O ct 2 00 5 Distribution of Damped Lyman - α Absorbers in a Λ Cold Dark Matter Universe

We examine the ‘rate-of-incidence’ distribution for damped Lyman-α absorbers (DLAs) as a function of halo mass, galaxy apparent magnitude, and impact parameter using a series of cosmological Smoothed Particle Hydrodynamics (SPH) simulations. Our simulations include radiative cooling and heating by a UV background, star formation, and feedback from supernovae and galactic winds. We find the DLA rate-of-incidence in our simulations at z = 3 is dominated (80− 90%) by faint galaxies with apparent magnitude RAB < 25.5. However, interestingly in a ‘strong wind’ run, we find that the differential distribution of DLA sight-lines is peaked at Mhalo = 10 12 hM⊙ (RAB ≃ 27) and the mean DLA halo mass is 〈MDLA〉 = 10 12.4 hM⊙ (RAB ≃ 26). These mass-scales are much larger than those if we ignore winds, because galactic feedback suppresses the DLA cross section in low mass halos and increases the relative contribution to the DLA incidence from more massive halos. But we caution that the mean DLA mass is biased towards a high value and is very different from the median halo mass unless the DLA distribution is highly peaked. The simulations also suggest that DLAs at z = 3 are more compact than present-day disk galaxies, and the impact parameter distribution is very narrow unless we limit searches for the galaxy hosting a DLA to only bright Lyman-break galaxies (LBGs). We find that the comoving number density of DLAs is higher than that of LBGs down to the magnitude limit of RAB = 30 mag if the physical radius of each DLA is smaller than 5 h 70 kpc. We discuss conflicts between current simulations and observations, and potential problems with hydrodynamic simulations based on the cold dark matter model. Subject headings: quasars: absorption lines — galaxies: ISM — stars: formation — galaxies: evolution – galaxies: formation – methods: numerical University of California, San Diego, Center for Astrophysics and Space Sciences, CA 9209 3-0424, U.S.A. Harvard University, 60 Garden Street, Cambridge, MA 02138, U.S.A. Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85740 Garching bei München, Germany