Confronting Simulations of Optically Thick Gas in Massive Halos with Observations at z=2-3

TL;DR

This study uses high-resolution simulations to compare the distribution of neutral hydrogen in galaxy halos at z=2-3 with observations, revealing discrepancies in massive halos and proposing a new observable to better understand the CGM.

Contribution

It provides the first detailed comparison of simulated and observed optically-thick gas in galaxy halos at high redshift, highlighting underpredictions in massive halos and introducing a new auto-correlation observable.

Findings

Simulations underpredict optically-thick gas in massive halos.

Covering fraction of LLSs varies weakly with halo mass.

Proposes auto-correlation function of absorbers as a new constraint.

Abstract

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.