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~ 2x10^11 - 4x10^12 M_sun at z~2. We find that both the
mass fraction of cool (T <= 3x10^4 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 >= 10^12 M_sun) 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 >= 5x10^11 M_sun) 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 auto-correlation
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.
