Cloud-scale molecular gas properties in 15 nearby galaxies

Abstract

We measure the velocity dispersion, sigma, and surface density, Sigma, of the molecular gas in nearby galaxies from CO spectral line cubes with spatial resolution 45-120. pc, matched to the size of individual giant molecular clouds. Combining 11 galaxies from the PHANGS-ALMA survey with four targets from the literature, we characterize similar to 30,000 independent sightlines where CO is detected at good significance. Sigma and sigma show a strong positive correlation, with the best-fit power-law slope close to the expected value for resolved, self-gravitating clouds. This indicates only a weak variation in the virial parameter alpha(vir) proportional to sigma(2)/Sigma, which is similar to 1.5-3.0 for most galaxies. We do, however, observe enormous variation in the internal turbulent pressure P-turb proportional to Sigma sigma(2), which spans similar to 5 dex across our sample. We find Sigma, sigma , and P-turb to be systematically larger in more massive galaxies. The same quantities appear enhanced in the central kiloparsec of strongly barred galaxies relative to their disks. Based on sensitive maps of M31 and M33, the slope of the sigma-Sigma relation flattens at Sigma less than or similar to 10M(circle dot) pc(-2), leading to high s for a given S and high apparent avir. This echoes results found in the Milky Way and likely originates from a combination of lower beam-filling factors and a stronger influence of local environment on the dynamical state of molecular gas in the low-density regime.