arXiv subjects

Hsiao-Wen Chen

Publications and source records attributed to Hsiao-Wen Chen.

At least 163 records · Page 9Linked to original sources

arXiv2004

Discovery of Massive Evolved Galaxies at z > 3 in the Hubble Ultra Deep Field

We have identified six early-type galaxies at z>2.8 in the central 5.76 arcmin^2 Hubble Ultra Deep Field NICMOS region based on a pronounced broad-band discontinuity between the NICMOS F110W and F160W bandpasses. These galaxies have red observed optical and near-infrared colors (AB_F775W - AB_F160W >= 2) that cannot be explained entirely by dust reddening (indicating advanced age), and their luminosities at rest-frame optical wavelengths suggest a substantial stellar mass. One of the galaxies is detected in X-ray and is likely to have an active nuclear region, while the rest are estimated to be at least 1 Gyr old at z ~ 3 and contain total stellar mass of (0.4 - 9.1) x 10^{10} h^{-2} M_\odot. We calculate a cumulative comoving stellar mass density of rho_star = 0.7 - 1.2 x 10^7 h M_\odot Mpc^{-3} for massive evolved galaxies of M > 10^9 h^{-2} M_\odot at z>2.5. Comparison of the stellar mass density confined in galaxies of different types shows that massive evolved galaxies contribute ~ 16 - 28% of total stellar mass density even at these early epochs. Finally, an analysis of their morphology observed in the ACS and NICMOS images shows that the active galaxy has compact structure, while the rest are less concentrated. The presence of massive evolved galaxies at z ~ 3, when the universe was only 2 Gyr old, suggests that early star formation may have been particularly efficient in massive halos.

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arXiv2004

The Gemini Deep Deep Survey: III. The abundance of massive galaxies 3-6 billion years after the Big Bang

Hierarchical galaxy formation is the model whereby massive galaxies form from an assembly of smaller units. The most massive objects therefore form last. The model succeeds in describing the clustering of galaxies, but the evolutionary history of massive galaxies, as revealed by their visible stars and gas, is not accurately predicted. Near-infrared observations (which allow us to measure the stellar masses of high-redshift galaxies) and deep multi-colour images indicate that a large fraction of the stars in massive galaxies form in the first 5 Gyr, but uncertainties remain owing to the lack of spectra to confirm the redshifts (which are estimated from the colours) and the role of obscuration by dust. Here we report the results of a spectroscopic redshift survey that probes the most massive and quiescent galaxies back to an era only 3 Gyr after the Big Bang. We find that at least two-thirds of massive galaxies have appeared since this era, but also that a significant fraction of them are already in place in the early Universe.

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arXiv2004

The Host Galaxy of GRB 031203: Implications of its low metallicity, low redshift, and starburst nature

We present Keck/NIRSPEC near-IR images and Magellan/IMACS optical spectroscopy of the host galaxy of GRB 031203. The host is an actively star-forming galaxy at z=0.1055 +/- 0.0001. This is the lowest redshift GRB to-date, aside from GRB 980425. From the hydrogen Balmer lines, we infer an extinction of A_V = 3.62 +/- 0.25 or a total reddening E_T(B-V) = 1.17 +/- 0.1 toward the sightline to the nebular regions. After correcting for reddening, we perform an emission-line analysis and derive an ISM temperature of T=13400+/-2000K and electron density of n_e = 300 cm^(-3). These imply a metallicity [O/H]=-0.72+/-0.15 dex and a roughly solar abundance pattern for N, Ne, S, and Ar. Integrating Ha, we infer a dust-corrected star formation rate (SFR) of > 11 Msol/yr. These observations have the following implications: (1) the galaxy has a low K'-band luminosity L ~ L^*/5, typical of GRB host galaxies; (2) the low redshift indicates GRB 031203 had an isotropic-equivalent gamma-ray energy release smaller than all previous confirmed GRB events. The burst discovery raises the likelihood of identifying many additional low z, low flux events with Swift; (3) the large SFR, low metallicity, and the inferred hard radiation field is suggestive of massive star formation, supporting the collapsar model; (4) several lines of evidence argue against the identification of GRB 031203 as an X-ray flash event.

