Resurrection of Traditional Luminosity Evolution Models to Explain Faint Field Galaxies

We explore the nature of the evolution of faint field galaxies by assuming that the local luminosity function is not well defined. We use a non-negative least squares technique to derive a near optimal set of local luminosity functions for different spectral types of galaxies by fitting to the observed optical and near-infrared counts, B-R colors, and redshift distributions for galaxies with 15 < B < 27. We report here the results of using only traditional luminosity evolution (ie. the photometric evolution of stars in a galaxy over time given reasonable assumptions of the form of the star formation history for various galaxy types), and simple galaxy reddening and find excellent fits to the observed data to B = 27. We conclude that models more exotic than traditional luminosity evolution are not yet required to explain existing faint field galaxy data and thus the need for contributions by mergers or new populations of galaxies is at least 5x less than previously estimated.Comment: 9 pages + 1 table + 4 figures; uuencoded tar compressed postscript; to be published in The Astrohysical Journal Letter

Counter-Evolution of Faint Field Galaxies

We adopt a new approach to explore the puzzling nature of faint blue field galaxies. Instead of assuming that the local luminosity function is well defined, we first determine whether any non-evolving set of luminosity functions for different spectral types of galaxies is compatible with the observed marginal distributions in optical and near-infrared counts, B - R colors, and redshifts. Exploiting a non-negative least squares method, we derive a new no-evolution model that is found to fit all the observations surprisingly well. We conclude that models more exotic than mild luminosity evolution, such as those requiring rapid evolution in star formation rates, disappearing dwarf galaxy populations, high values of the cosmological constant, rapid mergers, or substantial non-conservation of galaxy numbers with time, are no longer as compelling.Comment: 10 pages + 4 figures (appended), AAS LaTeX v3.0, Ap.J. Letters in press, UCO/Lick Bulletin #126

Locating Star-Forming Regions in Quasar Host Galaxies

We present a study of the morphology and intensity of star formation in the host galaxies of eight Palomar-Green quasars using observations with the Hubble Space Telescope. Our observations are motivated by recent evidence for a close relationship between black hole growth and the stellar mass evolution in its host galaxy. We use narrow-band [O II] $\lambda$3727, H$\beta$, [O III] $\lambda$5007 and Pa$\alpha$ images, taken with the WFPC2 and NICMOS instruments, to map the morphology of line-emitting regions, and, after extinction corrections, diagnose the excitation mechanism and infer star-formation rates. Significant challenges in this type of work are the separation of the quasar light from the stellar continuum and the quasar-excited gas from the star-forming regions. To this end, we present a novel technique for image decomposition and subtraction of quasar light. Our primary result is the detection of extended line-emitting regions with sizes ranging from 0.5 to 5 kpc and distributed symmetrically around the nucleus, powered primarily by star formation. We determine star-formation rates of order a few tens of M$_\odot$/yr. The host galaxies of our target quasars have stellar masses of order $10^{11}$ M$_\odot$ and specific star formation rates on a par with those of M82 and luminous infrared galaxies. As such they fall at the upper envelope or just above the star-formation mass sequence in the specific star formation vs stellar mass diagram. We see a clear trend of increasing star formation rate with quasar luminosity, reinforcing the link between the growth of the stellar mass of the host and the black hole mass found by other authors.Comment: Accepted for publication in M.N.R.A.

A Chandra−SwiftChandra-Swift View of Point Sources in Hickson Compact Groups: High AGN fraction but a dearth of strong AGNs

