The Nature of Starburst Activity in M821

We present new evolutionary synthesis models of M82 based mainly onobservations consisting of near-infrared integral field spectroscopy andmid-infrared spectroscopy. The models incorporate stellar evolution, spectralsynthesis, and photoionization modeling, and are optimized for 1-45 micronobservations of starburst galaxies. The data allow us to model the starburstregions on scales as small as 25 pc. We investigate the initial mass function(IMF) of the stars and constrain quantitatively the spatial and temporalevolution of starburst activity in M82. We find a typical decay timescale forindividual burst sites of a few million years. The data are consistent with theformation of very massive stars (> 50-100 Msun) and require a flattening of thestarburst IMF below a few solar masses assuming a Salpeter slope at highermasses. Our results are well matched by a scenario in which the globalstarburst activity in M82 occurred in two successive episodes each lasting afew million years, peaking about 10 and 5 Myr ago. The first episode took placethroughout the central regions of M82 and was particularly intense at thenucleus while the second episode occurred predominantly in a circumnuclear ringand along the stellar bar. We interpret this sequence as resulting from thegravitational interaction M82 and its neighbour M81, and subsequent bar-drivenevolution. The short burst duration on all spatial scales indicates strongnegative feedback effects of starburst activity, both locally and globally.Simple energetics considerations suggest the collective mechanical energyreleased by massive stars was able to rapidly inhibit star formation after theonset of each episode.Comment: 48 pages, incl. 16 Postscript figures; accepted for publication in the Astrophysical Journa