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arXiv2004

HE 0047-1756: A new gravitationally lensed double QSO

The quasar HE 0047-1756, at z=1.67, is found to be split into two images 1.44" apart by an intervening galaxy acting as a gravitational lens. The flux ratio for the two components is roughly 3.5:1, depending slightly upon wavelength. The lensing galaxy is seen on images obtained at 800 nm and 2.1 μ; there is also a nearby faint object which may be responsible for some shear. The spectra of the two quasar images are nearly identical, but the emission line ratio between the two components scale differently from the continuum. Moreover, the fainter component has a bluer continuum slope than the brighter one. We argue that these small differences are probably due to microlensing. There are hints of an Einstein ring emanating from the brighter image toward the fainter one.

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arXiv2004

The Gemini Deep Deep Survey: I. Introduction to the Survey, Catalogs and Composite Spectra

The Gemini Deep Deep Survey (GDDS) is an ultra-deep (K<20.6 mag, I<24.5 mag) redshift survey targeting galaxies in the "redshift desert" between 1<z<2. The primary goal of the survey is to constrain the space density at high redshift of evolved high-mass galaxies. We obtained 309 spectra in four widely-separated 30 arcmin^2 fields using the Gemini North telescope and the Gemini Multi-Object Spectrograph (GMOS). The spectra define a one-in-two sparse sample of the reddest and most luminous galaxies near the I-K vs. I color-magnitude track mapped out by passively evolving galaxies in the redshift interval 0.8<z<1.8. This sample is augmented by a one-in-seven sparse sample of the remaining high-redshift galaxy population. Typical exposures times were 20-30 hours per field (in Nod & Shuffle mode), and the resulting spectra are the deepest ever obtained. In this paper we present our sample of 309 spectra, along with redshifts, identifications of spectral features, and photometry. The infrared selection underlying the survey means that the GDDS is observing not only star-forming galaxies, as in most high-redshift galaxy surveys, but also quiescent evolved galaxies. The median redshift of the whole GDDS sample is z=1.1. Together with the data and catalogs, we present a summary of the criteria for selecting the GDDS fields, the rationale behind our mask designs, an analysis of the completeness of the survey, and a description of the data reduction procedures used. All data from the GDDS are publicly available. (ABRIDGED)

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arXiv2003

Rapid Growth of Massive Galaxies: A Paradox for Hierarchical Formation Models

On behalf of the survey teams I summarize the designs and results of the Las Campanas Infrared Survey and Gemini Deep Deep Survey, both of which were initiated to understand the nature of red galaxies and to study the history of stellar mass assembly. Our results from luminosity function analysis, ISM absorption line measurements, and spectral synthesis modeling show that near-infrared selected galaxies at 1<z<2 are not only massive and abundant but also old and metal enriched, indicating rapid formation of massive systems at higher redshifts.

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arXiv2003

The Nature of Damped Ly-alpha Absorbing Galaxies at z<=1--A Photometric Redshift Survey of Damped Ly-alpha Absorbers

