Spatially extended and high-velocity dispersion molecular component in
  spiral galaxies: single-dish vs. interferometric observations
  Recent studies of the molecular medium in nearby galaxies have provided
mounting evidence that the molecular gas can exist in two phases: one that is
clumpy and organized as molecular clouds and another one that is more diffuse.
This last component has a higher velocity dispersion than the clumpy one. In
order to investigate these two molecular components further, we compare the
fluxes and line widths of CO in NGC 4736 and NGC 5055, two nearby spiral
galaxies for which high-quality interferometric as well as single-dish data
sets are available. Our analysis leads to two main results: 1) Employing three
different methods, we determine the flux recovery of the interferometer as
compared to the single-dish to be within a range of 35-74% for NGC4736 and
81-92% for NGC5055, and 2) when focusing on high (SNR>5) lines of sight, the
single-dish line widths are larger by ~(40+-20)% than the ones derived from
interferometric data; which is in agreement with stacking all lines of sight.
These results point to a molecular gas component that is distributed over
spatial scales larger than 30"(~1kpc), and is therefore filtered out by the
interferometer. The available observations do not allow us to distinguish
between a truly diffuse gas morphology and a uniform distribution of small
clouds that are separated by less than the synthesized beam size (~3" or
~100pc), as they would both be invisible for the interferometer. This high
velocity dispersion component has a dispersion similar to what is found in the
atomic medium, as traced through observations of the HI line.
