Boosting Lya and HeII 1640A Line Fluxes from Pop III Galaxies:
  Stochastic IMF Sampling and Departures from Case-B
  We revisit calculations of nebular hydrogen Lya and HeII 1640A line strengths
for population III galaxies, undergoing continuous and bursts of star
formation. We focus on initial mass functions (IMFs) motivated by recent
theoretical studies, which generally span a lower range of stellar masses than
earlier works. We also account for case-B departures and the stochastic
sampling of the IMF. In agreement with previous works, we find that departures
from case-B can enhance the Lya flux by a factor of a few, but we argue that
this enhancement is driven mainly by collisional excitation and ionization, and
not due to photoionization from the n = 2 state of atomic hydrogen. The
increased sensitivity of the Lya flux to the high-energy end of the galaxy
spectrum makes it more subject to stochastic sampling of the IMF. The latter
introduces a dispersion in the predicted nebular line fluxes around the
deterministic value by as much as a factor of ~4. In contrast, the stochastic
sampling of the IMF has less impact on the emerging Lyman Werner (LW) photon
flux. When case-B departures and stochasticity effects are combined, nebular
line emission from population III galaxies can be up to one order of magnitude
brighter than predicted by 'standard' calculations that do not include these
effects. This enhances the prospects for detection with future facilities such
as JWST and large, groundbased telescopes.
