The First Galaxies: Signatures of the Initial Starburst
  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 10^8 M_Sun 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 H_alpha, Ly_alpha, and He II 1640, under the simple
assumption that the stellar population does not evolve over the first ~3 Myr of
the starburst. Although only < 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 1640 is much more strongly correlated with the
total stellar mass. The ratio of the luminosity in He II 1640 to that in
Ly_alpha or H_alpha is found to be a good indicator of the IMF in many cases.
The ratio of observable fluxes is F_1640/F_Halpha ~ 1 for clusters of 100 M_Sun
Pop III stars and F_1640/F_Halpha ~ 0.1 for clusters of 25 M_Sun Pop III stars.
The EW of the He II 1640 emission line is the most reliable IMF indicator, its
value varying between ~ 20 and ~ 200 angstrom 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. Instead, the JWST may discover only more massive, and hence more
chemically evolved, galaxies which host normal, Pop I/II, star formation.
