FirstLight IV: Diversity in sub-L${}_{\ast }$ galaxies at cosmic dawn

Abstract

Using a large sample of sub-L${}_{\ast }$ galaxies, with similar UV magnitudes, M${}_{\mathrm{UV}}\simeq -19$ at $z\simeq 6$, 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$1 dex range in equivalent width of the [OIII]$\hat{I}»$5007 line. Variations in nebular metallicity and ionization parameter within HII regions lead to a scatter in the equivalent widths and [OIII]/H$\hat{I}\pm$ line ratio at a fixed sSFR. [OIII]-bright emitters ([OIII]/H$\hat{I}\pm$>1) have higher ionization parameters and/or higher metallicities than H$\hat{I}\pm$-bright ([OIII]/H$\hat{I}\pm$<1) galaxies. According to the surface brightness maps in both [OIII] and H$\hat{I}\pm$, [OIII]-bright emitters are more compact than H$\hat{I}\pm$-bright galaxies. H$\hat{I}\pm$ 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$\hat{I}\pm$-emitting gas is significantly more extended than [OIII].

9 pages, 5 figures, accepted at MNRAS. The website of the FirstLight database can be found at http://odin.ft.uam.es/FirstLight/index.html