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We resolve 182 individual giant molecular clouds (GMCs) larger than 2. 5 × 10⁵ M ☉ in the inner disks of 5 large nearby spiral galaxies (NGC 2403, NGC 3031, NGC 4736, NGC 4826, and NGC 6946) to create the largest such sample of extragalactic GMCs within galaxies analogous to the Milky Way. Using a conservatively chosen sample of GMCs most likely to adhere to the virial assumption, we measure cloud sizes, velocity dispersions, and ^ (12) CO (J = 1-0) luminosities and calculate cloud virial masses. The average conversion factor from CO flux to H₂ mass (or X_ (CO) ) for each galaxy is 1-2 × 10^ (20) cm^ (–2) (K km s^ (–1) ) ^ (–1), all within a factor of two of the Milky Way disk value (~2 × 10^ (20) cm^ (–2) (K km s^ (–1) ) ^ (–1) ). We find GMCs to be generally consistent within our errors between the galaxies and with Milky Way disk GMCs; the intrinsic scatter between clouds is of order a factor of two. Consistent with previous studies in the Local Group, we find a linear relationship between cloud virial mass and CO luminosity, supporting the assumption that the clouds in this GMC sample are gravitationally bound. We do not detect a significant population of GMCs with elevated velocity dispersions for their sizes, as has been detected in the Galactic center. Though the range of metallicities probed in this study is narrow, the average conversion factors of these galaxies will serve to anchor the high metallicity end of metallicity-X_ (CO) trends measured using conversion factors in resolved clouds; this has been previously possible primarily with Milky Way measurements.
Donovan et al. (Fri,) studied this question.