Molecular Hydrogen in High-redshift Damped Lyman-{\alpha} Absorbers

TL;DR

This study explores the presence of molecular hydrogen in high-redshift circumgalactic gas, using simulations and models to understand its detectability and role in damped Lyα absorbers.

Contribution

The paper introduces an analytic model for HI-to-H$_2$ transitions integrated with high-resolution galaxy simulations to assess molecular content in the high-redshift CGM.

Findings

Detectable H$_2$ sightlines extend about 0.1 R_vir, regardless of redshift.

Low H$_2$ detection rates in DLAs are explained by low CGM densities and strong UV fields.

Molecular fractions in the CGM are much lower than in the ISM at similar HI columns.

Abstract

Simulations predict that circumgalactic hydrogen gas surrounding massive ($M_{\rm{halo}}^{z=1}=10^{12}-10^{13}\ M_{\odot}$) galaxies at $z\sim4$ may be predominantly neutral, and could produce damped Ly$\alpha$ absorbers (DLAs) along sight-lines to background quasars \citep{Stern2021}. A circumgalactic medium (CGM) origin for DLAs naturally explains high redshift HI absorption-selected galaxy detections at physical separations much greater than the likely extents of the galaxy disks \citep{Neeleman2017, Neeleman2019}. The observed $z\sim 4$ DLA HI column densities are large and comparable to interstellar (ISM) gas columns at which substantial molecular hydrogen (H$_2$) abundances occur. We therefore investigate the possible molecular content of high-redshift CGM gas, and its potential detectability via (rest-frame) far-ultraviolet (UV) absorption line studies. For this purpose we develop an analytic sub-grid model for HI-to-H$_2$ transitions and incorporate the model with zoom-in FIRE-2 simulations of evolving high-$z$ galaxies. We include dust absorption and scattering computations for the transfer of photodissociating Lyman-Werner (LW) band radiation. We find that the typical extents of detectable H$_2$ sightlines are $\approx 0.1\, R_{\rm vir}$, independent of redshift from $z=2.5$ to 5. We argue that a CGM origin for DLAs naturally explains the low detection rates of H$_2$ in DLA observations, as the low CGM densities and relatively strong far-UV fields lead to molecular fractions much lower than observed in the ISM at comparable HI columns.