Analysis of Filaments in the Serpens South Molecular Cloud

We present the N2H+(J=1-0) map of the Serpens South molecular cloud obtained as part of the CARMA Large Area Star Formation Survey (CLASSy). The observations cover 250 square arcminutes and fully sample structures from 3000 AU to 3 pc with a velocity resolution of 0.16 km/s, and they can be used to constrain the origin and evolution of molecular cloud filaments. The spatial distribution of the N2H+ emission is characterized by long filaments that resemble those observed in the dust continuum emission by Herschel. However, the gas filaments are typically narrower such that, in some cases, two or three quasi-parallel N2H+ filaments comprise a single observed dust continuum filament. The difference between the dust and gas filament widths casts doubt on Herschel ability to resolve the Serpens South filaments. Some molecular filaments show velocity gradients along their major axis, and two are characterized by a steep velocity gradient in the direction perpendicular to the filament axis. The observed velocity gradient along one of these filaments was previously postulated as evidence for mass infall toward the central cluster, but these kind of gradients can be interpreted as projection of large-scale turbulence.Comment: 12 pages, 4 figures, published in ApJL (July 2014