VLA-ALMA Spectroscopic Survey in the Hubble Ultra Deep Field (VLASPECS):
  Total Cold Gas Masses and CO Line Ratios for z=2-3 "Main Sequence" Galaxies
  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
submillimeter/Millimeter Array (ALMA) 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 10^10 (alpha_CO/3.6) Msun. We also
measure a median CO(J=3-2) to CO(J=1-0) line brightness temperature ratio of
r_31 = 0.84 +/- 0.26, and a CO(J=7-6) to CO(J=1-0) ratio range of r_71 <0.05 to
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.
