2020ApJ...896L..21R

Using the NSF's Karl G. Jansky Very Large Array (VLA), we report six detections of CO(J = 1 - 0) emission and one upper limit in z = 2-3 galaxies originally detected in higher-J CO emission in the Atacama Large Millimeter/submillimeter Array Spectroscopic Survey in the Hubble Ultra Deep Field (ASPECS). From the CO(J = 1 - 0) line strengths, we measure total cold molecular gas masses of M_gas = (2.4-11.6) x 10¹⁰ (α_CO/3.6)M_☉. We also measure a median CO(J = 3 - 2) to CO(J = 1 - 0) line brightness temperature ratio of r₃₁ = 0.84 ± 0.26, and a CO(J = 7 - 6) to CO(J = 1 - 0) ratio range of r₇₁ < 0.05 to r₇₁ = 0.17. These results suggest that CO(J = 3 - 2) selected galaxies may have a higher CO line excitation on average than CO(J = 1 - 0) selected galaxies, based on the limited, currently available samples from the ASPECS and VLA CO Luminosity Density at High Redshift (COLDz) surveys. This implies that previous estimates of the cosmic density of cold gas in galaxies based on CO(J = 3 - 2) measurements should be revised down by a factor of ≃2 on average based on assumptions regarding CO excitation alone. This correction further improves the agreement between the best currently existing constraints on the cold gas density evolution across cosmic history from line scan surveys, and the implied characteristic gas depletion times.