Wide-field HST/ACS images of M81: The Population of Compact Star
  Clusters
  We study the population of compact stellar clusters (CSCs) in M81, using the
HST/ACS images in the filters F435W, F606W and F814W covering, for the first
time, the entire optical extent of the galaxy. Our sample contains 435 clusters
of FWHM less than 10 ACS pixels (9 pc). The sample shows the presence of two
cluster populations, a blue group of 263 objects brighter than B=22 mag, and a
red group of 172 objects, brighter than B=24 mag. Based on the analysis of
colour magnitude diagrams and making use of simple stellar population models,
we find the blue clusters are younger than 300 Myr with some clusters as young
as few Myr, and the red clusters are as old as globular clusters. The
luminosity function of the blue group follows a power-law distribution with an
index of 2.0, typical value for young CSCs in other galaxies. The power-law
shows unmistakable signs of truncation at I=18.0 mag (M_I=-9.8 mag), which
would correspond to a mass-limit of 4x10^4 M_solar if the brightest clusters
are younger than 10 Myr. The red clusters have photometric masses between 10^5
to 2x10^7 M_solar for the adopted age of 5 Gyr and their luminosity function
resembles very much the globular cluster luminosity function in the Milky Way.
The brightest GC in M81 has M_B^0=-10.3 mag, which is ~0.9 mag brighter than
w-Cen, the most massive GC in the Milky Way.
