Arthur M. Wolfe

On the Kinematics of the Damped Lyman-α Protogalaxies

We present the first results of an ongoing program to investigate the kinematic properties of high-redshift damped Lyα systems. Because damped Lyα systems are widely believed to be the progenitors of current massive galaxies, an analysis of their kinematics allows a direct test of galaxy formation scenarios. Specifically, the kinematic history of protogalactic gas is a sensitive discriminator among competing theories of galaxy formation. We use the HIRES echelle spectrograph on the Keck 10 m telescope to obtain accurate, high-resolution spectra of 17 damped Lyα systems. We focus on unsaturated, low-ion transitions such as Si II 1808, since these accurately trace the velocity fields of the neutral gas dominating the baryonic content of the damped systems. The velocity profiles: (1) comprise multiple narrow components; (2) are asymmetric in that the component with strongest absorption tends to lie at one edge of the profile; and (3) exhibit a nearly uniform distribution of velocity widths between 20 and 200 km s-1. In order to explain these characteristics, we consider several physical models proposed to explain the damped Lyα phenomenon, 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 sight lines through these model systems to derive simulated low-ion profiles. We develop four test statistics that focus on the symmetry and velocity widths of the profiles to distinguish among the models. Comparing the distributions of test statistics from the simulated profiles with those calculated from the observed profiles, we determine that the models in which the damped Lyα gas is distributed in galactic halos and in spherically infalling gas, are ruled out at more than 99.9% confidence. A model in which dwarf galaxies are simulated by slowly rotating hot disks is ruled out at 97% confidence. More important, we demonstrate that the cold dark matter (CDM) model, as developed by Kauffmann (1996) is inconsistent with the damped Lyα data at more than the 99.9% confidence level. This is because the CDM model predicts the interception cross section of damped Lyα systems to be dominated by systems with rotation speeds too slow to be compatible with the data. This is an important result, because slow rotation speeds are generic traits of protogalaxies in most hierarchical cosmologies. We find that models with disks that rotate rapidly and are thick are the only tested models consistent with the data at high confidence levels. A relative likelihood ratio test indicates disks with rotation speeds, vrot 0.1 are ruled out with 99% confidence, where σcc is the velocity dispersion of the gas. We describe an independent test of the cold disk hypothesis. The test makes use of the redshift of emission lines sometimes detected in damped Lyα systems, as well as the absorption profiles. The test potentially distinguishes between damped systems that are (1) large rotating disks detected in absorption and emission, in which case a systematic relation exists between emission redshift and absorption velocity profile, and (2) emitting galaxies surrounded by satellite galaxies detected in damped Lyα absorption, in which case the relation between emission and absorption redshifts is random. Finally, we emphasize a dilemma stimulated by our findings. Specifically, while the kinematics of the damped Lyα systems strongly favor a cold disklike configuration, the low metallicities and Type II supernova abundance patterns of damped Lyα systems argue for a hot halo-like configuration. We speculate on how this dilemma might be resolved.

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. We have obtained high-resolution, high signal-to-noise spectra of 10 thick disk stars with the HIRES spectrograph on the 10 m Keck I telescope. 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 toward 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. The principal results of our analysis are as follows. (1) All 10 stars exhibit enhanced α/Fe ratios with O, Si, and Ca showing tentative trends of decreasing overabundances with increasing [Fe/H]. In contrast, the Mg and Ti enhancements are constant. (2) The light elements Na and Al are enhanced in these stars. (3) With the exception of Ni, Cr, and possibly Cu, the iron-peak elements show significant departures from the solar abundances. The stars are deficient in Mn, but overabundant in V, Co, Sc, and Zn. (4) The heavy elements Ba and Y are consistent with solar abundances, but Eu is significantly enhanced. If the trends of decreasing O, Si, and Ca with increasing [Fe/H] are explained by the onset of Type Ia SN, then the thick disk stars formed over the course of 1 Gyr. We argue that this formation time-scale would rule out most dissipational collapse scenarios for the formation of the thick disk. Models which consider the heating of an initial thin disk—either through gradual heating mechanisms or a sudden merger event—are favored. These observations provide new tests of theories of nucleosynthesis in the early universe. In particular, the enhancements of Sc, V, Co, and Zn may imply overproduction during an enhanced α-rich freeze out fueled by neutrino-driven winds. Meanwhile, the conflicting trends for Mg, Ti, Ca, Si, and O pose a difficult challenge to our current understanding of nucleosynthesis in Type Ia and Type II SN. The Ba/Eu ratios favor r-process dominated enrichment for the heavy elements, consistent with the ages (tage > 10 Gyr) expected for these stars. Finally, we discuss the impact of the thick disk abundances on interpretations of the abundance patterns of the damped Lyα systems. The observations of mildly enhanced Zn/Fe imply an interpretation for the damped systems which includes a dust depletion pattern on top of a Type II SN enrichment pattern. We also argue that the S/Zn ratio is not a good indicator of nucleosynthetic processes.

