  Current observational evidence suggests that the star formation rate (SFR)
efficiency of neutral atomic hydrogen gas measured in Damped Ly-alpha 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 HI 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.
