Damped Lyman α systems in high-resolution hydrodynamical simulations

We investigate the properties of damped Lyman alpha systems (DLAs) using high-resolution and large box-size cosmological hydrodynamical simulations of a Lambda cold dark matter model. The numerical code used is a modification of gadget-2 with a self-consistent implementation of the metal enrichment mechanism. We explore the numerical convergence of some relevant physical quantities and we vary the parameters describing the properties of galactic winds, the initial stellar mass function, the linear dark matter power spectrum and the metal enrichment pattern of the intergalactic medium (IGM) around DLAs. We focus on the properties of dark matter haloes that are likely to be the hosts of DLAs systems: we predict relatively low star formation rates (similar to 0.01-0.1 M-circle dot yr(-1)) and metallicities around 0.1 Z(circle dot), at least for the bulk of our haloes of masses between 10(9) and 10(10) h(-1) M-circle dot hosting DLAs. For more massive haloes metallicities and star formation rates depend on the specific wind model. We find that strong galactic winds with speed of about 600 km s(-1), in an energy-driven wind scenario, are needed in order to match the observed column density distribution function for DLAs and the evolution of the neutral hydrogen content with redshift. The momentum-driven implementation of the galactic wind model, that relates the speed and mass load in the wind to the properties of the dark matter haloes, shows a behaviour which is intermediate between the energy-driven galactic winds of small (similar to 100 km s(-1)) and large (similar to 600 km s(-1)) velocities. At z = 3 the contribution of haloes of masses between 10(9) and 10(10) h(-1) M-circle dot, for DLAs below 10(20.8) cm(-2), to the column density distribution function, is significant. By interpolating physical quantities along line-of-sights through massive haloes we qualitatively show how different galactic wind models impact on the IGM around DLAs. Furthermore, we analyse statistics related to the velocity widths of Si ii associated to DLAs: while the expanding shells of gaseous matter associated to the wind can account for the observed velocities, the metallicity in the wind seems to be rather clumpy and this produces an underestimation of the observed velocity widths. We outline possible solutions to this problem.