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 ~ 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 H2 gas masses of 8.3 ± 1.9 × 1010 M ☉, 5.6 ± 1.4 × 1010 M ☉, and 1.23 ± 0.34 × 1011 M ☉ for BzK-4171, BzK-21000, and BzK-16000, respectively, assuming a conversion αCO = 3.6 M ☉ (K km s–1 pc2)–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+0.35 –0.22, 1.22+0.61 –0.36, and 0.41+0.23 –0.13 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+0.28 –0.19. This suggests that, on average, the gas in these galaxies is thermalized up to J = 2, has star formation efficiencies of ~100 L ☉ (K km s–1 pc2)–1, and gas consumption timescales of ~0.4 Gyr, unlike submillimeter galaxies and quasi-stellar objects at high redshifts.