Evolved Super Star Clusters in M82

Abstract

We present high-resolution optical and near-infrared HST observations of two adjacent regions in the fossil starburst region in M82, B1 and B2. Age estimates date the cluster population in the fossil starburst between ~2 x 10^8 and ~10 x 10^9 years, assuming solar metallicity. The star cluster population in B2 is more heavily affected by internal extinction than that in B1. Although our cluster size estimates indicate that they are gravitationally bound, and not unlike Galactic globular clusters, their luminosity distribution is significantly fainter than both super star cluster luminosity functions (LFs) and the Galactic globular cluster LF. This makes them unlikely proto-globular clusters, since their luminosities will fade further before they are of similar age as the Galactic globular cluster population. If the compact H-alpha-bright sources in M82 B are Type II supernova remnants (SNRs), they set an upper limit to the end of the starburst in region B2, about 500 pc from the galaxy's core, of ~50 Myr. Region B1, about 1000 pc from the core, lacks good SNR candidates and is evidently somewhat older. This suggests star formation in the galaxy has propagated inward toward the present-day intense starburst core.