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We revisit calculations of nebular hydrogen Lya and HeII 1640A line strengthsfor population III galaxies, undergoing continuous and bursts of starformation. We focus on initial mass functions (IMFs) motivated by recenttheoretical studies, which generally span a lower range of stellar masses thanearlier works. We also account for case-B departures and the stochasticsampling of the IMF. In agreement with previous works, we find that departuresfrom case-B can enhance the Lya flux by a factor of a few, but we argue thatthis enhancement is driven mainly by collisional excitation and ionization, andnot due to photoionization from the n = 2 state of atomic hydrogen. Theincreased sensitivity of the Lya flux to the high-energy end of the galaxyspectrum makes it more subject to stochastic sampling of the IMF. The latterintroduces a dispersion in the predicted nebular line fluxes around thedeterministic value by as much as a factor of ~4. In contrast, the stochasticsampling of the IMF has less impact on the emerging Lyman Werner (LW) photonflux. When case-B departures and stochasticity effects are combined, nebularline emission from population III galaxies can be up to one order of magnitudebrighter than predicted by 'standard' calculations that do not include theseeffects. This enhances the prospects for detection with future facilities suchas JWST and large, groundbased telescopes.

Boosting Lya and HeII 1640A Line Fluxes from Pop III Galaxies: Stochastic IMF Sampling and Departures from Case-B