Searching for Compact Star Clusters in M81 Using HST/ACS Images

We study the stellar cluster population in M81, using the HST/ACS images in the filters F435W, F606W and F814W that cover a total field of view of approximately 140 square arcmin. We present details about the selection criteria, which were based both on morphological and photometrical features. The extracted sample of stellar clusters shows the presence of two cluster populations, a blue cluster group (young) with 560 objects, and a red cluster group (old) with 120 objects. The young group lacks clusters more massive than 10000 solar masses, that are present in large numbers in its neighbor M82. The luminosity distribution function of the young group follows a power-law distribution with an index of 2.0, whereas that for the red group resembles very much the globular cluster luminosity function in the Milky Way. Assuming an age of 5 Gyr, these red clusters have masses between 0.1 to 10 million solar masses.