Damped Lyman α systems in high-resolution hydrodynamical simulations

Abstract

We investigate the properties of Damped Lyman-alpha systems (DLAs) using high-resolution and large box-size cosmological hydrodynamical simulations of a LambdaCDM model. The numerical code used is a modification of GADGET-2 with a self consistent implementation of the metal enrichment mechanism (Tornatore et al. 2007). 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 IGM (Intergalactic Medium) around DLAs. We find that strong galactic winds with speed of about 600 km/s, 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 (100 km/s) and large (600 km/s) velocities. At z=3 the contribution of haloes of masses between 10^9 and 10^10 h^-1 M_sun, 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 SiII associated to DLAs: 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.