arXiv subjects

Jason X. Prochaska

Publications and source records attributed to Jason X. Prochaska.

At least 109 records · Page 6Linked to original sources

arXiv2001

The UCSD HIRES/KeckI Damped Lya Abundance Database: I. The Data

We present new chemical abundance measurements of 16 damped Lya systems at z>1.5 and update our previous abundance analyses. The entire database presented here was derived from HIRES observations on the Keck I telescope, reduced with the same software package, and analysed with identical techniques. Altogether, we present a large, homogeneous database of chemical abundance measurements for protogalaxies in the early universe, ideal for studying a number of important aspects of galaxy formation. In addition, we have established an online directory for this database and will continuously update the results.

astro-ph↗

arXiv2001

The UCSD HIRES/KeckI Damped Lya Abundance Database: II. The Implications

We present a comprehensive analysis of the damped Lya abundance database presented in the first paper of this series. This database provides a homogeneous set of abundance measurements for many elements including Si, Cr, Ni, Zn, Fe, Al, S, Co, O, and Ar from 38 damped Lya systems with z > 1.5. With little exception, these damped Llya systems exhibit very similar relative abundances. There is no significant correlation in X/Fe with [Fe/H] metallicity and the dispersion in X/Fe is small at all metallicity. We search the database for trends indicative of dust depletion and in a few cases find strong evidence. Specifically, we identify a correlation between [Si/Ti] and [Zn/Fe] which is unambiguous evidence for depletion. We present a discussion on the nucleosynthetic history of the damped Lya systems by focusing on abundance patterns which are minimally affected by dust depletion. We find [Si/Fe] -> +0.25 dex as [Zn/Fe] -> 0 and that the [Si/Fe] values exhibit a plateau of ~+0.3 dex at [Si/H] < -1.5 dex. Together these trends indicate significant alpha-enrichment in the damped Lya systems at low metallicity, an interpretation further supported by the observed O/Fe, S/Fe and Ar/Fe ratios. We also discuss Fe-peak nucleosynthesis and the odd-even effect. To assess the impact of dust obscuration, we present estimates of the dust-to-gas ratios for the damped Lya sightlines and crudely calculate dust extinction corrections. The distribution of extinction corrections suggests the effects of dust obscuration are minimal and that the population of 'missing' damped systems has physical characteristics similar to the observed sample. We update our investigation on the chemical evolution of the early universe in neutral gas. [significantly abridged]

astro-ph↗

arXiv2001

Are Simulations of CDM Consistent with Galactic-Scale Observations at High Redshift?

We compare new observations on the kinematic characteristics of the damped Lya systems against results from numerical SPH simulations to test the predictions of hierarchical galaxy formation. This exercise is particularly motivated by recent numerical results on the cross-section of damped Lya systems. Our analysis focuses on the velocity widths of ~50 low-ion absorption profiles from our sample of z>1.5 damped Lya systems. The results indicate that current numerical simulations fail to match the damped Lya observations at high confidence levels (>99.9%). Although we do not believe that our results present an insurmountable challenge to the paradigm of hierarchical cosmology, the damped Lya observations suggest that current numerical SPH simulations overlook an integral aspect of galaxy formation.

astro-ph↗

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.

astro-ph↗

arXiv2001

First Investigation of the Clustering Environment of Damped Lyman Alpha Absorbers at z=4

We report the first observations of the clustering environment of damped Lyman alpha absorption systems at z=4. Color selection and photometric redshifts were used to select 44 candidate Lyman-break galaxies brighter than I_AB=25.5 from deep BRI images of the 35 sq. arcmin field containing the quasar BR 0951-04. Multislit spectroscopy of 35 candidate galaxies was performed and 8 of these candidates have been confirmed as z>3.5 Lyman-break galaxies. With only BRI photometry, the photometric redshifts are quite accurate for the spectroscopically confirmed galaxies but have a high rate of misclassification due to color degeneracies between Lyman-break galaxies and low-redshift ellipticals. Both of the z>3.5 galaxies found within 15'' of the quasar line-of-sight appear to be causing absorption systems in the quasar spectrum. We use a battery of statistical tests to look for clustering in the redshift histogram of the z>3.5 galaxies but do not find measurable clustering of these Lyman-break galaxies with the damped Lyman alpha absorbers. With a larger sample of galaxies, our method should determine the cross-correlation between these objects, which probes the bias and hence the mass of the damped Lyman alpha absorbers.

