Cold molecular gas in massive disk galaxies at z=1.5
  We report the detection of the CO J=1-0 emission line in three near-infrared
selected star-forming galaxies at z~1.5 with the Very Large Array (VLA) and the
Green Bank telescope (GBT). These observations directly trace the bulk of
molecular gas in these galaxies. We find H_2 gas masses of 8.3 \pm 1.9 x
10^{10} M_sun, 5.6 \pm 1.4 x 10^{10} M_sun and 1.23 \pm 0.34 x 10^{11} M_sun
for BzK-4171, BzK-21000 and BzK-16000, respectively, assuming a conversion
alpha_CO=3.6 M_sun (K km s^{-1} pc^{2})^{-1}. We combined our observations with
previous CO 2-1 detections of these galaxies to study the properties of their
molecular gas. We find brightness temperature ratios between the CO 2-1 and CO
1-0 emission lines of 0.80_{-0.22}^{+0.35}, 1.22_{-0.36}^{+0.61} and
0.41_{-0.13}^{+0.23} for BzK-4171, BzK-21000 and BzK-16000, respectively. At
the depth of our observations it is not possible to discern between
thermodynamic equilibrium or sub-thermal excitation of the molecular gas at
J=2. However, the low temperature ratio found for BzK-16000 suggests
sub-thermal excitation of CO already at J=2. For BzK-21000, a Large Velocity
Gradient model of its CO emission confirms previous results of the
low-excitation of the molecular gas at J=3. From a stacked map of the CO 1-0
images, we measure a CO 2-1 to CO 1-0 brightness temperature ratio of
0.92_{-0.19}^{+0.28}. This suggests that, on average, the gas in these galaxies
is thermalized up to J=2, has star-formation efficiencies of ~100 L_sun (K km
s^{-1} pc^2)^{-1} and gas consumption timescales of ~0.4 Gyr, unlike SMGs and
QSOs at high redshifts.
