2008ApJ...679..404M

We present the results obtained from an objective search for stellar clusters, both in the currently active nuclear starburst region, and in the poststarburst disk of M82. Images obtained with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST) in F435W (B), F555W (V), and F814W (I) filters were used in the search for the clusters. We detected 653 clusters, of which 393 are located outside the central 450 pc in the poststarburst disk of M82. The luminosity function of the detected clusters shows an apparent turnover at B=22 mag (M_B=-5.8), which we interpret from Monte Carlo simulations as due to incompleteness in the detection of faint clusters, rather than an intrinsic lognormal distribution. We derived a photometric mass of every detected cluster from models of simple stellar populations assuming a mean age of either 8 (nuclear clusters) or 100 (disk clusters) million years old. The mass functions of the disk (older) and the nuclear (younger) clusters follow power laws, the former being marginally flatter (α=1.5±0.1) than the latter (α=1.8±0.1). The distribution of sizes (FWHM) of clusters brighter than the apparent turnover magnitude (mass ≳2x10⁴ M_☉) can be described by a lognormal function. This function peaks at 10 pc for clusters more massive than 10⁵ M_☉, whereas for lower masses, the peak is marginally shifted to larger values for the younger and smaller values for the older clusters. The observed trend toward flattening of the mass function with age, together with an overabundance of older compact clusters, imply that cluster disruption in M82 is both dependent on the mass and size of the clusters.