astro-ph↗

arXiv2001

The First Detection of Co in a Damped Lyman Alpha System

The study of elemental abundances in Damped Lyman Alpha systems (DLAs) at high redshift represents one of our best opportunities to probe galaxy formation and chemical evolution at early times. By coupling measurements made in high z DLAs with our knowledge of abundances determined locally and with nucleosynthetic models, we can start to piece together the star formation histories of these galaxies. Here, we discuss the clues to galactic chemical evolution that may be gleaned from studying the abundance of Co in DLAs. We present high resolution echelle spectra of two QSOs, Q2206-199 and Q1223+17, both already known to exhibit intervening damped systems. These observations have resulted in the first ever detection of Co at high redshift, associated with the z= 1.92 DLA in the sightline towards Q2206-199. We find that the abundance of Co is approximately 1/4 solar and that there is a clear overabundance relative to iron, [Co/Fe] = +0.31 +/- 0.05. From the abundance of Zn, we determine that this is a relatively metal-rich DLA, with a metallicity approximately 1/3 solar. Therefore, this first detection of Co is similar to the marked overabundance relative to Fe seen in Galactic bulge and thick disk stars.

astro-ph↗

arXiv2001

The First Detection of Cobalt in a Damped Lyman Alpha System

We present the first ever detection of Cobalt in a Damped Lyman Alpha system (DLA) at z = 1.92. In addition to providing important clues to the star formation history of these high redshift galaxies, we discuss how studying the Co abundance in DLAs may also help to constrain models of stellar nucleosynthesis in a regime not probed by Galactic stars.

astro-ph↗

arXiv2001

The Deuterium to Hydrogen Abundance Ratio Towards a Fourth QSO: HS0105+1619

We report the measurement of the primordial D/H abundance ratio towards QSO \object. The column density of the hydrogen in the $z \simeq 2.536$ Lyman limit system is high, \lnhi $= 19.422 \pm 0.009$ \cmm, allowing for the deuterium to be seen in 5 Lyman series transitions. The measured value of the D/H ratio towards QSO \object is found to be D/H$ = 2.54 \pm 0.23 \times 10^{-5}$. The metallicity of the system showing D/H is found to be $\simeq 0.01$ solar, indicating that the measured D/H is the primordial D/H within the measurement errors. The gas which shows D/H is neutral, unlike previous D/H systems which were more highly ionized. Thus, the determination of the D/H ratio becomes more secure since we are measuring it in different astrophysical environments, but the error is larger because we now see more dispersion between measurements. Combined with prior measurements of D/H, the best D/H ratio is now D/H$ = 3.0 \pm 0.4 \times 10^{-5}$, which is 10% lower than the previous value. The new values for the baryon to photon ratio, and baryonic matter density derived from D/H are $η= 5.6 \pm 0.5 \times 10^{-10} $ and \ob $=0.0205 \pm 0.0018$ respectively.

astro-ph↗

arXiv2001

Galactic Chemical Abundances at z>3 I: First Results from the Echellette Spectrograph and Imager

We present the first results from an ongoing survey to discover and measure the metallicity of z>3 damped Lya systems with the Echellette Spectrograph and Imager (ESI) on the Keck II telescope. Our motivation arises from a recent study on the damped Lya systems suggesting only mild evolution in the cosmic metallicity from z~2 to 4. The Echellette Spectrograph and Imager, which provides two complementary spectroscopic modes, is the ideal instrument for a z>3 damped Lya survey. We describe our observing strategy and report on the discovery and analysis of 5 new z>3 damped Lya systems acquired in a single night of observing. These observations further support the principal conclusions of the previous study: (1) the cosmic metallicity in neutral gas inferred from the damped Lya systems does not evolve significantly from z~2 to 4; (2) the unweighted metallicity exhibits a statistically significant decrease with increasing redshift; and (3) not a single damped Lya system has a metallicity below [Fe/H]=-3. We discuss the implications of these results and comment on recent theoretical studies which attempt to explain the observations.

astro-ph↗

arXiv2000

Ionized Gas in Damped Lyman Alpha Protogalaxies: II. Comparison Between Models and the Kinematic Data

We test semi-analytic models for galaxy formation with accurate kinematic data of damped Lyman alpha protogalaxies (DLAs) presented in the companion paper I. The models envisage centrifugally supported exponential disks at the centers of dark matter halos which are filled with ionized gas undergoing radial infall to the disks. The halo masses are drawn from cross-section weighted mass distributions predicted by CDM cosmogonies, or by the null hypothesis (TF model) that the dark matter mass distribution has not evolved since z ~ 3. In our models, C IV absorption lines detected in DLAs arise in infalling ionized clouds while the low-ion absorption lines arise from neutral gas in the disks. Using Monte Carlo methods we find: (a) The CDM models are incompatible with the low-ion statistics at more than 99% confidence whereas some TF models cannot be excluded at more than 88% confidence. (b) Both CDM and TF models agree with the observed distribution of C IV velocity widths. (c) The CDM models generate differences between the mean velocities of C IV and low ion profiles in agreement with the data, while the TF model produces differences in the means that are too large. (d) Both CDM and TF models produce ratios of C IV to low-ion velocity widths that are too large. (e) Both CDM and TF models generate C IV versus low-ion cross-correlation functions incompatible with the data. While it is possible to select model parameters resulting in consistency with the data, the disk-halo configuration assumed in both cosmogonies still does not produce significant overlap in velocity space between C IV low-ion velocity profiles. We conjecture that including angular momentum of the infalling clouds will increase the overlap between C IV and low-ion profiles.

