The Star Cluster Population of M51: II. Age distribution and relations among the derived parameters

Abstract

We use archival {____it Hubble Space Telescope} observations of broad-band images from the ultraviolet (F255W-filter) through the near infrared (NICMOS F160W-filter) to study the star cluster population of the interacting spiral galaxy M51. We obtain age, mass, extinction, and effective radius estimates for 1152 star clusters in a region of ____sim 7.3 ____times 8.1 kpc centered on the nucleus and extending into the outer spiral arms. In this paper we present the data set and exploit it to determine the age distribution and relationships among the fundamental parameters (i.e. age, mass, effective radius). Using this dataset we find: {____it i}) that the cluster formation rate seems to have had a large increase ____sim 50-70 Myr ago, which is coincident with the suggested {____it second passage} of its companion, NGC 5195, {____it ii}) a large number of extremely young ($<$ 10 Myr) star clusters, which we interpret as a population of unbound clusters of which a large majority will disrupt within the next ____sim10 Myr, and {____it iii)} that the distribution of cluster sizescan be well approximated by a power-law with exponent, -____eta = -2.2 ____pm 0.2, which is very similar to that of Galactic globular clusters, indicating that cluster disruption is largely independent of cluster radius. In addition, we have used this dataset to search for correlations among the derived parameters. In particular, we do not find any strong trends between the age and mass, mass and effective radius, nor between the galactocentric distance and effective radius. There is, however, a strong correlation between the age of a cluster and its extinction, with younger clusters being more heavily reddened than older clusters