The Age-Metallicity Relation of the Universe in Neutral Gas: The First 100 Damped Ly? Systems

We discuss accurate metallicity measurements for 125 damped Lyα (DLA) systems at 0.5 < z < 5, including ≈50 new measurements from our recently published Echellette Spectrograph and Imager surveys. This data set is analyzed to determine the age-metallicity relation of neutral gas in the universe. Contrary to previous analyses, this sample shows statistically significant evolution in the cosmic mean metallicity. The best linear fit for metallicity evolution is -0.26 ± 0.07 dex per unit redshift. The DLA systems continue to maintain a floor in metallicity of ≈1/700 solar, independent of observational effects. This metallicity threshold limits the prevalence of primordial gas in high-redshift galaxies and stresses the correspondence between damped systems and star formation. Finally, we comment on an apparent missing metals problem: the mean metallicity of the damped systems is ≈10 times lower than the value expected from their inferred star formation history. This problem is evident in current theoretical treatments of galaxy formation and may indicate a serious flaw in our understanding of the interplay between star formation and metal enrichment.

The UCSD HIRES/Keck I Damped Lyα Abundance Database.* II. The Implications

We present a comprehensive analysis of the damped Lyα (DLA) 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 DLA systems with zabs > 1.5. With little exception, these DLA 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. Following Hou and colleagues, we present [X/Si] abundances against [Si/H] + log N(H ) and note trends of decreasing X/Si with increasing [Si/H] + log N(H ) which argue for dust depletion. Similarly, comparisons of [Si/Fe] and [Si/Cr] against [Si/H] indicate significant depletion at [Si/H] > -1 but suggest essentially dust-free damped systems at [Si/H] 3.5, where small number statistics dominate the N(H I)-weighted mean; and (2) there is no evolution in the mean [Fe/H] metallicity from z = 1.7 to 3.5 but possibly a marked drop at higher redshift. We conclude with a general discussion on the physical nature of the DLA systems. We stress the uniformity of the DLA chemical abundances which indicates that the protogalaxies identified with DLA systems have very similar enrichment histories, i.e., a nearly constant relative contribution from Type Ia and Type II supernovae. The DLA systems also show constant relative abundances within a given system, which places strict constraints on the mixing timescales of the damped systems and may pose a great challenge to the protogalactic clump scenarios favored by hierarchical galaxy formation.

Chemical Abundances of the Damped Lyα Systems at z > 1.5

We present chemical abundance measurements for 19 damped Lyα systems observed with the high-resolution echelle spectrograph (HIRES) on the 10 m W.M. Keck Telescope. We perform a detailed analysis of every system, deriving ionic column densities for all unblended metal-line transitions. Our principal goal is to investigate the abundance patterns of the damped systems and thereby determine the underlying physical processes that dominate their chemical evolution. We place particular emphasis on gauging the relative importance of two complementary effects often invoked to explain the damped Lyα abundances (1) nucleosynthetic enrichment from Type II supernovae and (2) an interstellar medium-like (ISM-like) dust-depletion pattern. Similar to the principal results of Lu et al., our observations lend support both for dust depletion and Type II supernova (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 Lyα systems. Meanwhile, the relative abundances of Al, Si, and Cr versus 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., 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 Lyα abundance patterns. We also examine the metallicity of the damped Lyα systems both with respect to Zn/H and Fe/H. Our results confirm previous surveys by Pettini and collaborators, i.e., [ Zn/H ] = -1.15±0.15 dex. In contrast with other damped Lyα surveys at z > 1.5, we do not formally observe an evolution of metallicity with redshift, although we stress that this result is based on the statistics from a small sample of high-z damped systems.

