ALMA-LEGUS. I. The Influence of Galaxy Morphology on Molecular Cloud Properties

We present a comparative study of the molecular gas in two galaxies from the Legacy ExtraGalactic UV Survey(LEGUS)sample: barred spiral NGC 1313 andflocculent spiral NGC 7793. These two galaxies have similarmasses, metallicities, and star formation rates, but NGC 1313 is forming significantly more massive star clustersthan NGC 7793, especially young massive clusters(<10 Myr,>10(4)M(circle star)). Using Atacama Large Millimeter/submillimeter Array(ALMA)CO(2-1)observations of the two galaxies with the same sensitivity and resolution(13 pc), we directly compare the molecular gas in these two similar galaxies to determine the physical conditionsresponsible for their large disparity in cluster formation. Byfitting size-line width relations for the clouds in eachgalaxy, wefind that NGC 1313 has a higher intercept than NGC 7793, implying that its clouds have higher kineticenergies at a given size scale. NGC 1313 also has more clouds near virial equilibrium than NGC 7793, which maybe connected to its higher rate of massive cluster formation. However, these virially bound clouds do not show astronger correlation with young clusters than with the general cloud population. Wefind surprisingly smalldifferences between the distributions of molecular cloud populations in the two galaxies, though the largest ofthose differences is that NGC 1313 has higher surface densities and lower freefall times.