Properties of Resolved Star Clusters in M51

Abstract

We present a study of compact star clusters in the nearby pair of interacting galaxies NGC 5194/5195 (M51), based on multifilter Hubble Space Telescope (HST) WFPC2 archival images. We have detected ~400 isolated, resolved clusters in five HST WFPC2 fields of the two-galaxy system. Due to our requirement that the clusters be detected based only on their morphology, which results in the selection of relatively isolated objects, we estimate that we are missing the majority (by a factor of 4?6) of clusters younger than ~10?Myr due to the extreme crowding in the spiral arms and star-forming regions. Hence, we focus on the cluster population older than 10?Myr. An age distribution of the detected clusters shows a broad peak between 100 and 500?Myr, which is consistent with the crossing times of the companion galaxy NGC?5195 through the NGC?5194 disk estimated in both single- and multiple-passage dynamical models. We estimate that the peak contains ~2.2?2.5 times more clusters than expected from a constant rate of cluster formation over this time interval. While there is also evidence for individual peaks near 100 and 500?Myr in the cluster age distribution (consistent with the predictions of multiple-passage models), this result requires verification. We estimate the effective radii of our sample clusters and find a median value of ~3?4 pc. In addition, we see correlations of (increasing) cluster size with cluster mass (with a best-fit slope of 0.14 ? 0.03) at the ~4 ? level, and with cluster age (0.06 ? 0.02) at the 3 ? level. Finally, we report for the first time the discovery of faint, extended star clusters in the companion, NGC?5195, an SB0 galaxy. These have red [(V - I) > 1.0] colors and effective radii >7?pc, and are scattered over the disk of NGC?5195. Our results therefore indicate that NGC?5195 is currently the third known barred lenticular galaxy to have formed so-called faint fuzzy star clusters.