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

Abstract

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 (NH I ≿ 1017.2 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 (Mvir ∼ 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 Mvir ≾ 1010 M☉ that future observations aim to constrain, providing key insights into the physics of structure formation and gas assembly.