arXiv subjects

Jason X. Prochaska

Publications and source records attributed to Jason X. Prochaska.

118 records · Page 7Linked to original sources

arXiv1998

Making Damped Lyman Alpha Systems in Semi-Analytic Models

The velocity profiles of weak metal absorption lines can be used to observationally probe the kinematic state of gas in damped Lyman-alpha systems. Prochaska and Wolfe have argued that the flat distribution of velocity widths combined with the asymmetric line profiles indicate that the DLAS are disks with large rotation velocities. An alternative explanation has been proposed by Haehnelt, Steinmetz, and Rauch, in which the observed large velocity widths and asymmetric profiles can be produced by lines of sight passing through two or more clumps each having relatively small internal velocity dispersions. We investigate the plausibility of this scenario in the context of semi-analytic models based on hierarchical merging trees and including simple treatments of gas dynamics, star formation, supernova feedback, and chemical evolution. We find that all the observed properties of the metal-line systems including the distribution of velocities and the asymmetric profiles, can be reproduced by lines of sight passing through sub-clumps that are bound within larger virialized dark matter halos. In order to produce enough multiple hits, we find that the cold gas must be considerably more extended than the optical radius of the proto-galaxies, perhaps even beyond the tidal radius of the sub-halo. This could occur due to tidal stripping or supernova-driven outflows.

astro-ph↗

arXiv1998

Investigating the Metal Line Systems at Z=1.9 toward J2233-606 in the HDF-S

The combination of STIS and optical spectroscopy with STIS imaging of the field surrounding J2233-606 afford a unique opportunity to study the physical nature of the Quasar Absorption Line systems. We present an analysis of the ionization state, chemical abundances, and kinematic characteristics of two metal-line systems at z=1.92 and z=1.94 toward J2233-606. We focus on these two systems because (i) the observations provide full coverage of the Lyman series, hence an accurate determination of their HI column densities and (ii) the metal-line transitions allow a measurement of their ionization state. Line-profile fits of the Lyman series for the two systems indicate log N(HI) = 17.15 \pm 0.02 cm^-2 evenly distributed between two components for the z=1.92 system and log N(HI) = 16.33 \pm 0.04 cm^-2 for the z=1.94 system. By comparing observed ionic ratios of C and Si against calculations performed with the CLOUDY software package, we find the ionization state is high and well constrained in both systems. Applying ionization corrections to the measured ionic column densities, we determine that the systems exhibit significantly different metallicities: ~ 1/50 solar and < 1/200 solar for the two components of z=1.92 and ~40% solar at z=1.94. The properties of the z=1.92 system are consistent with a low metallicity galaxy (e.g. a dwarf galaxy) as well as absorption by large scale structure of the IGM. On the other hand, the high metallicity of the z=1.94 system suggests significant star formation and is therefore most likely associated with a galactic system. Furthermore, we predict it is the most likely to be observed with the STIS imaging and follow-up observations. If a galaxy is identified, our results provide direct measurements on the properties of the ISM for a z~2 galaxy.

astro-ph↗

arXiv1998

The Physical Nature of the Lyman Limit Systems

We analyze Keck HIRES observations of a Lyman limit system at z=2.652 toward Q2231-00. These observations afford the most comprehensive study of the physical properties of a LL system to date. By comparing the ionic column densities for Fe^+, Fe^{++}, Si^+, and Si^{3+} against calculations derived from the CLOUDY software package, we have strictly constrained the ionization state of this system. This has enabled us to calculate accurate abundances of a Lyman limit system for the first time at z > 2, e.g., [Fe/H] = -0.5 \pm 0.1. We also derive a total hydrogen column density, log N(H) = 20.73 \pm 0.2, which is comparable to values observed for the damped lya systems. The system is special for exhibiting CII* 1335 absorption, allowing an estimate of the electron density, n_e = 6.5 \pm 1.3 x 10^{-2} cm^{-3}. Coupling this measurement with our knowledge of the ionization state, we derive the following physical properties: (1) hydrogen volume density, n_H = 5.9 \pm 1.2 x 10^{-2} cm^{-3}, (2) path length, l = 3 \pm 1.6 kpc, and (3) ionizing intensity, log J_{912} = -20.22 \pm 0.21. We point out that a number of the physical properties (e.g. [Fe/H], N(H), n_H) resemble those observed for the damped lya systems, which suggests this system may be the photoionized analog of a damped system. The techniques introduced in this Letter should be applicable to a number of Lyman limit systems and therefore enable a survey of their chemical abundances and other physical properties.

