Searcharxiv⌕ Search

arXiv subjects

H. -W. Chen

Publications and source records attributed to H. -W. Chen.

47 records · Page 3Linked to original sources

The mass-metallicity relation at z~0.7

The ISM metallicity and the stellar mass are examined in a sample of 66 galaxies at 0.4<z<1, selected from the Gemini Deep Deep Survey (GDDS) and the Canada-France Redshift Survey (CFRS). We observe a mass-metallicity relation similar to that seen in z~0.1 SDSS galaxies, but displaced towards higher masses and/or lower metallicities. Using this sample, and a small sample of z~2.3 LBGs, a redshift dependent mass-metallicity relation is proposed which describes the observed results.

astro-ph↗

Cosmic Star Formation History and its Dependence on Galaxy Stellar Mass

We examine the cosmic star formation rate (SFR) and its dependence on galaxy stellar mass over the redshift range 0.8 < z < 2 using data from the Gemini Deep Deep Survey (GDDS). The SFR in the most massive galaxies (M > 10^{10.8} M_sun) was six times higher at z = 2 than it is today. It drops steeply from z = 2, reaching the present day value at z ~ 1. In contrast, the SFR density of intermediate mass galaxies (10^{10.2} < M < 10^{10.8} M_sun) declines more slowly and may peak or plateau at z ~ 1.5. We use the characteristic growth time t_SFR = rho_M / rho_SFR to provide evidence of an associated transition in massive galaxies from a burst to a quiescent star formation mode at z ~ 2. Intermediate mass systems transit from burst to quiescent mode at z ~ 1, while the lowest mass objects undergo bursts throughout our redshift range. Our results show unambiguously that the formation era for galaxies was extended and proceeded from high to low mass systems. The most massive galaxies formed most of their stars in the first ~3 Gyr of cosmic history. Intermediate mass objects continued to form their dominant stellar mass for an additional ~2 Gyr, while the lowest mass systems have been forming over the whole cosmic epoch spanned by the GDDS. This view of galaxy formation clearly supports `downsizing' in the SFR where the most massive galaxies form first and galaxy formation proceeds from larger to smaller mass scales.

astro-ph↗

Mission: Impossible (Escape from the Lyman Limit)

We investigate the intrinsic opacity of high-redshift galaxies to outgoing ionising photons using high-quality photometry of a sample of 27 spectroscopically-identified galaxies of redshift 1.9<z<3.5 in the Hubble Deep Field. Our measurement is based on maximum-likelihood fitting of model galaxy spectral energy distributions-including the effects of intrinsic Lyman-limit absorption and random realizations of intervening Lyman-series and Lyman-limit absorption-to photometry of galaxies from space- and ground-based broad-band images. Our method provides several important advantages over the methods used by previous groups, including most importantly that two-dimensional sky subtraction of faint-galaxy images is more robust than one-dimensional sky subtraction of faint-galaxy spectra. We find at the 3sigma statistical confidence level that on average no more than 4% of the ionising photons escape galaxies of redshift 1.9<z<3.5. This result is consistent with observations of low- and moderate-redshift galaxies but is in direct contradiction to a recent result based on medium-resolution spectroscopy of high-redshift (z~3) galaxies. Dividing our sample in subsamples according to luminosity, intrinsic ultraviolet colour, and redshift, we find no evidence for selection effects that could explain such discrepancy. Even when all systematic effects are included, the data could not realistically accomodate any escape fraction value larger than ~15%.

astro-ph↗

The Las Campanas IR Survey. II. Photometric redshifts, comparison with models and clustering evolution

