FirstLight II: Star formation rates of primeval galaxies from z=5-15

TL;DR

This study uses extensive cosmological simulations to analyze the star formation histories of early galaxies from redshift 5 to 15, revealing diverse burst behaviors and their impact on galaxy evolution.

Contribution

It provides the first detailed statistical analysis of star formation bursts in primeval galaxies during cosmic dawn, highlighting their characteristics and effects on galaxy growth.

Findings

First galaxies spend 70% of their time in star formation bursts.

Typical burst at z=6 has sSFR of 5-15 Gyr^-1 with ~100 Myr duration.

High sSFR bursts reach 20-30 Gyr^-1 with shorter timescales.

Abstract

In the FirstLight project, we have used ~300 cosmological, zoom-in simulations to determine the star-formation histories of distinct first galaxies with stellar masses between Ms=10^6 and 3 x 10^9 Msun during cosmic dawn (z=5-15). The evolution of the star formation rate (SFR) in each galaxy is complex and diverse, characterized by bursts of star formation. Overall, first galaxies spend 70% of their time in SF bursts. A sample of 1000 of these bursts indicates that the typical burst at z=6 has a specific SFR (sSFR) maximum of 5-15 Gyr^-1 with an effective width of ~100 Myr, one tenth of the age of the Universe at that redshift. A quarter of the bursts populate a tail with very high sSFR maxima of 20-30 Gyr^-1 and significantly shorter timescales of ~40-80 Myr. This diversity of bursts sets the mean and the mass-dependent scatter of the star-forming main sequence. This scatter is driven by a population of low-mass, Ms < 10^8 Msun, quiescent galaxies. The mean sSFR and the burst maximum at fixed mass increase with redshift, with the later always being a factor ~2 higher than the former. This implies sSFR maxima of ~20-60 Gyr^-1 at z=9-10. The SFR histories are publicly available at the FirstLight website.