astro-ph↗

arXiv1998

Chemical Abundances of the Damped Lya Systems at z>1.5

We present chemical abundance measurements for 19 damped lya systems observed with HIRES on the 10m W.M. Keck Telescope. Our principal goal is to investigate the abundance patterns of the damped systems and thereby determine the underlying physical processes which dominate their chemical evolution. We place particular emphasis on gauging the relative importance of two complementary effects often invoked to explain the damped lya abundances: (1) nucleosynthetic enrichment from Type II supernovae and (2) an ISM-like dust depletion pattern. Similar to the principal results of Lu et al. (1996), our observations lend support both for dust depletion and Type II SN enrichment. Specifically, the observed overabundance of Zn/Fe and underabundance of Ni/Fe relative to solar abundances suggest significant dust depletion within the damped lya systems. Meanwhile, the relative abundances of Al, Si, and Cr vs. Fe are consistent with both dust depletion and Type II supernova enrichment. Our measurements of Ti/Fe and the Mn/Fe measurements from Lu et al. (1996), however, cannot be explained by dust depletion and indicate an underlying Type II SN pattern. Finally, the observed values of [S/Fe] are inconsistent with the combined effects of dust depletion and the nucleosynthetic yields expected for Type II supernovae. This last result emphasizes the need for another physical process to explain the damped lya abundance patterns. We also examine the metallicity of the damped lya systems both with respect to Zn/H and Fe/H. Our results confirm previous surveys by Pettini and collaborators, i.e., [ ] = -1.15 +/- 0.15 dex. [abridged]

astro-ph↗

arXiv1998

Protogalactic Disk Models of Damped Lya Kinematics

We present new observational results on the kinematics of the damped lya systems. Our full sample is now comprised of 31 low-ion profiles and exhibits similar characteristics to the sample from Paper I. The primary exception is that the new distribution of velocity widths includes values out to a maximum of nearly 300 km/s, approx 100 km/s greater than the previous maximum. These high velocity width systems will significantly leverage models introduced to explain the damped lya systems. Comparing the characteristics from low-redshift and high-redshift sub-samples, we find no evidence for significant evolution in the kinematic properties of protogalaxies from z = 2.0 - 3.3. The new observations give greater statistical significance to the main conclusions of our first paper. In particular, those models inconsistent with the damped lya observations in Paper I are ruled out at even higher levels of confidence. At the same time, the observations are consistent with a population of rapidly rotating, thick disks (the TRD model) at high redshift. Buoyed by the success of the TRD model, we investigate it more closely by considering more realistic disk properties. Our goal is to demonstrate the statistical power of the damped lya observations by investigating the robustness of the TRD model. In particular, we study the effects of warping, realistic rotation curves, and photoionization on the kinematics of disks in the TRD model. The principal results are: (1) disk warping has only minimal effect on the kinematic results, primarily influencing the effective disk thickness, (2) the TRD model is robust to more realistic rotation curves; (3) the effects of photoionization require thicker disks to give consistent velocity width distributions. [abridged]

astro-ph↗

arXiv1997

On the Evolution of Damped Lyman Alpha Systems to Galactic Disks

The mean metallicity of the thick disk of the Galaxy is 0.5 dex higher than that of the damped Lyman alpha systems. This has been interpreted to argue that stars in the former do not arise out of gas in the latter. Using new metallicity and H I column-density data we show the metal-rich damped systems do contain sufficient baryons at the thick-disk metallicity to account for the stellar masses of thick disks. Comparing our kinematic data with the metallicities we show that damped Lyman alpha systems exhibiting the largest profile velocity widths span a narrow range of high metallicities, while systems with small velocity widths span a wider range of metallicities. This is naturally explained by passage of the damped Lyman alpha sightlines through rapidly rotating disks with negative radial gradients in metallicity. The systematically lower N(H I) of systems with high velocity widths indicates (a) the gaseous disks have centrally located holes, and (b) an apparent inconsistency with the protogalactic clump model for damped Lyman alpha systems. The higher metallicity of systems with low N(H I) further implies that stars rather than gas dominate the baryonic content of the most metal-rich damped systems.

astro-ph↗

arXiv1997

Damped Lyman Alpha Systems at High Redshift and Models of Protogalactic Disks

We employ observationally determined intrinsic velocity widths and column densities of damped Lyman-alpha systems at high redshift to investigate the distribution of baryons in protogalaxies within the context of a standard cold dark matter model. We proceed under the assumption that damped Lyman alpha systems represent a population of cold, rotationally supported, protogalactic disks and that the abundance of protogalactic halos is well approximated by a cold dark matter model with critical density and vanishing cosmological constant. Using conditional cross sections to observe a damped system with a given velocity width and column density, we compare observationally inferred velocity width and column density distributions to the corresponding theoretically determined distributions for a variety of disk parameters and CDM normalizations. In general, we find that the observations can not be reproduced by the models for most disk parameters and CDM normalizations. Whereas the column density distribution favors small disks with large neutral gas fraction, the velocity width distribution favors large and thick disks with small neutral gas fraction. The possible resolutions of this problem in the context of this CDM model may be: (1) an increased contribution of rapidly rotating disks within massive dark matter halos to damped Lyman-alpha absorption or (2) the abandoning of simple disk models within this CDM model for damped Lyman-alpha systems at high redshift. Here the first possibility may be achieved by supposing that damped Lya system formation only occurs in halos with fairly large circular velocities and the second possibility may result from a large contribution of mergers and double-disks to damped Lya absorption at high redshift.

