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

TL;DR

This study revisits nebular line flux calculations for Pop III galaxies, highlighting how stochastic IMF sampling and case-B departures can significantly increase line brightness, improving detection prospects with future telescopes.

Contribution

It introduces the combined effects of stochastic IMF sampling and case-B departures into nebular line flux predictions for Pop III galaxies, showing they can boost line brightness substantially.

Findings

Case-B departures can enhance Lya flux by a factor of a few.

Stochastic IMF sampling causes up to a fourfold dispersion in nebular line fluxes.

Combined effects can make nebular emission up to ten times brighter, aiding detection.

Abstract

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.