The Fossil Starburst in M82

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Abstract
We present high-resolution HST imaging in the optical (WFPC2) and near-infrared (NICMOS) of a disk region 1 kpc northeast 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 interpretation. M82 thus presents us with the opportunity to observe both active and evolved starburst environments at close range. The surface brightness of M82 B is well above normal for galactic disks and comparable to the core surface brightnesses in spiral galaxies. Its intrinsic surface brightness at an age of 10 Myr was comparable to that found in the present-day nuclear starburst, indicating an event of comparable amplitude. We find a large, evolved system of super star clusters in M82 B. Using size as a criterion to distinguish cluster candidates from point sources, we identify a total of 113 super star cluster candidates. We use a two-color BVI diagram and evolutionary spectral synthesis models to separately estimate the extinction and age of each cluster. The clusters range in absolute magnitude from M = -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, which presumably induced the starburst. 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 α = -1.2 ± 0.3, similar to that of other young cluster systems in interacting galaxies. There is tentative evidence for broadening of the luminosity function due to dynamical destruction of lower mass clusters. Cluster sizes (2.34 ≤ Reff 10 pc, or 2.4 Rcore 7.9 pc) and estimated masses (a median of 105 M⊙) 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.