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

TL;DR

This study uses CO observations of z~2 galaxies to compare molecular gas excitation in different galaxy types, finding no significant differences and exploring implications for galaxy evolution and star formation.

Contribution

It provides the first combined analysis of CO(1-0) and CO(3-2) line ratios across diverse galaxy populations at z~2, clarifying excitation differences and their impact on gas property estimates.

Findings

No significant difference in CO excitation between SMGs and AGN host galaxies.

Reproduction of CO excitation and star formation efficiency correlations.

Schmidt-Kennicutt index consistent with N=1, increasing to ~1.2 with low-z galaxies included.

Abstract

We present CO(1-0) observations obtained at the Karl G. Jansky Very Large Array (VLA) for 14 z~2 galaxies with existing CO(3-2) measurements, including 11 galaxies which contain active galactic nuclei (AGN) and three submillimeter galaxies (SMGs). We combine this sample with an additional 15 z~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, 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 found in the literature, instead finding 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 obtain indices consistent with N=1 for the various sub-populations. However, including low-z "normal" galaxies increases our best-fit Schmidt-Kennicutt index to N~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.