The LCIR Survey, using the Cambridge IR Survey Instrument (CIRSI), reaches H \~ 20-21 over ~1 deg^2. We present results for 744 arcmin^2 in which public UBVRI data exist. We compare optical-IR colours with predictions of a semi-analytic hierarchical model (SAM) and find reasonable agreement. We also determine photometric redshifts, finding a median z of z_m ~ 0.55. We compare N(z) of different spectral types with models, showing that the observations are inconsistent with simple PLE models while the SAM provides a reasonable fit to the total N(z) but underestimates the number of z ~ 1 red spectral types. We also present N(z) for samples of red objects (EROs). We find that EROs with R - H > 4 and H < 20.5 have z_m ~ 1; redder EROs have higher z_m. For 19 < H < 20, EROs with R - H > 4 comprise ~18% of the observed galaxy population while in the SAM they contribute only ~4%. We also determine the angular correlation function w(theta) for magnitude, colour, spectral type and photo-z selected samples and use the estimated N(z) to derive the spatial clustering xi(r). Parametrizing xi(r) by xi(r_c,z)=(r_c/r_*(z))^(-1.8) (r_c comoving), we find that r_*(z) increases by ~1.5-2 times from z = 0 to z ~ 1.2. We interpret this as a selection effect - the galaxies selected at z ~ 1.2 are intrinsically very luminous. When galaxies are selected by absolute magnitude we find no evidence for evolution in r_* over this z range. Extrapolated to z = 0, we find r_*(0) ~ 6.5 h^-1Mpc for red galaxies and r_*(0) ~ 2-4 h^-1Mpc for blue galaxies. We also find that while the angular clustering amplitude of EROs with R - H > 4 or I - H > 3 is up to four times that of the whole galaxy population, the spatial clustering length r_*(z=1) is ~7.5-10.5 h^-1Mpc which is only a factor of ~1.7 times r_* for non-EROs lying in a similar z and luminosity range.

astro-ph↗

Error analysis of the photometric redshift tecnique

We present a calculation of the systematic component of the error budget in the photometric redshift technique. We make use of it to describe a simple technique that allows for the assignation of confidence limits to redshift measurements obtained through photometric methods. We show that our technique, through the calculation of a redshift probability function, gives complete information on the probable redshift of an object and its associated confidence intervals. This information can and must be used in the calculation of any observable quantity which makes use of the redshift.

astro-ph↗

The z ~ 1.2 Galaxy Luminosity Function from The LCIR Survey

We present results from the Las Campanas Infrared Survey, designed to identify a statistically significant sample of z>=1 galaxies using photometric redshift techniques. Here we summarize the design and strategies of the survey and present the first estimate of the galaxy luminosity function at z>=1 based on H-band selected galaxies identified in our survey. Results of number count studies and luminosity function measurements indicate that most early-type galaxies were already in place by z~1.2 with a modest space density evolution and a mild luminosity evolution over that expected from passive evolution.

astro-ph↗

The Las Campanas IR Survey: Early Type Galaxy Progenitors Beyond Redshift One

(Abridged) We have identified a population of faint red galaxies from a 0.62 square degree region of the Las Campanas Infrared Survey whose properties are consistent with their being the progenitors of early-type galaxies. The optical and IR colors, number-magnitude relation and angular clustering together indicate modest evolution and increased star formation rates among the early-type field population at redshifts between one and two. The counts of red galaxies with $H$ magnitudes between 17 and 20 rise with a slope that is much steeper than that of the total H sample. The surface density of red galaxies drops from roughly 3000 per square degree at H = 20.5, I-H > 3 to ~ 20 per square degree at H = 20, I-H > 5. The V-I colors are approximately 1.5 magnitudes bluer on average than a pure old population and span a range of more than three magnitudes. The colors, and photometric redshifts derived from them, indicate that the red galaxies have redshift distributions adequately described by Gaussians with sigma_z ~ 0.2$ centered near redshift one, with the exception that galaxies having $V-I<1.6$ and $I-H>3$ are primarily in the 1.5 < z < 2 range. We find co-moving correlation lengths of 9-10 Mpc at z ~ 1, comparable to, or larger than, those found for early-type galaxies at lower redshifts. A simple photometric evolution model reproduces the counts of the red galaxies, with only a ~ 30% decline in the underlying space density of early-type galaxies at z ~ 1.2. We suggest on the basis of the colors, counts, and clustering that these red galaxies are the bulk of the progenitors of present day early-type galaxies.