We study the nature of damped Lya absorption (DLA) systems at z<=1 using a sample of 11 DLA galaxies, for which accurate redshift measurements are available. We demonstrate that the precision of photometric redshifts is sufficient for identifying DLA galaxies, because DLAs are rare and their intrinsically high column density implies a small impact parameter of the host galaxy to the QSO line of sight. We adopt this first large DLA galaxy sample to study the neutral gas cross section of intermediate-redshift galaxies and examine the optical properties of DLA galaxies at z<=1. The results of our study are: (1) the extent of neutral gas around intermediate-redshift galaxies scales with B-band luminosity as R/R_* = [L_B/L_{B_*}]^β with R_*=24-30 h^{-1} kpc and β= 0.26_{-0.06}^{+0.24} at N(HI)=10^{20} cm^{-2}; (2) the observed incidence of the DLAs versus the B-band luminosity of the DLA galaxies is consistent with models derived from adopting a known galaxy B-band luminosity function and the best-fit scaling relation of the neutral gas cross section at M_B - 5\log h <= -17; (3) comparison of the observed and predicted number density of DLAs supports that luminous galaxies can explain most of the DLAs found in QSO absorption line surveys and a large contribution of dwarfs (M_B - 5\log h >= -17) to the total neutral gas cross section is not necessary; (4) of the 11 DLAs studied, 45% are disk dominated, 22% are bulge dominated, 11% are irregular, and 22% are in galaxy groups, indicating that galaxies that give rise to the DLAs span a wide range of morphological types and arise in a variety of galaxy environment; (Abridged)

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arXiv2002

The Las Campanas Infrared Survey. IV. The Photometric Redshift Survey and the Rest-frame R-band Galaxy Luminosity Function at 0.5 <= z <= 1.5

(Abridged) We present rest-frame R-band galaxy luminosity function measurements for three different redshift ranges: 0.5<=z<=0.75, 0.75<=z<=1.0, and 1.0<=z<=1.5. Our measurements are based on photometric redshifts for ~3000 H-band selected galaxies with apparent magnitudes 17<=H<=20 from the Las Campanas Infrared Survey. We show that our photometric redshifts are accurate with an RMS dispersion between the photometric and spectroscopic redshifts of σ_z/(1+z)~0.08. Using galaxies identified in the Hubble Deep Field South and Chandra Deep Field South regions, we find, respectively, that (7.3\pm 0.2) % and (16.7\pm 0.4)% of the H<=20 galaxies are at z>=1. We first demonstrate that the systematic uncertainty inherent in the luminosity function measurements due to uncertainties in photometric redshifts is non-negligible and therefore must be accounted for. We then develop a technique to correct for this systematic error by incorporating the redshift error functions of individual galaxies in the luminosity function analysis. The redshift error functions account for the non-gaussian characteristics of photometric redshift uncertainties. They are the products of a convolution between the corresponding redshift likelihood functions of individual galaxies and a Gaussian distribution function that characterizes template-mismatch variance. We demonstrate, based on a Monte Carlo simulation, that we are able to completely recover the bright end of the intrinsic galaxy luminosity function using this technique. Finally, we calculate the luminosity function separately for the total H-band selected sample and for a sub-sample of early-type galaxies that have a best-fit spectral type of E/S0 or Sab from the photometric redshift analysis.

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arXiv2002

Galaxies Associated with z~4 Damped Lya Systems: I. Imaging and Photometric Selection

This paper describes the acquisition and analysis of imaging data for the identification of galaxies associated with z~4 damped Lya systems. We present deep BRI images of three fields known to contain four z~4 damped systems. We discuss the reduction and calibration of the data, detail the color criteria used to identify z~4 galaxies, and present a photometric redshift analysis to complement the color selection. We have found no galaxy candidates closer to the QSO than 7'' which could be responsible for the damped Lya systems. Assuming that at least one of the galaxies is not directly beneath the QSO, we set an upper limit on this damped Lya system of L < L*/4. Finally, we have established a web site to release these imaging data to the public.

