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

Abstract

Using the GAEA semi-analytic model, we analyse the connection between Damped Ly α systems (DLAs) and H I in galaxies. Our state-of-the-art semi-analytic model is tuned to reproduce the local galaxy H I 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<SUP>5</SUP> random lines of sight in a composite simulated volume: dark matter haloes with log$(\frac{M_{200}}{ {\rm M}_{\odot }}) \ge 11.5$ are extracted from the Millennium Simulation, while for $9.2 \le \log (\frac{M_{200}}{ \mathrm{M}_{\odot }})\lt 11.5$ we use the Millennium II, and for $8 \le \log (\frac{M_{200}}{\mathrm{M}_{\odot }}) \lt 9.2$ 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 H I 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 $M_{200} \ge {10}^{11} \, \mathrm{M}_{\odot }$ give the major contribution to Ω<SUB>DLA</SUB>, and the typical DLA host halo mass is $\sim \!{10}^{11} \, \mathrm{M}_{\odot }$. 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.