Formation of Massive Population III Galaxies through Photoionization Feedback: A Possible Explanation for CR7

TL;DR

This paper proposes that photoionization feedback in early universe dark matter halos can lead to the formation of massive Population III galaxies, potentially explaining the observed properties of CR7 with a new formation scenario.

Contribution

It introduces a novel formation pathway for massive Pop III galaxies via photoionization feedback, supported by analytical estimates of their abundance.

Findings

Massive Pop III galaxies can form in halos around 10^9 M_sun due to photoionization feedback.

Estimated number density of such Pop III galaxies at z=6.6 is about 10^-7 Mpc^-3.

This formation channel explains the large Pop III star mass observed in CR7.

Abstract

We explore the formation of massive high-redshift Population III (Pop III) galaxies through photoionization feedback. We consider dark matter halos formed from progenitors that have undergone no star formation as a result of early reionization and photoevaporation caused by a nearby galaxy. Once such a halo reaches $\approx 10^9~M_\odot$, corresponding to the Jeans mass of the photoheated intergalactic medium (IGM) at $z\approx 7$, pristine gas is able to collapse into the halo, potentially producing a massive Pop III starburst. We suggest that this scenario may explain the recent observation of strong He II 1640~\AA~line emission in CR7, which is consistent with $\sim 10^7~M_\odot$ of young Pop III stars. Such a large mass of Pop III stars is unlikely without the photoionization feedback scenario, because star formation is expected to inject metals into halos above the atomic cooling threshold ($\sim 10^8~M_\odot$ at $z \approx 7$). We use merger trees to analytically estimate the abundance of observable Pop III galaxies formed through this channel, and find a number density of $\approx 10^{-7}~{\rm Mpc^{-3}}$ at $z=6.6$ (the redshift of CR7). This is approximately a factor of ten lower than the density of Ly$\alpha$ emitters as bright as CR7.