The HST colours of high-redshift population III galaxies with strong Lyman alpha emission

TL;DR

This paper proposes a method to identify high-redshift Population III galaxies with strong Lyman alpha emission using their unique colours in Hubble Space Telescope imaging, potentially revealing these early universe objects.

Contribution

It introduces a colour-based identification technique for Population III galaxies with strong Lyman alpha emission at high redshifts, even with minimal stellar mass contribution.

Findings

Population III galaxies can be distinguished by their unusually blue J-H colours at z=8-10.

Some objects in current Y-band dropout samples match the predicted colour criteria.

The method is effective even if Population III stars contribute as little as 0.1% to 1% of stellar mass.

Abstract

Population III galaxies, made partly or exclusively of metal-free stars, are predicted to exist at high redshifts and may produce very strong Lya emission. A substantial fraction of these Lya photons are likely absorbed in the intergalactic medium at z>6, but recent simulations suggest that significant Lya emission may be detectable up to z~8.5, i.e. well into the reionization epoch. Here, we argue that high-redshift population III galaxies with strong Lya emission can be identified in Hubble Space Telescope imaging data because of their unusual colours. We quantify this effect in some of the filters used in Y-band dropout searches for galaxies at z~8 and find that population III galaxies with high Lya fluxes may exhibit much bluer J-H colours at z=8-10 than any normal type of galaxy at these redshifts. This colour signature can arise even if pop III stars account for as little as ~1e-3 to ~1e-2 of the stellar mass in these galaxies. Some of the anomalously blue objects reported in current Y-band dropout samples do in fact meet the colour criteria for Lya-emitting population III galaxies.