Giant Molecular Cloud Catalogues for PHANGS-ALMA: Methods and Initial
  Results
  We present improved methods for segmenting CO emission from galaxies into
individual molecular clouds, providing an update to the CPROPS algorithms
presented by Rosolowsky & Leroy (2006; arXiv:astro-ph/0601706 ). The new code
enables both homogenization of the noise and spatial resolution among data,
which allows for rigorous comparative analysis. The code also models the
completeness of the data via false source injection and includes an updated
segmentation approach to better deal with blended emission. These improved
algorithms are implemented in a publicly available python package, PYCPROPS. We
apply these methods to ten of the nearest galaxies in the PHANGS-ALMA survey,
cataloguing CO emission at a common 90 pc resolution and a matched noise level.
We measure the properties of 4986 individual clouds identified in these
targets. We investigate the scaling relations among cloud properties and the
cloud mass distributions in each galaxy. The physical properties of clouds vary
among galaxies, both as a function of galactocentric radius and as a function
of dynamical environment. Overall, the clouds in our target galaxies are
well-described by approximate energy equipartition, although clouds in stellar
bars and galaxy centres show elevated line widths and virial parameters. The
mass distribution of clouds in spiral arms has a typical mass scale that is
2.5x larger than interarm clouds and spiral arms clouds show slightly lower
median virial parameters compared to interarm clouds (1.2 versus 1.4).
