We present a study of compact star clusters in the nearby pair of interacting galaxies NGC 5194/95 ( M51 ) , based on multifilter Hubble Space Telescope WFPC2 archival images .
We have detected \sim 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 4–6 ) of clusters younger than \sim 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 100–500 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 \sim 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 Myr 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 \sim 3–4 pc .
Additionally , we see correlations of ( increasing ) cluster size with cluster mass ( with a best fit slope of 0.14 \pm 0.03 ) at the \sim 4 \sigma level , and with cluster age ( 0.06 \pm 0.02 ) at the 3 \sigma 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 , effective radii > 7 pc , and are scattered over the disk of NGC 5195 .
Our results indicate that NGC 5195 is therefore the third known barred lenticular galaxy to have formed so-called “ faint fuzzy ” star clusters .