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Recent studies of the molecular medium in nearby galaxies have providedmounting evidence that the molecular gas can exist in two phases: one that isclumpy and organized as molecular clouds and another one that is more diffuse.This last component has a higher velocity dispersion than the clumpy one. Inorder to investigate these two molecular components further, we compare thefluxes and line widths of CO in NGC 4736 and NGC 5055, two nearby spiralgalaxies for which high-quality interferometric as well as single-dish datasets are available. Our analysis leads to two main results: 1) Employing threedifferent methods, we determine the flux recovery of the interferometer ascompared to the single-dish to be within a range of 35-74% for NGC4736 and81-92% for NGC5055, and 2) when focusing on high (SNR>5) lines of sight, thesingle-dish line widths are larger by ~(40+-20)% than the ones derived frominterferometric data; which is in agreement with stacking all lines of sight.These results point to a molecular gas component that is distributed overspatial scales larger than 30"(~1kpc), and is therefore filtered out by theinterferometer. The available observations do not allow us to distinguishbetween a truly diffuse gas morphology and a uniform distribution of smallclouds that are separated by less than the synthesized beam size (~3" or~100pc), as they would both be invisible for the interferometer. This highvelocity dispersion component has a dispersion similar to what is found in theatomic medium, as traced through observations of the HI line.

Spatially extended and high-velocity dispersion molecular component in spiral galaxies: single-dish vs. interferometric observations