Detection of the radiation emitted from the first galaxies at z \geq 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 10 ^ { 8 } { \mathrm { M } } _ { \odot } dwarf galaxy at z \sim 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 H \alpha , Ly \alpha , and He ii \lambda 1640 , under the simple assumption that the stellar population does not evolve over the first \sim 3 Myr of the starburst .
Although only \la 40 percent of the gas in the central 100 pc 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 He ii -ionizing photons , the luminosity emitted in He ii \lambda 1640 is much more strongly correlated with the total stellar mass .
The ratio of the luminosity in He ii \lambda 1640 to that in Ly \alpha or H \alpha 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 _ { \lambda 1640 } /F _ { H \alpha } \sim 1 for clusters of 100 { \mathrm { M } } _ { \odot } Pop III stars and F _ { \lambda 1640 } /F _ { H \alpha } \sim 0.1 for clusters of 25 { \mathrm { M } } _ { \odot } Pop III stars .
The EW of the He ii \lambda 1640 emission line is the most reliable IMF indicator , its value varying between \sim 20 and \sim 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 gravtitational 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 .