FirstLight IV: Diversity in sub-L$_*$ galaxies at cosmic dawn

TL;DR

This study uses the FirstLight simulations to explore the diversity of sub-L* galaxies at cosmic dawn, revealing significant variations in star formation, nebular properties, and emission line characteristics during the end of reionization.

Contribution

It provides new insights into the diversity of galaxy properties at high redshift, highlighting the roles of metallicity, ionization, and morphology in emission line variations.

Findings

40-fold variation in specific star-formation rate

Correlation between [OIII] brightness and galaxy compactness

Extended Hα emission compared to [OIII] in galaxies

Abstract

Using a large sample of sub-L$_*$ galaxies, with similar UV magnitudes, M$_{\rm UV}\simeq -19$ at $z\simeq6$, extracted from the FirstLight simulations, we show the diversity of galaxies at the end of the reionization epoch. We find a factor $\sim$40 variation in the specific star-formation rate (sSFR). This drives a $\sim$1 dex range in equivalent width of the [OIII]$\lambda$5007 line. Variations in nebular metallicity and ionization parameter within HII regions lead to a scatter in the equivalent widths and [OIII]/H$\alpha$ line ratio at a fixed sSFR. [OIII]-bright emitters ([OIII]/H$\alpha$>1) have higher ionization parameters and/or higher metallicities than H$\alpha$-bright ([OIII]/H$\alpha$<1) galaxies. According to the surface brightness maps in both [OIII] and H$\alpha$, [OIII]-bright emitters are more compact than H$\alpha$-bright galaxies. H$\alpha$ luminosity is higher than [OIII] if star formation is distributed over extended regions. OIII dominates if it is concentrated in compact clumps. In both cases, the H$\alpha$-emitting gas is significantly more extended than [OIII].