The Shocking Truth: The small contribution to hydrogen reionization from gravitational infall

TL;DR

This paper evaluates the role of gravitational infall shocks in hydrogen reionization, concluding they contribute minimally but could influence the clustering of ionizing sources detectable by future 21cm observations.

Contribution

It provides the first quantitative assessment of hydrogen reionization contribution from gravitational infall shocks, showing it is minor but impacts source clustering.

Findings

Shocks ionize only a few percent of hydrogen by z~6.

Fast accretion shocks produce more biased ionizing radiation than stars.

Potential for future 21cm experiments to detect clustering bias modifications.

Abstract

It is commonly thought that stars are responsible for reionizing the Universe. However, deep searches for star-forming galaxies during the epoch of reionization have not yet found sufficient galaxies to provide the necessary ionising flux. Fast accretion shocks associated with gravitational infall of baryons during the formation of galaxies have recently been proposed as an alternative method of generating the required ionising photons. In this Letter we assess the contribution to hydrogen reionization from shocked gas associated with gravitational infall. We find that shocks can ionize no more than a few percent of the cosmic hydrogen by z~6. However, the small fraction of ionizing radiation produced by fast accretion shocks would be significantly more biased than that associated with stars, leading to a modification of the luminosity weighted source clustering by ~10%. This modification of the bias may be measurable with future precision experiments utilising the redshifted 21cm line to study the distribution of hydrogen during the reionization era.