Effects of Ultraviolet Background and Local Stellar Radiation on the H_I Column Density Distribution

TL;DR

This study uses cosmological simulations to show that the ultraviolet background significantly influences the H_I column density distribution in damped Ly-alpha systems at z=3, with local stellar radiation having minimal effect.

Contribution

It demonstrates that accurately modeling the UVB, rather than the optically thin approximation, is crucial for reproducing observed H_I column density distributions in DLAs.

Findings

Proper UVB treatment improves agreement with observations.

Local stellar radiation has little impact on f(N_HI).

Optically thin approximation is inadequate for simulating neutral gas ionization.

Abstract

We study the impact of ultraviolet background (UVB) radiation field and the local stellar radiation on the H_I column density distribution f(N_HI) of damped Ly-alpha systems (DLAs) and sub-DLAs at z=3 using cosmological smoothed particle hydrodynamics simulations. We find that, in the previous simulations with an optically thin approximation, the UVB was sinking into the H_I cloud too deeply, and therefore we underestimated the f(N_HI) at 19 < log(N_HI) < 21.2 compared to the observations. If the UVB is shut off in the high-density regions with n_gas > 6 x 10^{-3} cm^{-3}, then we reproduce the observed f(N_HI) at z=3 very well. We also investigate the effect of local stellar radiation by post-processing our simulation with a radiative transfer code, and find that the local stellar radiation does not change the f(N_HI) very much. Our results show that the shape of f(N_HI) is determined primarily by the UVB with a much weaker effect by the local stellar radiation and that the optically thin approximation often used in cosmological simulation is inadequate to properly treat the ionization structure of neutral gas in and out of DLAs. Our result also indicates that the DLA gas is closely related to the transition region from optically-thick neutral gas to optically-thin ionized gas within dark matter halos.