COLD MOLECULAR GAS IN MASSIVE, STAR-FORMING 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 similar to 1.5 with the Very Large Array and the Green Bank Telescope. These observations directly trace the bulk of molecular gas in these galaxies. We find H-2 gas masses of 8.3 +/- 1.9 x 10(10) M-circle dot, 5.6 +/- 1.4 x 10(10) M-circle dot, and 1.23 +/- 0.34 x 10(11) M-circle dot for BzK-4171, BzK-21000, and BzK-16000, respectively, assuming a conversion alpha(CO) = 3.6 M-circle dot (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 similar to 100 L-circle dot (K km s(-1) pc(2))(-1), and gas consumption timescales of similar to 0.4 Gyr, unlike submillimeter galaxies and quasi-stellar objects at high redshifts.