ALMA shows that gas reservoirs of star-forming disks over the past 3
  billion years are not predominantly molecular
  Cold hydrogen gas is the raw fuel for star formation in galaxies, and its
partition into atomic and molecular phases is a key quantity for galaxy
evolution. In this Letter, we combine Atacama Large Millimeter/submillimeter
Array and Arecibo single-dish observations to estimate the molecular-to-atomic
hydrogen mass ratio for massive star-forming galaxies at $z\sim$ 0.2 extracted
from the HIGHz survey, i.e., some of the most massive gas-rich systems
currently known. We show that the balance between atomic and molecular hydrogen
in these galaxies is similar to that of local main-sequence disks, implying
that atomic hydrogen has been dominating the cold gas mass budget of
star-forming galaxies for at least the past three billion years. In addition,
despite harboring gas reservoirs that are more typical of objects at the cosmic
noon, HIGHz galaxies host regular rotating disks with low gas velocity
dispersions suggesting that high total gas fractions do not necessarily drive
high turbulence in the interstellar medium.
