Tidally - induced Super Star Clusters in M 82

Using new HST imaging, we identify a large, evolved system of super star clusters in a disk region just outside the starburst core in the prototypical starburst galaxy M82, " M82 B. " This region has been suspected to be a fossil starburst site in which an intense episode of star formation occurred over 100 Myr ago, which is now confirmed by our derived age distribution. It 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. 1. M82, the prototypical starburst galaxy Observations from radio to X-rays show evidence for a tidally-induced starburst in the center of M82 (e.g., Telesco 1988) In fact, there is now evidence that M82 has undergone multiple episodes of intense star formation (cf. Marcum & O'Connell 1996, de Grijs, O'Connell & Gallagher 2001). " M82 B, " the fossil starburst region, has exactly the properties of an evolved starburst with a similar amplitude to the active burst (Marcum & O'Connell 1996). Thus, M82 is a unique starburst galaxy, since no other galaxy offers the opportunity to study two discrete starbursts at such close range! By analogy with the HST results from the active starburst region (O'Connell et al. 1994, 1995), we expected M82 B to have contained a complement of luminous super star clusters. It is possible that most of the star formation in starbursts takes place in the form of such concentrated clusters; in M82, we do not observe any outside the starburst regions. The combination of observations of both the active and the fossil starburst sites in M82 provides a unique physical environment for the study of the stellar and dynamical evolution of star cluster systems 1