Star formation rate and metallicity of damped Lyman α absorbers in cosmological smoothed particle hydrodynamics simulations

Abstract

We study the distribution of the star formation rate (SFR) and metallicity of damped Lyman alpha absorbers (DLAs) in the redshift range z= 0-4.5 using cosmological smoothed particle hydrodynamics (SPH) simulations of the Lambda cold dark matter model. Our simulations include standard radiative cooling and heating with a uniform ultraviolet background, star formation, supernova (SN) feedback, as well as a phenomenological model for feedback by galactic winds. The latter allows us to examine, in particular, the effect of galactic outflows on the distribution of the SFR and metallicity of DLAs. We employ a 'conservative entropy' formulation of SPH which alleviates numerical overcooling effects that affected earlier simulations. In addition, we utilize a series of simulations of varying box-size and particle number to investigate the impact of numerical resolution on our results.We find that there is a positive correlation between the projected stellar mass density and the neutral hydrogen column density (N(H I)) of DLAs for high N(H I) systems, and that there is a good correspondence in the spatial distribution of stars and DLAs in the simulations. The evolution of typical star-to-gas mass ratios in DLAs can be characterized by an increase from approximately 2 at z= 4.5 to 3 at z= 3, to 10 at z= 1 and finally to 20 at z= 0. We also find that the projected SFR density in DLAs follows the Kennicutt law well at all redshifts, and the simulated values are consistent with the recent observational estimates of this quantity by Wolfe, Prochaska & Gawiser. The rate of evolution in the mean metallicity of simulated DLAs as a function of redshift is mild, and is consistent with the rate estimated from observations. The predicted metallicity of DLAs is generally subsolar in our simulations, and there is a significant scatter in the distribution of DLA metallicity for a given N(H I). However, we find that the median metallicity of simulated DLAs is close to that of Lyman-break galaxies, and is higher than the values typically observed for DLAs by nearly an order of magnitude. This discrepancy with observations could be due to an inadequate treatment of the SN feedback or the multiphase structure of the gas in our current simulations. Alternatively, the current observations might be missing the majority of the high metallicity DLAs due to selection effects.