The H I covering fraction of Lyman Limit Systems in FIRE haloes

Atomic hydrogen (H I) serves a crucial role in connecting galactic-scale properties such as star formation with the large-scale structure of the Universe. While recent numerical simulations have successfully matched the observed covering fraction of H I near Lyman Break Galaxies (LBGs) and in the foreground of luminous quasars at redshifts z ≾ 3, the low-mass end remains as-of-yet unexplored in observational and computational surveys. We employ a cosmological, hydrodynamical simulation (FIREbox) supplemented with zoom-in simulations (MassiveFIRE) from the Feedback In Realistic Environments (FIRE) project to investigate the H I covering fraction of Lyman Limit Systems (N_{H I} ≿ 10^17.2 cm⁻²) across a wide range of redshifts (z = 0 − 6) and halo masses (10⁸ − 10¹³ M_☉ at z = 0, 10⁸ − 10¹¹ M_☉ at z = 6) in the absence of feedback from active galactic nuclei. We find that the covering fraction inside haloes exhibits a strong increase with redshift, with only a weak dependence on halo mass for higher mass haloes. For massive haloes (M_vir ∼ 10¹¹ − 10¹² M_☉), the radial profiles showcase scale-invariance and remain independent of mass. The radial dependence is well captured by a fitting function. The covering fractions in our simulations are in good agreement with measurements of the covering fraction in LBGs. Our comprehensive analysis unveils a complex dependence with redshift and halo mass for haloes with M_vir ≾ 10¹⁰ M_☉ that future observations aim to constrain, providing key insights into the physics of structure formation and gas assembly.