  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.
