Vestige of the Star Cluster Burst in M51
  We present a study of the star cluster formation in M51 based on the image
data taken with Advanced Camera for Surveys (ACS) and Wide Field Planetary
Camera 2 (WFPC2) of Hubble Space Telescope (HST). We have derived the star
cluster formation rate using the ages and masses of about 2,000 star clusters
estimated by comparing photometric data (F336W, F435W, F555W, and F814W) with
theoretical population synthesis models. The star cluster formation rate
increased significantly during the period of 100 ~ 250 Myr ago. This period
roughly coincides with the epoch of dynamical encounters of two galaxies, NGC
5194 and NGC 5195, expected by theoretical models. The age distribution of the
star clusters also shows two peaks at about 100 Myr and 250 Myr ago. The star
cluster mass ranges from 10^3 to 10^6 M_{\odot} and the mass function can be
represented by a power law with an index ranging from \alpha = -2.23 \pm 0.34
for t<10 Myr to \alpha = -1.37 \pm 0.11 for t>100 Myr. The mass function of
star clusters older than 10 Myr also appears to display the steepest
distribution with \alpha \approx -1.50 at around 200 Myr ago, near the expected
epoch of the galaxy interaction. We also confirm the correlations of cluster
size increasing with cluster mass (with a best fit slope of 0.16 \pm 0.02), and
with cluster age (0.14 \pm 0.03).
