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.

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