The Hubble Space Telescope colours of high-redshift Population III galaxies with strong Lya emission

Population III (pop 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 similar to 8.5, i.e. well into the reionization epoch. Here, we argue that high-redshift pop 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 approximate to 8 and find that pop III galaxies with high Lya fluxes may exhibit much bluer J-H colours at z approximate to 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 similar to 10(-3) to 10(-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 pop III galaxies.