Reversal of Fortune: Increased Star Formation Efficiencies in the Early Histories of Dwarf Galaxies?

TL;DR

This paper proposes that dwarf galaxies had higher star formation efficiencies in their early histories, with a two-phase assembly process that contrasts with their lower efficiencies observed at later times.

Contribution

It introduces a two-phase model of dwarf galaxy assembly, showing early high star formation efficiency using combined observational and simulation data.

Findings

Early dwarf galaxy halos had high star formation efficiencies.

Dwarf galaxies' star formation efficiency increased during initial assembly phases.

Data suggests dwarf galaxies were among the most efficient star-forming systems early in the universe.

Abstract

On dwarf galaxy scales, the different shapes of the galaxy stellar mass function and the dark halo mass function require a star-formation efficiency (SFE) in these systems that is currently more than 1 dex lower than that of Milky Way-size halos. Here, we argue that this trend may actually be reversed at high redshift. Specifically, by combining the resolved star-formation histories of nearby isolated dwarfs with the simulated mass-growth rates of dark matter halos, we show that the assembly of these systems occurs in two phases: (1) an early, fast halo accretion phase with a rapidly deepening potential well, characterized by a high SFE; and (2) a late slow halo accretion phase where, perhaps as a consequence of reionization, the SFE is low. Nearby dwarfs have more old stars than predicted by assuming a constant or decreasing SFE with redshift, a behavior that appears to deviate qualitatively from the trends seen amongst more massive systems. Taken at face value, the data suggest that, at sufficiently early epochs, dwarf galaxy halos above the atomic cooling mass limit can be among the most efficient sites of star formation in the universe.