Damped Ly α absorbers and atomic hydrogen in galaxies: the view of the GAEA model

Using the GAEA semi-analytic model, we analyse the connection between Damped Ly alpha systems (DLAs) and HI in galaxies. Our state-of-the-art semi-analytic model is tuned to reproduce the local galaxy HI mass function, and that also reproduces other important galaxy properties, including the galaxy mass-gas metallicity relation. To produce catalogues of simulated DLAs we throw 10(5) random lines of sight in a composite simulated volume: dark matter haloes with log(M-200/M-circle dot) >= 11.5 are extracted from the Millennium Simulation, while for 9.2 <= log(M-200/M-circle dot) < 11.5 we use the Millennium II, and for 8 <= log(M-200/M-circle dot) < 9.2 a halo occupation distribution model. At 2 < z < 3, where observational data are more accurate, our fiducial model predicts the correct shape of the column density distribution function, but its normalization falls short of the observations, with the discrepancy increasing at higher redshift. The agreement with observations is significantly improved increasing both the HI masses and the disc radii of model galaxies by a factor of 2, as implemented 'a posteriori' in our 2M-2R model. In the redshift range of interest, haloes with M-200 >= 10(11) M-circle dot give the major contribution to Omega(DLA), and the typical DLA host halo mass is similar to 10(11) M-circle dot. The simulated DLA metallicity distribution is in relatively good agreement with observations, but our model predicts an excess of DLAs at low metallicities. Our results suggest possible improvements for the adopted modelling of the filtering mass and metal ejection in low-mass haloes.