Large Molecular Gas Reservoirs in Ancestors of Milky Way-Mass Galaxies 9
  Billion Years Ago
  The gas accretion and star-formation histories of galaxies like the Milky Way
remain an outstanding problem in astrophysics. Observations show that 8 billion
years ago, the progenitors to Milky Way-mass galaxies were forming stars 30
times faster than today and predicted to be rich in molecular gas, in contrast
with low present-day gas fractions ($<$10%). Here we show detections of
molecular gas from the CO(J=3-2) emission (rest-frame 345.8 GHz) in galaxies at
redshifts z=1.2-1.3, selected to have the stellar mass and star-formation rate
of the progenitors of today's Milky Way-mass galaxies. The CO emission reveals
large molecular gas masses, comparable to or exceeding the galaxy stellar
masses, and implying most of the baryons are in cold gas, not stars. The
galaxies' total luminosities from star formation and CO luminosities yield long
gas-consumption timescales. Compared to local spiral galaxies, the
star-formation efficiency, estimated from the ratio of total IR luminosity to
CO emission,} has remained nearly constant since redshift z=1.2, despite the
order of magnitude decrease in gas fraction, consistent with results for other
galaxies at this epoch. Therefore the physical processes that determine the
rate at which gas cools to form stars in distant galaxies appear to be similar
to that in local galaxies.
