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

TL;DR

This study compares molecular gas properties in two similar galaxies with different star cluster formation rates, revealing that galaxy morphology influences molecular cloud dynamics and star formation efficiency.

Contribution

It provides the first detailed comparison of molecular cloud properties in barred and flocculent spiral galaxies at high resolution, linking cloud dynamics to star cluster formation.

Findings

NGC 1313 has higher cloud kinetic energies than NGC 7793.

More virialized clouds are present in NGC 1313, correlating with higher massive cluster formation.

Molecular cloud distributions are similar, but NGC 1313 has higher surface densities and lower free-fall times.

Abstract

We present a comparative study of the molecular gas in two galaxies from the LEGUS sample: barred spiral NGC 1313 and flocculent spiral NGC 7793. These two galaxies have similar masses, metallicities, and star formation rates, but NGC 1313 is forming significantly more massive star clusters than NGC 7793, especially young massive clusters (<10 Myr, >10^4 Msol). Using ALMA CO(2-1) observations of the two galaxies with the same sensitivities and resolutions of 13 pc, we directly compare the molecular gas in these two similar galaxies to determine the physical conditions responsible for their large disparity in cluster formation. By fitting size-linewidth relations for the clouds in each galaxy, we find that NGC 1313 has a higher intercept than NGC 7793, implying that its clouds have higher kinetic energies at a given size scale. NGC 1313 also has more clouds near virial equilibrium than NGC 7793, which may be connected to its higher rate of massive cluster formation. However, these virially bound clouds do not show a stronger correlation with young clusters than that of the general cloud population. We find surprisingly small differences between the distributions of molecular cloud populations in the two galaxies, though the largest of those differences are that NGC 1313 has higher surface densities and lower free-fall times.