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arXiv2001

The Las Campanas Infrared Survey. III. The H-band Imaging Survey and the Near-Infrared and Optical Photometric Catalogs

(Abridged) The Las Campanas Infrared Survey, based on broad-band optical and near-infrared photometry, is designed to robustly identify a statistically significant and representative sample of evolved galaxies at redshifts z>1. We have completed an H-band imaging survey over 1.1 square degrees of sky in six separate fields. The average 5 sigma detection limit in a four arcsecond diameter aperture is H ~ 20. Here we describe the design of the survey, the observation strategies, data reduction techniques, and object identification procedures. We present sample near-infrared and optical photometric catalogs for objects identified in two survey fields. We perform object detection in all bandpasses and identify ~ 54,000 galaxies over 1,408 square arcminutes of sky in the two fields. Of these galaxies, ~ 14,000 are detected in the H-band and ~ 2,000 have the colors of evolved galaxies, I - H >3, at z > 1. We find that (1) the differential number counts N(m) for the H-band detected objects has a slope of 0.44 at H < 19 and 0.27 for H > 19. In addition, we find that (2) the differential number counts for the H detected red objects has a slope of 0.85 at H < 20 and 0.32 at H > 20, with a mean surface density ~ 3,000 degree^{-2} mag^{-1} at H=20. Finally, we find that (3) galaxies with red optical to near-IR colors (I-H > 3) constitute ~ 20% of the H detected galaxies at H ~ 21, but only 2% at H = 19. We show that red galaxies are strongly clustered, which results in a strong field to field variation in their surface density. Comparisons of observations and predictions based on various formation scenarios indicate that these red galaxies are consistent with mildly evolving early-type galaxies at z ~ 1, although with a significant amount of on-going star formation as indicated by the large scatter in their V-I colors.

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arXiv2001

The Star Formation Rate Intensity Distribution Function--Implications for the Cosmic Star Formation Rate History of the Universe

We address the effects of cosmological surface brightness dimming on observations of faint galaxies by examining the distribution of "unobscured" star formation rate intensities versus redshift. We use the star formation rate intensity distribution function to assess the ultraviolet luminosity density versus redshift, based on our photometry and photometric redshift measurements of faint galaxies in the HDF and the HDF--S WFPC2 and NICMOS fields. We find that (1) previous measurements have missed a dominant fraction of the ultraviolet luminosity density of the universe at high redshifts by neglecting cosmological surface brightness dimming effects, which are important at redshifts larger than z = 2, (2) the incidence of the highest intensity star forming regions increases monotonically with redshift, and (3) the ultraviolet luminosity density plausibly increases monotonically with redshift through the highest redshifts observed. By measuring the spectrum of the luminosity density versus redshift, we also find that (4) previous measurements of the ultraviolet luminosity density at redshifts z < 2 must be reduced by a factor 2 to allow for the spectrum of the luminosity density between rest-frame wavelengths 1500 and 2800 A. And by comparing with observations of high-redshift damped Lyman-alpha absorption systems detected toward background QSOs, we further find that (5) the distribution of star formation rate intensities matches the distribution of neutral hydrogen column densities at redshifts z = 2 through 5, which establishes a quantitative connection between high-redshift galaxies and high column density gas and suggests that high-redshift damped Lyman-alpha absorption systems trace lower star formation rate intensity regions of the same galaxies detected in star light in the HDF and HDF--S.

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arXiv2001

Properties of a Chemically Enriched Lya Absorption System at z=0.167 and Implications for the Total Baryons in Extended Gaseous Envelopes Around Galaxies

We present a detailed analysis of the chemical abundances, ionization state, and origin of a partial Lyman limit system (N(HI) ~ 10^{16} cm^{-2}) at low redshift (z=0.167 towards PKS0405-1219). We also present the first estimate of the total baryons in extended gaseous envelopes around galaxies based on a simple photoionization model for the Lya absorbers. We find that extended gaseous envelopes can explain between 5--20% of the total baryons predicted by the big bang nucleosynthesis model.

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arXiv2001

PMN J0134-0931: A gravitationally lensed quasar with an unusual radio morphology

The radio-loud quasar PMN J0134-0931 was discovered to have an unusual morphology during our search for gravitational lenses. In VLA and MERLIN images, there are 5 compact components with maximum separation 681 millarcseconds. All of these components have the same spectral index from 5 GHz to 43 GHz. In a VLBA image at 1.7 GHz, a curved arc of extended emission joins two of the components in a manner suggestive of gravitational lensing. At least two of the radio components have near-infrared counterparts. We argue that this evidence implies that J0134-0931 is a gravitational lens, although we have not been able to devise a plausible model for the foreground gravitational potential. Like several other radio-loud lenses, the background source has an extraordinarily red optical counterpart.

