On the Kinematics of the Damped Lyman Alpha Protogalaxies

TL;DR

This study investigates the kinematic properties of high-redshift damped Lyman-alpha systems to test galaxy formation models, finding that rapidly rotating cold disk models best match observed data, challenging some CDM predictions.

Contribution

It provides the first comprehensive kinematic analysis of damped Lyman-alpha systems and tests physical models against observed velocity profiles, favoring rapidly rotating cold disk scenarios.

Findings

Rapidly rotating cold disk model fits the data well

Disks must have rotation speeds >180 km/s at 99% confidence

Standard CDM models are inconsistent with the observed high rotation speeds

Abstract

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.