Wide-field HST/ACS images of M81: the population of compact star clusters

Abstract

We study the population of compact star clusters (CSCs) in M81, using the Hubble Space Telescope/Advanced Camera for Surveys (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 full width at half-maximum less than 10 ACS pixels (9pc). The sample shows the presence of two cluster populations, a blue group of 263 objects brighter than B=22mag, and a red group of 172 objects, brighter than B=24mag. On the basis of analysis of colour-magnitude diagrams and making use of simple stellar population models, we find the blue clusters are younger than 300Myr with some clusters as young as few Myr, and the red clusters are as old as globular clusters (GCs). 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=18.0mag (M_I=-9.8mag), which would correspond to a mass limit of if the brightest clusters are younger than 10Myr. The red clusters have photometric masses between 10⁵ and for the adopted age of 5Gyr and their luminosity function resembles very much the GC luminosity function in the Milky Way. The brightest GC in M81 has M⁰_B=-10.3mag, which is ~0.9mag brighter than Cen, the most massive GC in the Milky Way.