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arXiv2001

Discovery of the Galaxy Proximity Effect and Implications for Measurements of the Ionizing Background Radiation at Low Redshifts

We present an analysis of galaxy and QSO absorption line pairs toward 24 QSOs at redshifts between z~0.2 and 1 in an effort to establish the relationship between galaxies and absorption lines in physical proximity to QSOs. We demonstrate the existence of a galaxy proximity effect, in that galaxies in the vicinities of QSOs do not show the same incidence and extent of gaseous envelopes as galaxies far from QSOs. We show that the galaxy proximity effect exists to galaxy-QSO velocity separations of ~ 3000 km/s, much larger than the size of a typical cluster (~ 1000 km/s), i.e. it is more comparable to the scale of the sphere of influence of QSO ionizing radiation rather than the scale of galaxy-QSO clustering. This indicates that the QSO ionizing radiation rather than some dynamical effect from the cluster environment is responsible for the galaxy proximity effect. We combine previous findings that (1) many or most Lya absorption lines arise in extended galaxy envelopes, and (2) galaxies cluster around QSOs to show that the magnitude of the Lya forest proximity effect is underestimated. Consequently, determinations of the UV ionizing background intensity using the proximity effect are likely overestimated. We use the galaxy-QSO cross-correlation function measured from our data to estimate the magnitude of this overestimate and find that it could be as high as a factor of 20 at z<1. This can have strong implications for models of the origin and evolution of the ionizing background, and may indicate that QSOs produce sufficient ionizing flux at all redshifts to account for the entire background radiation field.

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arXiv2001

The Gaseous Extent of Galaxies and the Origin of Lya Absorption Systems. V. Optical and Near-Infrared Photometry of Lya-absorbing Galaxies at z < 1

(Abridged) We present results of a program to obtain and analyze HST WFPC2 images and ground-based images of galaxies identified in an imaging and spectroscopic survey of faint galaxies in fields of HST spectroscopic target QSOs. We confirm and improve the results presented by Lanzetta et al. (1995) and Chen et al. (1998) that (1) extended gaseous envelopes are a common and generic feature of galaxies of a wide range of luminosity and morphological type, (2) the extent of tenuous gas (N(H I) > 10^{14} cm^{-2} around galaxies scales with galaxy B-band luminosity as r\propto L_B^{0.39+/-0.09}, and (3) galaxy interactions do not play an important role in distributing tenuous gas around galaxies in most cases. We further demonstrate that (4) the gaseous extent of galaxies scales with galaxy K-band luminosity as r\propto L_K^{0.28+/-0.08}, and (5) tenuous gas around typical L_* galaxies is likely to be distributed in spherical halos of radius ~ 180 h^{-1} kpc of covering factor of nearly unity. Because galaxies of all morphological types possess extended gaseous halos and because the extent of tenuous gas around galaxies scales with galaxy K-band luminosity, we argue that galaxy mass---rather than recent star-formation activity---is likely to be the dominant factor that determines the extent of tenuous gas around galaxies. Nevertheless, applying the scaling relationship between the extent of Lya absorbing gas around galaxies and galaxy B-band luminosity, the results of our analysis also suggest that the number density evolution of Lya absorption systems may serve to constrain the evolution of the comoving galaxy B-band luminosity density (at least for the redshift interval between z~0 and z~1 that has been studied in our survey).

