Clusters, Clouds, and Correlations: Relating Young Clusters to Giant Molecular Clouds in M33 and M31

Abstract

Using young clusters and giant molecular clouds (GMCs), we constrain timescales and distances involved in the star formation process in the Triangulum (M33) and Andromeda (M31) galaxies. The young star clusters come from the PHATTER catalogue, with 1214 clusters in M33 and the PHAT catalogue, with 1249 clusters in M31. These cluster catalogues have the ages of their members measured using colour-magnitude diagram fitting. In M33, we compare these clusters to 444 GMCs identified from an ALMA 12CO(2-1) survey with 35 pc resolution. In M31, we compare the clusters to 251 GMCs identified from a CARMA 12CO(1-0) survey with 20 pc resolution. By comparing the separations between clusters and their nearest GMC, we find that younger clusters are preferentially located near GMCs, while older clusters have separations consistent with a random distribution. Performing two-point correlation on our clusters and GMCs reveals that younger clusters are likely to be found at small separations from other young clusters. In M33, where the cluster catalogue contains younger clusters, we find through cross-correlation analysis that GMCs and clusters ≤10 Myr are statistically associated. This association between GMCs and clusters decreases until the clusters are ≈ 18 Myr, where the clusters and GMCs then appear randomly distributed for older clusters. This transition to a more random distribution can be explained by a simple drift model where the clusters have a velocity dispersion of 5-10 km/s with respect to the surrounding gas. By finding the number of spatially overlapping young clusters and GMCs in M33, we estimate that clusters spend 4-6 Myr emitting feedback into their parent GMC. We then estimate that GMCs in M33 have a total lifetime of 11-15 Myr.