A Portrait of Cold Gas in Galaxies at 60pc Resolution and a Simple
  Method to Test Hypotheses That Link Small-Scale ISM Structure to Galaxy-Scale
  Processes
  The cloud-scale density, velocity dispersion, and gravitational boundedness
of the interstellar medium (ISM) vary within and among galaxies. In turbulent
models, these properties play key roles in the ability of gas to form stars.
New high fidelity, high resolution surveys offer the prospect to measure these
quantities across galaxies. We present a simple approach to make such
measurements and to test hypotheses that link small-scale gas structure to star
formation and galactic environment. Our calculations capture the key physics of
the Larson scaling relations, and we show good correspondence between our
approach and a traditional "cloud properties" treatment. However, we argue that
our method is preferable in many cases because of its simple, reproducible
characterization of all emission. Using, low-J 12CO data from recent surveys,
we characterize the molecular ISM at 60pc resolution in the Antennae, the Large
Magellanic Cloud, M31, M33, M51, and M74. We report the distributions of
surface density, velocity dispersion, and gravitational boundedness at 60pc
scales and show galaxy-to-galaxy and intra-galaxy variations in each. The
distribution of flux as a function of surface density appears roughly lognormal
with a 1sigma width of ~0.3 dex, though the center of this distribution varies
from galaxy to galaxy. The 60pc resolution line width and molecular gas surface
density correlate well, which is a fundamental behavior expected for virialized
or free-falling gas. Varying the measurement scale for the LMC and M31, we show
that the molecular ISM has higher surface densities, lower line widths, and
more self-gravity at smaller scales.
