Distribution of damped Lyα absorbers in a Λ cold dark matter universe

We present the results of a numerical study of a galactic wind model and its implications on the properties of damped Ly alpha absorbers (DLAs) using cosmological hydrodynamic simulations. We vary both the wind strength and the internal parameters of the wind model in a series of smoothed particle hydrodynamics (SPH) simulations that include radiative cooling and heating by a UV background, star formation, and feedback from supernovae and galactic winds. To test our simulations, we examine the DLA rate of incidence'' as a function of halo mass, galaxy apparent magnitude, and impact parameter. We find that the statistical distribution of DLAs does not depend on the exact values of internal numerical parameters that control the decoupling of hydrodynamic forces when the gas is ejected from star-forming regions, although the exact spatial distribution of neutral gas may vary for individual halos. The DLA rate of incidence in our simulations at z = 3 is dominated (80%-90%) by the faint galaxies with apparent magnitude RAB < 25.5. However, interestingly in a strong wind'' run, the differential distribution of DLA sight lines is peaked at M-halo = 10(12) h(-1) M-circle dot (R-AB similar or equal to 27), and the mean DLA halo mass is < M-DLA > = 10(12.4) h(-1) M-circle dot (R-AB similar or equal to 26). The DLAs in our simulations are more compact than the present-day disk galaxies, and the impact parameter distribution is very narrow unless we limit the search for the host galaxy to only bright Lyman break galaxies (LBGs). We discuss conflicts between current simulations and observations, and potential problems with hydrodynamic simulations based on the cold dark matter model.