Abstract:
With an infrared luminosity of 3x1010 L⊙ within its central part, M82 is a prime example of a starburst. Due to its proximity, 3.2 Mpc, it is possible to investigate in details its structure (1"=16pc). Oriented almost edge-on it can be seen that the starburst region is the base of a prominent outflow which feeds its halo. The starburst region containing dense gas and dust the observations at optical and near infrared wavelengths suffer extinction. In contrast using radio recombination lines (RRL) it is possible to probe the ionized gas in depth within the starburst, the source of the ionization originating from the presence of numerous massive young stars. At millimeter wavelengths the RRLs trace the dense component of the plasma. Anantharamiah et al. suggested that, in the case of the starburst Arp220, the millimeter RRL emission comes from numerous ultra-compact HII regions (UCHII); this has stringent consequences to estimate the star formation rate (SFR) and recent history (SFH): the present day SFR would have been sustained since several Myr or alternatively been occurring in multiple events of short duration but at extremely high SFR. This result is only suggestive given the uncertainties. I'll present observations of M82 at H40alpha which give a direct evidence that the RRLs observed at high frequencies are associated to the dense components of the ISM. I'll show that not only the UCHII regions contribute to the RRL emission but also PDRs, the skin of the dense molecular clumps which are exposed to an intense and hard ambient radiation field. Adopting such a morphology this has important consequences to determine the SFR and SFH. The lifetime of the UCHII phase may have been significantly underestimated and that would explain the observed artificially low excitation of the plasma and the near correspondence between the spatial distribution of the dense plasma and of the supernova remnants.
Coffee and tea will be served 20 minutes prior to the seminar.