2009MNRAS.399...37J

Detection of the radiation emitted from the first galaxies at z ≥ 10 will be made possible in the next decade, with the launch of the James Webb Space Telescope (JWST). We carry out cosmological radiation hydrodynamics simulations of Population III (Pop III) starbursts in a 10⁸M_☉ dwarf galaxy at z ∼ 12.5. For different star formation efficiencies and stellar initial mass functions (IMFs), we calculate the luminosities and equivalent widths (EWs) of the recombination lines Hα, Lyα and Heii λ1640, under the simple assumption that the stellar population does not evolve over the first ∼3 Myr of the starburst. Although only ≲40 per cent of the gas in the central 100pc of the galaxy is photoionized, we find that photoheating by massive stars causes a strong dynamical response, which results in a weak correlation between luminosity emitted in hydrogen recombination lines and the total mass in stars. However, owing to the low escape fraction of Heii-ionizing photons, the luminosity emitted in Heii λ1640 is much more strongly correlated with the total stellar mass. The ratio of the luminosity in Heii λ1640 to that in Lyα or Hα is a complex function of the density field and the star formation rate, but is found to be a good indicator of the IMF in many cases. The ratio of observable fluxes is F_λ1640/F_Hα∼ 1 for clusters of 100M_☉ Pop III stars and F_λ1640/F_Hα ∼ 0.1 for clusters of 25M_☉ Pop III stars. The EW of the Heii λ1640 emission line is the most reliable IMF indicator, its value varying between ∼20 and ∼200 AA for a massive and very massive Pop III IMF, respectively. Even the bright, initial stages of Pop III starbursts in the first dwarf galaxies will likely not be directly detectable by the JWST, except in cases where the flux from these galaxies is strongly magnified through gravitational lensing. Instead, the JWST may discover already more massive, and hence more chemically evolved, galaxies in which primordial star formation has largely ceased, or is contaminated with more normal, Pop I/II, star formation.