astro-ph↗

The Gaseous Extent of Galaxies and The Origin of QSO Absorption Line Systems

We present results of an ongoing program to study the gaseous extent of galaxies and the origin of QSO absorption line systems. For \lya absorption systems, we find that absorption equivalent width depends strongly on galaxy impact parameter and galaxy $B$-band luminosity, and that the gaseous extent of individual galaxies scales with galaxy $B$-band luminosity as $r\propto L_B^{0.40\pm0.09}$. Applying the results to galaxies in the Hubble Deep Field to calculate the predicted number density of \lya absorption lines as a function of redshift and comparing it with observations, we find that at least 50% and perhaps as much as 100% of \lya absorption systems with $W\apg0.32$ Åcan be explained by the extended gaseous envelops of normal galaxies. The anti-correlation analysis has also been performed on \civ absorption line systems, and the results show that the ionized gas cross section scales with galaxy $B$-band luminosity as $r\propto L_B^{0.76\pm0.26}$.

astro-ph↗

The Gaseous Extent of Galaxies and the Origin of \lya Absorption Systems. III. Hubble Space Telescope Imaging of \lya-Absorbing Galaxies at z < 1

We present initial results of a program to obtain and analyze HST WFPC2 images of galaxies identified in an imaging and spectroscopic survey of faint galaxies in fields of HST spectroscopic target QSOs. We measure properties of 87 galaxies, of which 33 are associated with corresponding \lya absorption systems and 24 do not produce corresponding \lya absorption lines to within sensitive upper limits. Considering only galaxy and absorber pairs that are likely to be physically associated and excluding galaxy and absorber pairs within 3000 \kms of the background QSOs leaves 26 galaxy and absorber pairs and seven galaxies that do not produce corresponding \lya absorption lines to within sensitive upper limits. Redshifts of the galaxy and absorber pairs range from 0.0750 to 0.8912 with a median of 0.3718, and impact parameter separations of the galaxy and absorber pairs range from 12.4 to $157.4 h^{-1}$ kpc with a median of $62.4 h^{-1}$ kpc. The primary result of the analysis is that the amount of gas encountered along the line of sight depends on the galaxy impact parameter and B-band luminosity but does not depend strongly on the galaxy average surface brightness, disk-to-bulge ratio, or redshift. This result confirms and improves upon the anti-correlation between \lya absorption equivalent width and galaxy impact parameter found previously by Lanzetta et al. (1995). There is no evidence that galaxy interactions play an important role in distributing tenuous gas around galaxies in most cases. Galaxies might account for all \lya absorption systems with $W > 0.3$ Å, but this depends on the unknown luminosity function and gaseous cross sections of low-luminosity galaxies as well as on the uncertainties of the observed number density of \lya absorption systems.

astro-ph↗

The Gaseous Extent of Galaxies and the Origin of \lya Absorption Systems at z < 1

We present initial results of a program to obtain and analyze HST WFPC2 images of galaxies in fields of HST spectroscopic target QSOs. The goal of the program is to investigate how the properties of \lya absorption systems observed in the spectra of background QSOs vary with the properties of intervening galaxies. We found that \lya absorption equivalent width depends strongly on galaxy impact parameter and galaxy B-band luminosity, and that the gaseous extent of individual galaxies scales with galaxy B-band luminosity by $r\propto L_B^{0.35\pm0.10}$.

astro-ph↗

The Compared Number Density of High-Redshift Galaxies and Lyman αClouds

We use our catalog of photometric redshifts in the Hubble Deep Field (HDF) to estimate the Luminosity Function (LF) of galaxies up to z=2. Using the obtained LF and a relationship between luminosity and halo size, we calculate the expected density of galactic halo crossings for any arbitrary line of sight. This density is then compared with the known one of Lyman αlines, showing that the observed density of galaxies is enough to account for the observed absorption lines.

astro-ph↗