Abstract

Encoded in the 21-cm HI line are key properties of molecular cloud evolution, including the mechanisms of molecular cloud formation, the structure of cloud envelopes, and the signatures of stellar feedback that destroy molecular clouds. Observationally characterizing these processes requires a large uniform sample of molecular clouds, mapping galactic scales, with sufficient resolution to resolve individual molecular clouds (<100 pc).  These requirements make the Local Group galaxies (<1 Mpc) the only targets where current telescopes can map both the atomic (HI) and molecular (CO) ISM. In this talk, I will present a detailed analysis of HI and CO in M33, the nearest moderately-inclined spiral galaxy, on 80 pc scales. Our analysis has two key results: (i) M33 has only marginal evidence for a thick diffuse molecular disc, unlike other nearby galaxies; and (ii) the line-of-sight HI and CO line widths are strongly correlated but still show intrinsic scatter. This scatter arises from regional variations, yet does not change with the GMC evolutionary state. I will discuss ongoing work to characterize the source of regional variations in the line width relation, including the search for spatial variations in turbulent properties and using higher resolution observations.