We report the detection of the CO J = 1 - 0 emission line in three near-infrared selected star-forming galaxies at z \sim 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 \times 10 ^ { 10 } M _ { \sun } , 5.6 \pm 1.4 \times 10 ^ { 10 } M _ { \sun } and 1.23 \pm 0.34 \times 10 ^ { 11 } M _ { \sun } for BzK-4171 , BzK-21000 and BzK-16000 , respectively , assuming a conversion \alpha _ { \mathrm { 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 \sim 100 L _ { \sun } ( K km s ^ { -1 } pc ^ { 2 } ) ^ { -1 } and gas consumption timescales of \sim 0.4 Gyr , unlike SMGs and QSOs at high redshifts .