On the evolution of the HI column density distribution in cosmological simulations

TL;DR

This study uses cosmological simulations with radiative transfer to analyze neutral hydrogen distribution after reionization, revealing smoother transitions due to recombination radiation and matching observed column density distributions across redshifts.

Contribution

It introduces fitting functions for the photoionization rate based on simulations, improving post-processing accuracy and providing insights into hydrogen self-shielding and ionization processes.

Findings

Simulations match observed hydrogen column density distributions.

Reproduces the lack of evolution in certain Lyman systems below redshift 3.

Predicts strong damped Lyα systems track star formation rate density.

Abstract

We use a set of cosmological simulations combined with radiative transfer calculations to investigate the distribution of neutral hydrogen in the post-reionization Universe. We assess the contributions from the metagalactic ionizing background, collisional ionization and diffuse recombination radiation to the total ionization rate at redshifts z=0-5. We find that the densities above which hydrogen self-shielding becomes important are consistent with analytic calculations and previous work. However, because of diffuse recombination radiation, whose intensity peaks at the same density, the transition between highly ionized and self-shielded regions is smoother than what is usually assumed. We provide fitting functions to the simulated photoionization rate as a function of density and show that post-processing simulations with the fitted rates yields results that are in excellent agreement with the original radiative transfer calculations. The predicted neutral hydrogen column density distributions agree very well with the observations. In particular, the simulations reproduce the remarkable lack of evolution in the column density distribution of Lyman limit and weak damped Ly\alpha\ systems below z = 3. The evolution of the low column density end is affected by the increasing importance of collisional ionization with decreasing redshift. On the other hand, the simulations predict the abundance of strong damped Ly\alpha\ systems to broadly track the cosmic star formation rate density.