The nature of damped HI absorbers probed by cosmological simulations: satellite accretion and outflows

We use state-of-the-art cosmological zoom simulations to explore the distribution of neutral gas in and around galaxies that gives rise to high column density \ion{H}{i} \mbox{Ly-$\alpha$} absorption (formally, sub-DLAs and DLAs) in the spectra of background quasars. Previous cosmological hydrodynamic simulations under-predict the mean projected separations $(b)$ of these absorbers relative to the host, and invoke selection effects to bridge the gap with observations. On the other hand, single lines of sight (LOS) in absorption cannot uniquely constrain the galactic origin. Our simulations match all observational data, with DLA and sub-DLA LOS existing over the entire probed parameter space ($-4\lesssim $[M/H]$\lesssim 0.5$, $b<50$ kpc) at all redshifts ($z\sim 0.4 - 3.0$). We demonstrate how the existence of DLA LOS at $b\gtrsim 20-30$ kpc from a massive host galaxy require high numerical resolution, and that these LOS are associated with dwarf satellites in the main halo, stripped metal-rich gas and outflows. Separating the galaxy into interstellar ("\ion{H}{i} disc") and circumgalactic ("halo") components, we find that both components significantly contribute to damped \ion{H}{i} absorption LOS. Above the sub-DLA (DLA) limits, the disc and halo contribute with $\sim 60 (80)$ and $\sim 40 (20)$ per cent, respectively. Our simulations confirm analytical model-predictions of the DLA-distribution at $z\lesssim 1$. At high redshift ($z\sim 2-3$) sub-DLA and DLAs occupy similar spatial scales, but on average separate by a factor of two by $z\sim 0.5$. On whether sub-DLA and DLA LOS sample different stellar-mass galaxies, such a correlation can be driven by a differential covering-fraction of sub-DLA to DLA LOS with stellar mass. This preferentially selects sub-DLA LOS in more massive galaxies in the low-$z$ universe.