The Fossil Starburst in M82
  (Abridged) We present high-resolution HST imaging in the optical (WFPC2) and near-infrared (NICMOS) of a disk region 1 kpc NE of the starburst core in the nearby galaxy M82. This region, M82 ``B,'' has been suspected to be a fossil starburst site in which an intense episode of star formation occurred over 100 Myr ago, and our new observations confirm this intepretation. We find a large, evolved system of super star clusters in M82 B; we identify a total of 113 super star cluster candidates. The clusters range in absolute magnitude from M_V^0 = -6 to -10, with a peak at -7.5. The derived age distribution suggests steady, continuing cluster formation at a modest rate at early times (> 2 Gyr ago), followed by a concentrated formation episode ~600 Myr ago and more recent suppression of cluster formation. The peak episode coincides with independent dynamical estimates for the last tidal encounter with M81. Our J and H band observations resolve the bright giant population in M82's disk for the first time. Star formation evidently continued in M82 B until about 20-30 Myr ago, but none is found associated with the youngest generations in the nuclear starburst (age <= 15 Myr). After correcting the cluster luminosity function to a fiducial age of 50 Myr, we find that the bright end is characterized by a power-law slope with alpha = -1.2 +/- 0.3. Cluster sizes (2.34 <= Reff <= 10 pc, or 2.4 <= Rcore <= 7.9 pc) and estimated masses (a median of 10^5 Msun) are consistent with values found for young super star cluster populations in M82's core and other galaxies and with the progenitors of globular clusters. 
Richard de Grijs (1), Robert W. O'Connell (1), and John S. Gallagher,   III (2) ((1) Astronomy Department, University of Vir...