astro-ph↗

arXiv2000

Ionized Gas in Damped Lyman Alpha Protogalaxies: I. Model-Independent Inferences From Kinematic Data

We investigate the kinematics of ionized and neutral gas in a sample of 35 damped Lyman alpha protogalaxies (DLAs) with HIRES on the Keck I 10 m telescope. Velocity profiles with resolution of ~ 8 km/s are obtained for high ions such as C IV and Si IV, and for intermediate ions such as Al III. Combining these profiles with accurate low-ion (e.g., Fe II) profiles, we investigate the kinematic state of DLAs in the redshift range 1.8 < z < 4.4 by comparisons between data for various ion pairs. We find the DLAs comprise distinct kinematic subsystems: a low ion subsystem in which the low ions are physically associated with intermediate ions, and a high subsystem containing neither low nor intermediate ions. The evidence for two subsystems stems from (a) differences between the widths of the velocity profiles, (b) misalignment in velocity space between the narrow components comprising the profiles in each subsystem, and (c) significant dissimilarities between the mean velocities of the high ion and low ion velocity profiles. In every case we find that test statistics such as velocity width and various asymmetry parameters distribute differently for low ions than for high ions. Despite misalignment in velocity space, the low and high ion kinematic subsystems are interrelated. This is indicated by detection of a statistically significant C IV versus low-ion cross correlation function, and by a systematic effect where the C IV velocity widths are greater than or equal to the low ion velocity widths in 29 out of 32 systems. These phenomena are consistent with the location of the two subsystems in the same potential well.

astro-ph↗

arXiv2000

The Origin of a Chemically Enriched lya Absorption System at z=0.167

We present the first detailed analysis of the chemical abundances, ionization state, and origin of a partial Lyman limit system (N(\hI) ~ 10^{16} \cmjj) at low redshift (z=0.167 towards PKS0405-1219). Two galaxies at ~ 70 h^{-1} kpc projected distance to the QSO have been identified at the absorber redshift. We analyze an echelle spectrum of the QSO obtained with the Space Telescope Imaging Spectrograph and find that this absorption system exhibits associated lines produced by C^+, N^+, O^0, Si^+, Si^{++}, Si^{+3}, Fe^+, and Fe^{++}, and most interestingly, O^{+5} and N^{+4}. The results of our analysis show that the partial Lyman limit system traced by various ions is likely to be embedded in a collisionally ionized \ovi gas of larger spatial extent. Furthermore, the partial Lyman limit system appears to have a metallicity of {\em at least} 1/10 solar and most likely solar or super solar despite the fact that no luminous galaxies are seen within a projected distance ρ= 60 h^{-1} kpc to the QSO. Finally, adopting the temperature estimated for the hot gas T ~ 2.5\times 10^5 K and assuming a simple isothermal halo, we estimate that the galaxy or galaxy group that supports the extended gas may have a total mass ~ 1.5\times 10^{12} M_\odot and a gas number density \apl 3\times 10^{-5} cm^{-3}.

astro-ph↗

arXiv2000

The Galactic Thick Disk Stellar Abundances

We present first results from a program to measure the chemical abundances of a large (N>30) sample of thick disk stars with the principal goal of investigating the formation history of the Galactic thick disk. Our analysis confirms previous studies of O and Mg in the thick disk stars which reported enhancements in excess of the thin disk population. Furthermore, the observations of Si, Ca, Ti, Mn, Co, V, Zn, Al, and Eu all argue that the thick disk population has a distinct chemical history from the thin disk. With the exception of V and Co, the thick disk abundance patterns match or tend towards the values observed for halo stars with [Fe/H]~-1. This suggests that the thick disk stars had a chemical enrichment history similar to the metal-rich halo stars. With the possible exception of Si, the thick disk abundance patterns are in excellent agreement with the chemical abundances observed in the metal-poor bulge stars suggesting the two populations formed from the same gas reservoir at a common epoch. We discuss the implications for the formation of the thick disk, new insights presented for theories of nucleosynthesis, and the impact on interpretations of the abundance patterns of the damped Lya systems at high redshift. [significantly abridged]

