Cloud-Scale Molecular Gas Properties in 15 Nearby Galaxies
  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 $4$
targets from the literature, we characterize ${\sim}30,000$ 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
weak variation in the virial parameter
$\alpha_\mathrm{vir}\propto\sigma^2/\Sigma$, which is ${\sim}1.5$-$3.0$ for
most galaxies. We do, however, observe enormous variation in the internal
turbulent pressure $P_\mathrm{turb}\propto\Sigma\,\sigma^2$, which spans
${\sim}5\rm\;dex$ across our sample. We find $\Sigma$, $\sigma$, and
$P_\mathrm{turb}$ to be systematically larger in more massive galaxies. The
same quantities appear enhanced in the central kpc 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
$\Sigma\lesssim10\rm\;M_\odot\,pc^{-2}$, leading to high $\sigma$ for a given
$\Sigma$ and high apparent $\alpha_\mathrm{vir}$. 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.
