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Session 47 - Old Stellar Populations Beyond The Milky Way - II.
Oral session, Wednesday, June 11
North Main Hall A,
We present the first results of an ongoing program to investigate the kinematic characteristics of high redshift damped Ly\alpha systems. Because damped Ly\alpha 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. In turn, we aim to constrain different cosmogonies by making comparisons with both Press-Schechter predictions and N-Body simulations. We have collected a kinematically unbiased sample of 17 high signal-to-noise ratio, high resolution damped Ly\alpha spectra taken with HIRES on the 10 m W.M. Keck Telescope. Our study focuses on the unsaturated, low-ion transitions (e.g. Si II 1808) of these systems which reveal their kinematic traits. The profiles exhibit a nearly uniform distribution of velocity widths ranging from \approx 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 Ly\alpha systems, including rapidly rotating thick 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. We develop 4 statistical tests to distinguish between the models which focus on the symmetry and velocity widths of the profiles. Comparing the distributions of values from the simulated profiles with those calculated from the observed profiles, we determine that the rapidly rotating thick disk model is the only tested model consistent with the data at high confidence levels. A Relative Likelihood Test of the rapidly rotating thick disk model indicates the disks must have large rotation speeds (v_rot > 180 km/s at the 99% c.l.) and be relatively thick (the ratio of the vertical scale height to the radial scale length, h/R_d > 0.1). These characteristics are highly suggestive of the thick disk of the Milky Way.
Program listing for Wednesday