Through Thick and Thin - HI Absorption in Cosmological Simulations

TL;DR

This study uses advanced cosmological simulations to analyze neutral hydrogen absorption features at redshift 3, successfully matching observed data and revealing how self-shielding and molecular formation influence the column density distribution.

Contribution

It introduces a parameter-free, detailed simulation approach to model HI absorption systems, incorporating self-shielding and molecular hydrogen formation with high accuracy.

Findings

Simulation reproduces observed HI absorption distributions at z=3.

Self-shielding flattens the column density distribution for NHI > 10^18 cm-2.

Conversion to neutral gas and H2 formation steepen the distribution at higher column densities.

Abstract

We investigate the column density distribution function of neutral hydrogen at redshift z = 3 using a cosmological simulation of galaxy formation from the OverWhelmingly Large Simulations (OWLS) project. The base simulation includes gravity, hydrodynamics, star formation, supernovae feedback, stellar winds, chemodynamics, and element-by-element cooling in the presence of a uniform UV background. Self-shielding and formation of molecular hydrogen are treated in post-processing, without introducing any free parameters, using an accurate reverse ray-tracing algorithm and an empirical relation between gas pressure and molecular mass fraction. The simulation reproduces the observed z = 3 abundance of Ly-A forest, Lyman Limit and Damped Ly-A HI absorption systems probed by quasar sight lines over ten orders of magnitude in column density. Self-shielding flattens the column density distribution for NHI > 10^18 cm-2, while the conversion to fully neutral gas and conversion of HI to H2 steepen it around column densities of NHI = 10^20.3 cm-2 and NHI = 10^21.5 cm-2, respectively.