A Total Molecular Gas Mass Census in z~ 2-3 Star-forming Galaxies: Low-J CO Excitation Probes of Galaxies' Evolutionary States

Journal article, 2016

We present CO(1-0) observations obtained at the Karl G. Jansky Very Large Array for 14 z similar to 2 galaxies with existing CO(3-2) measurements, including 11 galaxies that. contain active galactic nuclei (AGNs) and three submillimeter galaxies (SMGs). We combine this sample with an additional 15 z similar to 2 galaxies from the literature that have both CO(1-0) and CO(3-2) measurements in order to evaluate differences in CO excitation between SMGs and AGN host galaxies, to. measure the effects of CO excitation on the derived molecular gas properties of these populations, and to look for correlations between the molecular gas excitation and other physical parameters. With our expanded sample of CO(3-2)/CO(1-0) line ratio measurements, we do not find a statistically significant difference in the mean line ratio between SMGs and AGN host galaxies as can be found in the literature; we instead find r(3,1) = 1.03. 0.50 for AGN host galaxies and r(3,1) = 0.78. 0.27 for SMGs (or r(3,1) = 0.90 +/- 0.40 for both populations combined). We also do not measure a statistically significant difference between the distributions of the line ratios for these populations at the p = 0.05 level, although this result is less robust. We find no excitation dependence on the index or offset of the integrated Schmidt-Kennicutt relation for the two CO lines, and we. obtain indices consistent with N = 1 for the various subpopulations. However, including low-z "normal" galaxies increases our best-fit Schmidt-Kennicutt index to N similar to 1.2. While we do not reproduce correlations between the CO line width and luminosity, we do reproduce correlations between CO excitation and star-formation efficiency.