Protogalactic Disk Models of Damped Lyα Kinematics

We present new observational results on the kinematics of the damped Lyα systems. Our full sample now comprises 31 low-ion profiles and exhibits characteristics similar to those of the sample from our previous paper.The primary exception is that the new distribution of velocity widths includes values out to a maximum of nearly 300 km s-1, ≈ 100 km s-1 greater than the previous maximum. These high velocity width systems will significantly leverage models introduced to explain the damped Lyα systems. Comparing the characteristics from low-redshift and high-redshift subsamples, we find no evidence for significant evolution in the kinematic properties of protogalaxies from z = 2.0 to z = 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 Lyα observations in the first paper 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, as predicted by cosmologies with early structure formation. 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 Lyα 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 the following: (1) Disk warping has only a minimal effect on the kinematic results, primarily influencing the effective disk thickness. (2) The TRD model is robust to more realistic rotation curves; we point out, however, that the rotation curve derived from centrifugal equilibrium with H I gas alone does not yield acceptable results but that rather flat rotation curves such as those generated by dark matter halos are required. (3) The effects of photoionization require thicker disks to give consistent velocity width distributions.

C II* Absorption in Damped Lyα Systems. I. Star Formation Rates in a Two-Phase Medium

We describe a technique that for the first time measures star formation rates (SFRs) in damped Lyα systems (DLAs) directly. We assume that massive stars form in DLAs and that the far-ultraviolet (FUV) radiation they emit heats the gas by the grain photoelectric mechanism. We infer the heating rate by equating it to the cooling rate measured by the strength of C II* λ1335.7 absorption. Since the heating rate is proportional to the product of the dust-to-gas ratio, the grain photoelectric heating efficiency, and the SFR per unit area, , we can deduce for DLAs in which the cooling rate and dust-to-gas ratio have been measured. We consider models in which the dust consists of carbonaceous grains and silicate grains. We present two-phase models in which the cold neutral medium (CNM) and warm neutral medium (WNM) are in pressure equilibrium. In the CNM model the line of sight passes through CNM and WNM gas, while in the WNM model the line of sight passes only through WNM gas. Since the grain photoelectric heating efficiency is at least an order of magnitude higher in the CNM than in the WNM, most of the C II* absorption arises in the CNM in the CNM model. We use the measured C II* absorption lines to derive for a sample of ≈30 DLAs in which κ has been inferred from element depletion patterns. We show that the inferred corresponds to an average over the star-forming volume of the DLA rather than to local star formation along the line of sight. We obtain the average and show that = 10-2.2 M☉ yr-1 kpc-2 for the CNM solution and = 10-1.3 M☉ yr-1 kpc-2 for the WNM solution. Interestingly, the SFR per unit area in the CNM solution is similar to that measured in the Milky Way interstellar medium.

C II* Absorption in Damped Lyα Systems. II. A New Window on the Star Formation History of the Universe

Starting from the star formation rate (SFR) per unit area, , determined for damped Lyα systems (DLAs) using the C II* method, we obtain the SFR per unit comoving volume, (z), at z ≈ 3. Pure warm neutral medium (WNM) solutions are ruled out since they generate more bolometric background radiation than observed, but the two-phase solutions dominated by the cold neutral medium (CNM) are consistent with the backgrounds. We find that (z) for DLAs agrees with the (z) for the Lyman break galaxies (LBGs). Although the mass of produced stars indicated by the SFRs is consistent with the current densities of known stellar populations, the mass of metals produced by z = 2.5 is 30 times larger than detected in absorption in DLAs. Of the three possible solutions to this missing metals problem, the most likely appears to be that star formation occurs in compact bulge regions. We search for evidence of feedback and find no correlations between and N(H I), but possible correlations between and low-ion velocity width and and metal abundance. We show that (1) the correlation between cooling rate and dust-to-gas ratio is positive evidence for grain photoelectric heating, (2) the cosmic microwave background (CMB) does not significantly populate the C II excited fine-structure states, and (3) the ratio of C II* to resonance-line optical depths is a sensitive probe of the multiphase structure of the DLA gas. We address recent arguments that DLAs are comprised only of WNM gas and show them to be inconclusive. Despite the rough agreement between (z) for DLAs and LBGs, current evidence indicates that these are distinct populations.

The ESI/Keck II damped Lyα abundance database

This paper presents chemical abundance measurements for 37 damped Ly? systems at z > 2.5 observed with the Echellette Spectrograph and Imager on the Keck II telescope. We measure the H I column densities of these damped systems through Voigt profile fits to their Ly? profiles, and we implement the apparent optical depth method to determine ionic column densities. Figures and tables of all relevant data are presented. A full analysis of the chemical enrichment history described by these observations will be presented in a future paper. This data set is also valuable for efficiently planning future echelle observations and for rough abundance pattern analyses. We aim to make this entire data set public within 3 years of this publication.

The UCSD HIRES/Keck I Damped Lyα Abundance Database. I. The Data

We present new chemical abundance measurements of 16 damped Lyα 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 analyzed 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 on-line directory for this database and will continuously update the results.