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arXiv2001

The Origin of C IV Absorption Systems at Redshifts z<1---Discovery of Extended C IV Envelopes Around Galaxies

(Abridged) We report the discovery of extended CIV gaseous envelopes around galaxies of a wide range of luminosity and morphological type. First, we show that CIV absorption systems are strongly clustered around galaxies on velocity scales of v < 250 km/s and impact parameter scales of rho < 100 h^{-1} kpc but not on larger velocity or impact parameter scales. Next, adopting measurements of galaxy properties presented in previous papers, we examine how properties of the CIV absorption systems depend on properties of the galaxies. On the basis of 14 galaxy and absorber pairs and 36 galaxies that do not produce corresponding CIV absorption lines to within sensitive upper limits, we find that: (1) Galaxies of a range of morphological type and luminosity appear to possess extended CIV gaseous envelopes of radius R ~ 100 h^{-1} kpc, with abrupt boundaries between the CIV absorbing and non-absorbing regions. (2) The extent of CIV-absorbing gas around galaxies scales with galaxy B-band luminosity as R \propto L_B^{0.5 +/- 0.1} but does not depend strongly on galaxy surface brightness, redshift, or morphological type. And (3) the covering factor of CIV clouds within ~ 100 h^{-1} kpc of galaxies is nearly unity, but there is a large scatter in the mean number of clouds encountered along the line of sight. The most significant implication of the study is that galaxies of a wide range of luminosity and morphological type are surrounded by chemically enriched gas that extends for at least ~ 100 h^{-1} kpc. We consider various scenarios that may have produced metals at large galactic distance and conclude that accreting satellites are most likely to be responsible for chemically enriched gas at large galactic distances to regular looking galaxies.

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arXiv2001

On the compared accuracy and reliability of spectroscopic and photometric redshift measurements

We present a comparison between the spectroscopic catalog of the HDF recently published by Cohen et al and the redshifts measured by our group for those objects using photometric techniques. We perform this comparison in order to characterize the errors associated to the photometric redshift technique. The spectroscopic sample includes over 140 objects in the HDF proper, representing the deepest, cleanest, most complete spectroscopic catalog ever compiled. We study each object for which our redshift and the one measured by Cohen et al disagree. In most cases the photometric evidence is strong enough to call for a careful review of the spectroscopic values, as they seem to be in error. It is possible to characterize the systematic errors associated to our technique, which when combined with the photometric errors allow us to obtain complete information on the redshift of each galaxy and its associated confidence interval, regardless of apparent magnitude. One of the main conclusions of this study is that, to date, all the redshifts from our published catalog that have been checked have been shown to be correct (within the stated confidence limits). This implies that our galaxy template set is a fair representation of the galaxy population at all redshifts (0<z<6) and magnitudes (R<24) explored to date. On the other hand, spectroscopy of faint sources is subject to unknown and uncharacterized systematic errors, that will in turn be transmitted to any photometric redshift technique which uses spectroscopic samples as calibration. Our analysis proves that photometric redshift techniques can and must be used to extend the range of applicability of the spectroscopic redshift measurements. (Abridged)

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arXiv2000

The Unusual Spectral Energy Distribution of a Galaxy Previously Reported to be at Redshift 6.68

Observations of distant galaxies are important both for understanding how galaxies form and for probing the physical conditions of the universe at the earliest epochs. It is, however, extremely difficult to identify galaxies at redshift z>5, because these galaxies are faint and exhibit few spectral features. In a previous work, we presented observations that supported the identification of a galaxy at redshift z = 6.68 in a deep STIS field. Here we present new ground-based photometry of the galaxy. We find that the galaxy exhibits moderate detections of flux in the optical B and V images that are inconsistent with the expected absence of flux at wavelength shortward of the redshifted Lyman-alpha emission line of a galaxy at redshift z>5. In addition, the new broad-band imaging data not only show flux measurements of this galaxy that are incompatible with the previous STIS measurement, but also suggest a peculiar spectral energy distribution that cannot be fit with any galaxy spectral template at any redshift. We therefore conclude that the redshift identification of this galaxy remains undetermined.

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