astro-ph↗

arXiv2000

On the Perils of Hyperfine Splitting: A reanalysis of Mn and Sc Abundance Trends

We investigate the impact of hyperfine splitting on stellar abundance analyses of Mn and Sc, and find that incorrect hfs treatment can lead to spurious abundance trends with metallicity. We estimate corrections to a recent study by Nissen et al. (2000), and find: (1) [Mn/Fe] is described by a bimodal distribution, with [Mn/Fe] ~ -0.3 for stars [Fe/H] < -0.7, and [Mn/Fe] ~ -0.05 for stars at higher metallicity, suggestive of a transition between halo/thick disk and thin disk populations. (2) The large majority of stars show nearly solar [Sc/Fe] ratios; although important deviations cannot be ruled-out.

astro-ph↗

arXiv2000

Metallicity Evolution in the Early Universe

Observations of the damped Lya systems provide direct measurements on the chemical enrichment history of neutral gas in the early universe. In this Letter, we present new measurements for four damped Lya systems at high redshift. Combining these data with [Fe/H] values culled from the literature, we investigate the metallicity evolution of the universe from z~1.5-4.5. Contrary to our expectations and the predictions of essentially every chemical evolution model, the N(HI)-weighted mean [Fe/H] metallicity exhibits minimal evolution over this epoch. For the individual systems, we report tentative evidence for an evolution in the unweighted [Fe/H] mean and the scatter in [Fe/H] with the higher redshift systems showing lower scatter and lower typical [Fe/H] values. We also note that no damped Lya system has [Fe/H] < -2.7 dex. Finally, we discuss the potential impact of small number statistics and dust on our conclusions and consider the implications of these results on chemical evolution in the early universe.

astro-ph↗

arXiv2000

What Damped Ly-alpha Systems Tell Us About the Radial Distribution of Cold Gas at High Redshift

We investigate the properties of damped Lyman-alpha systems (DLAS) in semi-analytic models, focusing on whether the models can reproduce the kinematic properties of low-ionization metal lines described by Prochaska & Wolfe (1997b, 1998). We explore a variety of approaches for modelling the radial distribution of the cold neutral gas associated with the galaxies in our models, and find that our results are very sensitive to this ingredient. If we use an approach based on Fall & Efstathiou (1980), in which the sizes of the discs are determined by conservation of angular momentum, we find that the majority of the DLAS correspond to a single galactic disc. These models generically fail to reproduce the observed distribution of velocity widths. In alternative models in which the gas discs are considerably more extended, a significant fraction of DLAS arise from lines of sight intersecting multiple gas discs in a common halo. These models produce kinematics that fit the observational data, and also seem to agree well with the results of recent hydrodynamical simulations. Thus we conclude that Cold Dark Matter based models of galaxy formation can be reconciled with the kinematic data, but only at the expense of the standard assumption that DLAS are produced by rotationally supported gas discs whose sizes are determined by conservation of angular momentum. We suggest that the distribution of cold gas at high redshift may be dominated by another process, such as tidal streaming due to mergers.

astro-ph↗

arXiv2000

Optical Counterparts to Damped Lyman Alpha Systems

Previously we have shown (Maller et al, 1998) that the kinematics of Damped Lyman Alpha Systems (DLAS) as measured by Prochaska and Wolfe (1998) can be reproduced in a multiple disk model (MDM) if the gaseous disks are of sufficient radial extent. Here we discuss this model's predictions for the relationship between DLAS and Lyman break galaxies (LBGs), which we here take to be objects at z~3 brighter than R=25.5. We expect that future observations of the correlations between DLAS and LBGs will provide a new data set able to discriminate between different theoretical models of the DLAS. Djorgovski (1997) has already detected a few optical counterparts and more studies are underway.

astro-ph↗

arXiv2000

Damped Ly-alpha Systems in Semi-Analytic Models: Sensitivity to dynamics, disk properties, and cosmology

Previously we have shown that it is possible to account for the kinematic properties of damped Lyman alpha systems (DLAS) in the context of semi-analytic models. In these models, hierarchical structure formation is approximated by constructing a merger tree for each dark matter halo. A natural consequence is that every virialized halo may contain not only a central galaxy, but also a number of satellite galaxies as determined by its merging history. Thus the kinematics of the DLAS arise from the combined effects of the internal rotation of gas disks and the motions between gas disks within a common halo. Here we investigate the sensitivity of this model to some of the assumptions made previously, including the modeling of satellite dynamics, the scale height of the gas, and the cosmology.

astro-ph↗