Very High Gas Fractions and Extended Gas Reservoirs in z=1.5 Disk
  Galaxies
  We present evidence for very high gas fractions and extended molecular gas
reservoirs in normal, near-infrared selected (BzK) galaxies at z~1.5, based on
multi-configuration CO[2-1] observations obtained at the IRAM PdBI. Six of the
six galaxies observed were securely detected. High resolution observations
resolve the CO emission in four of them, implying sizes of order of 6-11 kpc
and suggesting the presence of rotation. The UV morphologies are consistent
with clumpy, unstable disks, and the UV sizes are consistent with the CO sizes.
The star formation efficiencies are homogeneously low and similar to local
spirals - the resulting gas depletion times are ~0.5 Gyr, much higher than what
is seen in high-z submm galaxies and quasars. The CO luminosities can be
predicted to within 0.15 dex from the star formation rates and stellar masses,
implying a tight correlation of the gas mass with these quantities. We use
dynamical models of clumpy disk galaxies to derive dynamical masses. These
models are able to reproduce the peculiar spectral line shapes of the CO
emission. After accounting for the stellar and dark matter masses we derive gas
masses of 0.4-1.2x10^11 Msun. The conversion factor is very high:
alpha_CO=3.6+-0.8, consistent with the Galaxy but four times higher than that
of local ultra-luminous IR galaxies. The gas accounts for an impressive 50-65%
of the baryons within the galaxies' half light radii. We are witnessing truly
gas-dominated galaxies at z~1.5, a finding that explains the high specific SFRs
observed for z>1 galaxies. The BzK galaxies can be viewed as scaled-up versions
of local disk galaxies, with low efficiency star formation taking place inside
extended, low excitation gas disks. They are markedly different than local
ULIRGs and high-z submm galaxies, which have more excited and compact gas.