We present $Chandra$ X-ray point source catalogs for 9 Hickson Compact Groups (HCGs, 37 galaxies) at distances $34 - 89$ Mpc. We perform detailed X-ray point source detection and photometry, and interpret the point source population by means of simulated hardness ratios. We thus estimate X-ray luminosities ($L_X$) for all sources, most of which are too weak for reliable spectral fitting. For all sources, we provide catalogs with counts, count rates, power-law indices ($\Gamma$), hardness ratios, and $L_X$, in the full ($0.5-8.0$ keV), soft ($0.5-2.0$ keV) and hard ($2.0-8.0$ keV) bands. We use optical emission-line ratios from the literature to re-classify 24 galaxies as star-forming, accreting onto a supermassive black hole (AGNs), transition objects, or low-ionization nuclear emission regions (LINERs). Two-thirds of our galaxies have nuclear X-ray sources with $Swift$/UVOT counterparts. Two nuclei have $L_{X,{\rm 0.5-8.0 keV}}$~$> 10^{42}$ erg s$^{-1}$, are strong multi-wavelength AGNs and follow the known $\alpha_{\rm OX}-\nu L_{\nu,\rm near UV}$ correlation for strong AGNs. Otherwise, most nuclei are X-ray faint, consistent with either a low-luminosity AGN or a nuclear X-ray binary population, and fall in the "non-AGN locus" in $\alpha_{\rm OX}-\nu L_{\nu,\rm near UV}$ space, which also hosts other, normal, galaxies. Our results suggest that HCG X-ray nuclei in high specific star formation rate spiral galaxies are likely dominated by star formation, while those with low specific star formation rates in earlier types likely harbor a weak AGN. The AGN fraction in HCG galaxies with $M_R \le -20$ and $L_{X,{\rm 0.5-8.0 keV}} \ge 10^{41}$ erg s$^{-1}$ is $0.08^{+0.35}_{-0.01}$, somewhat higher than the $\sim 5$ fraction in galaxy clusters.Comment: 77 pages (emulateapj), 28 tables, 11 figures. Accepted by ApJS on March 5, 201

Morphology and evolution of emission line galaxies in the Hubble Ultra Deep Field

We investigate the properties and evolution of a sample of galaxies selected to have prominent emission lines in low-resolution grism spectra of the Hubble Ultra Deep Field (HUDF). These objects, eGRAPES, are late type blue galaxies, characterized by small proper sizes (R_50 < 2 kpc) in the 4350A rest-frame, low masses (5x10^9 M_sun), and a wide range of luminosities and surface brightnesses. The masses, sizes and volume densities of these objects appear to change very little up to a redshift of z=1.5. On the other hand, their surface brightness decreases significantly from z=1.5 to z=0 while their mass-to-light ratio increases two-folds. This could be a sign that most of low redshift eGRAPES have an older stellar population than high redshift eGRAPES and hence that most eGRAPES formed at higher redshifts. The average volume density of eGRAPES is (1.8 \pm 0.3)x10^{-3} Mpc^{-3} between 0.3 < z < 1.5. Many eGRAPES would formally have been classified as Luminous Compact Blue Galaxies (LCBGs) if these had been selected based on small physical size, blue intrinsic color, and high surface brightness, while the remainder of the sample discussed in this paper forms an extension of LCBGs towards fainter luminosities.Comment: Accepted, to appear in Ap

HI Properties of Low Luminosity Star-Forming Galaxies in the KPNO International Spectroscopic Survey

New HI observations are presented for a complete sample of 109 low luminosity star-forming galaxies taken from the KPNO International Spectroscopic Survey (KISS), the first CCD-based wide-field objective-prism survey for emission-line galaxies. This sample consists of all star-forming galaxies with M_B > -18.0 and cz < 11,000 km/s from the first Halpha-selected survey list. Overall, 97 out of 109 galaxies have been detected in HI. We confirm the weak trend of increasing gas richness with decreasing luminosity found by previous authors. Gas richness is also shown to be weakly anti-correlated with metallicity. The dependence of star formation rates (SFRs) and HI gas depletion timescales on metallicity is examined. The median solar metallicity based SFR and gas depletion timescale are 0.1639 M_sun/yr and 5 Gyrs, respectively. Corrections for variations in metallicity decreases SFRs by about 0.5 dex and increases gas depletion timescales by an average of about 8 Gyrs. The majority of galaxies in this sample still have large reservoirs of HI gas, and despite their large current star formation rates, could have formed stars in a quasi-continuous manner for a Hubble time. Finally, we present the first HI mass function for low luminosity star-forming galaxies and show that this subpopulation contributes 10-15% of the overall HI density in the local universe. We conclude that if the HI mass function of the Universe does indeed have a steeply rising low-mass slope as suggested by previous authors, it is not due to the population of low luminosity star-forming galaxies.Comment: 27 pages, 13 figures, 4 tables. Accepted for publication in the Astronomical Journa