Confronting simulations of optically thick gas in massive halos with observations at z = 2-3

Cosmological hydrodynamic simulations predict the physical state of baryons in the circumgalactic medium (CGM), which can be directly tested via quasar absorption line observations. We use high-resolution "zoom-in" simulations of 21 galaxies to characterize the distribution of neutral hydrogen around halos in the mass range M _vir ∼ 2 × 10¹¹ to 4 × 10¹² M _⊙ at z ∼ 2. We find that both the mass fraction of cool (T ≤ 3 × 10⁴ K) gas and the covering fraction of optically thick Lyman limit systems (LLSs) depend only weakly on halo mass, even around the critical value for the formation of stable virial shocks. The covering fraction of LLSs interior to the virial radius varies between f _c ∼ 0.05-0.2, with significant scatter among halos. Our simulations of massive halos (M _vir ≥ 10¹² M _⊙) underpredict the covering fraction of optically thick gas observed in the quasar CGM by a large factor. The reason for this discrepancy is unclear, but several possibilities are discussed. In the lower mass halos (M _vir ≥ 5 × 10¹¹ M _⊙) hosting star-forming galaxies, the predicted covering factor agrees with observations; however, current samples of quasar-galaxy pairs are too small for a conclusive comparison. To overcome this limitation, we propose a new observable: the small-scale autocorrelation function of optically thick absorbers detected in the foreground of close quasar pairs. We show that this new observable can constrain the underlying dark halos hosting LLSs at z ∼ 2-3, as well as the characteristic size and covering factor of the CGM.