Using the NSF ’ s Karl G. Jansky Very Large Array ( VLA ) , we report six detections of CO ( J =1 \to 0 ) emission and one upper limit in z =2–3 galaxies originally detected in higher- J CO emission in the Atacama Large submillimeter/Millimeter Array ( ALMA ) Spectroscopic Survey in the Hubble Ultra Deep Field ( ASPECS ) .
From the CO ( J =1 \to 0 ) line strengths , we measure total cold molecular gas masses of M _ { gas } =2.4–11.6 \times 10 ^ { 10 } ( \alpha _ { CO } /3.6 ) M _ { \odot } .
We also measure a median CO ( J =3 \to 2 ) to CO ( J =1 \to 0 ) line brightness temperature ratio of r _ { 31 } =0.84 \pm 0.26 , and a CO ( J =7 \to 6 ) to CO ( J =1 \to 0 ) ratio range of r _ { 71 } < 0.05 to 0.17 .
These results suggest that CO ( J =3 \to 2 ) selected galaxies may have a higher CO line excitation on average than CO ( J =1 \to 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 \to 2 ) measurements should be revised down by a factor of \simeq 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 .