2010ApJ...718..177A

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±1.9x10¹⁰ M_☉, 5.6±1.4x10¹⁰ M_☉, and 1.23±0.34x10¹¹ M_☉ for BzK-4171, BzK-21000, and BzK-16000, respectively, assuming a conversion α_CO = 3.6 M_☉(K km/s 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.pc²)^–1, and gas consumption timescales of ∼0.4 Gyr, unlike submillimeter galaxies and quasi-stellar objects at high redshifts.