astro-ph↗

arXiv1997

On the Kinematics of the Damped Lyman Alpha Protogalaxies

We present the first results of an ongoing program to investigate the kinematic characteristics of high redshift damped lya systems. Because damped lya systems are widely believed to be the progenitors of current massive galaxies, an analysis of their kinematic history allows a direct test of galaxy formation scenarios. We have collected a kinematically unbiased sample of 17 high S/N ratio, high resolution damped lya spectra taken with HIRES on the 10m W.M. Keck Telescope. Our study focuses on the unsaturated, low-ion transitions of these systems which reveal their kinematic traits. The profiles exhibit a nearly uniform distribution of velocity widths ranging from 20 - 200 km/s and a relatively high degree of asymmetry. In an attempt to explain these characteristics, we introduce several physical models, which have previously been attributed to damped lya systems, including rapidly rotating cold disks, slowly rotating hot disks, massive isothermal halos, and a hydrodynamic spherical accretion model. Using standard Monte Carlo techniques, we run sightlines through these model systems to derive simulated low-ion profiles. Comparing statistical measures of the simulated profiles with the observed profiles, we determine that the rapidly rotating cold disk model is the only tested model consistent with the data at high confidence levels. A Relative Likelihood Test of the rapidly rotating cold disk model indicates the disks must have large rotation speeds; v > 180 km/s at the 99% c.l. In turn, we demonstrate that the Cold Dark Matter Model, as developed by Kauffmann (1996), is inconsistent with the damped lya data at very high c.l. This is because the CDM Model does not predict a large enough fraction of rapidly rotating disks at z approx 2.5.

astro-ph↗

arXiv1996

A Keck HIRES Investigation of the Metal Abundances and Kinematics of Three Damped Lya Systems Toward Q2206-199

We present high resolution, high SNR spectra of the QSO Q2206-199 obtained with HIRES on the 10m W.M. Keck Telescope. Our analysis focuses on the two previously identified damped \lya systems found at $z=1.920$ and $z=2.076$. For each system, we measure accurate abundances. The $z=1.920$ system exhibits the highest metallicity we have measured for a damped \lya system. We report the first confident ($>5 σ$) detection of Ti in a QSO absorption line system. By contrast the $z=2.076$ system is the most metal poor we have analyzed, showing absorption features for only the strongest transitions. We find no positive evidence for the presence of dust in either system. The two damped systems exhibit significantly different kinematic characteristics, yet we contend the two systems are consistent with one physical description: that of a thick, rotating disk. We investigate a very strong Mg II system at $z=0.752$ which is very likely yet a third damped \lya system. The very weak Mn II and Ti II transitions have been positively measured and imply $\log \N{HI} > 19.0$. We analyze the abundance ratios [Mn/Fe] and [Ti/Fe] and their values are inconsistent with dust depletion, yet consistent with the abundance pattern detected for halo stars in the Galaxy (see Lu et al. 1996a). Finally, we identify a C IV system at $z=2.014$ that shows a very narrow feature in Si IV and C IV absorption. The corresponding $b$ values (5.5 \kms and 8.9 \kms for Si IV and C IV) for this component suggest a temperature of $4.7 \sci{4} \rm K$. Because collisional ionization can explain the observed abundances only for $T > 8 \sci{4} \rm K$, we contend these ions must have formed through a different physical process (e.g. photoionization).

astro-ph↗

arXiv1996

A Keck HIRES Investigation of the Metal Abundances and Kinematics of the z=2.46 Damped Lya System Toward Q0201+365

We present high resolution ($\approx 8$ \kms) spectra of the QSO Q0201+365 obtained with HIRES, the echelle spectrograph on the 10m W.M. Keck Telescope. Although we identify over $80\%$ of the absorption features and analyze several of the more complex metal-line systems, we focus our analysis on the damped \Lya system at $z=2.462$. Ionization simulations suggest the hydrogen in this system is significantly neutral and all of the observed metals are predominantly singly ionized. We measure accurate abundances for Fe, Cr, Si, Ni and place a lower limit on the abundance of Zn: [Fe/H] = $-0.830 \pm 0.051$, [Cr/H] = $-0.902 \pm 0.064$, [Si/H] = $-0.376 \pm 0.052$, [Ni/H] = $-1.002 \pm 0.054$ and [Zn/H] $> -0.562 \pm 0.064$. We give evidence suggesting the actual Zn abundance is [Zn/H] $\approx -0.262$, implying the highest metallicity observed at a redshift $z \geq 2$. The relative abundances of these elements remains constant over essentially the entire system ($\approx 150$ \kms in velocity space), suggesting it is well mixed. Furthermore, we use the lack of abundance variations to infer properties of the dust responsible for element depletion. Finally, we discuss the kinematic characteristics of this damped \Lya system, comparing and contrasting it with other systems. The low-ion line profiles span $\approx 200$ \kms in velocity space and have an asymmetric shape with the strongest feature on the red edge. These kinematic characteristics are consistent with a rotating disk model.

astro-ph↗