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 < 180 km s-1, and scale heights, h < 0.1 times the radial scale length Rd, are ruled out at the 99% confidence level. The most likely values of these parameters are vrot = 225 km s-1 and h = 0.3Rd. We also find that these disks must be “cold,” since models in which σcc/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.