Scaling relations for galaxies prior to reionization

Abstract

The first galaxies in the universe are the building blocks of all observed galaxies. We present scaling relations for galaxies forming at redshifts z ≥ 15 when reionization is just beginning. We utilize the "Rarepeak" cosmological radiation hydrodynamics simulation that captures the complete star formation history in over 3300 galaxies, starting with massive Population III stars that form in dark matter halos as small as 106 M⊙. We make various correlations between the bulk halo quantities, such as virial, gas, and stellar masses and metallicities and their respective accretion rates, quantifying a variety of properties of the first galaxies up to halo masses of 109 M⊙. Galaxy formation is not solely relegated to atomic cooling halos with virial temperatures greater than 104 K, where we find a dichotomy in galaxy properties between halos above and below this critical mass scale. Halos below the atomic cooling limit have a stellar mass-halo mass relationship log M ∗ ≃ 3.5 + 1.3log (M vir/107 M). We find a non-monotonic relationship between metallicity and halo mass for the smallest galaxies. Their initial star formation events enrich the interstellar medium and subsequent star formation to a median of 10-2 Z⊙ and 10-1.5 Z respectively, in halos of total mass 107 M⊙, which is then diluted by metal-poor inflows well beyond Population III pre-enrichment levels of 10-3.5 Z. The scaling relations presented here can be employed in models of reionization, galaxy formation, and chemical evolution in order to consider these galaxies forming prior to reionization.