The First Galaxies: Signatures of the Initial Starburst

TL;DR

This study uses cosmological simulations to analyze the signatures of the first starburst galaxies at high redshift, focusing on emission lines as indicators of stellar populations and initial mass functions, with implications for JWST observations.

Contribution

It provides detailed predictions of emission line ratios and equivalent widths for Pop III starbursts, linking them to stellar initial mass functions and galaxy detectability with JWST.

Findings

He II 1640 luminosity correlates strongly with stellar mass.

He II 1640 to Ly_alpha or H_alpha ratio indicates IMF type.

He II 1640 EW varies significantly with IMF, from ~20 to ~200 Å.

Abstract

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.