THE STAR FORMATION RATE EFFICIENCY OF NEUTRAL ATOMIC-DOMINATED HYDROGEN GAS IN THE OUTSKIRTS OF STAR-FORMING GALAXIES FROM z ∼ 1 TO z ∼ 3

Current observational evidence suggests that the star formation rate (SFR) efficiency of neutral atomic hydrogen gas measured in damped Lyα systems (DLAs) at ~ z 3 is more than 10 times lower than predicted by the Kennicutt–Schmidt (KS) relation. To understand the origin of this deficit, and to investigate possible evolution with redshift and galaxy properties, we measure the SFR efficiency of atomic gas at ~ z 1, ~ z 2, and ~ z 3 around star-forming galaxies. We use new robust photometric redshifts in the Hubble Ultra Deep Field to create galaxy stacks in these three redshift bins, and measure the SFR efficiency by combining DLA absorber statistics with the observed rest-frame UV emission in the galaxies’ outskirts. We find that the SFR efficiency of H I gas at > z 1 is ∼1%–3% of that predicted by the KS relation. Contrary to simulations and models that predict a reduced SFR efficiency with decreasing metallicity and thus with increasing redshift, we find no significant evolution in the SFR efficiency with redshift. Our analysis instead suggests that the reduced SFR efficiency is driven by the low molecular content of this atomic-dominated phase, with metallicity playing a secondary effect in regulating the conversion between atomic and molecular gas. This interpretation is supported by the similarity between the observed SFR efficiency and that observed in local atomic-dominated gas, such as in the outskirts of local spiral galaxies and local dwarf galaxies.