We present evidence for very high gas fractions and extended molecular gas reservoirs in normal , near-infrared selected ( BzK ) galaxies at z \sim 1.5 .
Our results are based on multi-configuration CO [ 2-1 ] observations obtained at the IRAM Plateau de Bure Interferometer .
All six star forming galaxies observed were detected at high significance .
High spatial resolution observations resolve the CO emission in four of them , implying sizes of the gas reservoirs of order of 6–11 kpc and suggesting the presence of ordered rotation .
The galaxies have UV morphologies consistent with clumpy , unstable disks , and UV sizes that are consistent with those measured in CO .
The star formation efficiencies are homogeneously low within the sample and similar to those of local spirals – the resulting gas depletion times are \sim 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 observed star formation rates and stellar masses , implying a tight correlation of the gas mass with these quantities .
We use new dynamical models of clumpy disk galaxies to derive dynamical masses for our sample .
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 molecular gas reservoirs with masses of 0.4–1.2 \times 10 ^ { 11 } M _ { \odot } .
The implied conversion ( CO luminosity-to-gas mass ) factor is very high : \alpha _ { CO } = 3.6 \pm 0.8 , consistent with a Galactic conversion factor but four times higher than that of local ultra-luminous IR galaxies that is typically used for high-redshift objects .
The gas mass in these galaxies is comparable to or larger than the stellar mass , and the gas accounts for an impressive 50–65 % of the baryons within the galaxies ’ half light radii .
We are thus witnessing truly gas-dominated galaxies at z \sim 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 .
These galaxies are markedly different than local ULIRGs and high-z submm galaxies and quasars , where higher excitation and more compact gas is found .