M82, Starbursts, Star Clusters, and the Formation of Globular Clusters

We observed the nearby starburst galaxy M82 in CO in the higher frequency(2--1) transition to achieve an angular resolution below 1 arc second or 17 pcat the target. We resolved the molecular gas into a large number of compactclouds, with masses ranging from about 2x10^3 to 2x10^6 solar masses. The massspectrum scales as N(M) ~ M^-1.5, similar to the mass spectra of young massivestar clusters suggesting that individual molecular clouds are transformed inthe starburst into individual star clusters. The larger clouds are surroundedby supernovae and HII regions suggesting that star formation proceeds from theoutside of the clouds and progresses inward consistent with triggering by asudden increase in external pressure. The clouds with internal star formationhave velocity gradients and inverse P-Cygni spectral line profiles indicatinginward motions of 35 kms consistent with shock driven compression. Diffusefree-free radio emission and X-ray emission around the clouds provides evidencefor superheated ionized gas sufficient to drive the compression. Clouds withspectral lines indicating expansion show little internal star formationsuggesting that the dynamics precedes and is responsible for the star formationrather than the inverse. M82 is known to be in interaction with neighboringM81. The overall picture is consistent with the formation of massive starclusters from individual giant molecular clouds crushed by a sudden galacticscale increase in external pressure generated by the changing dynamics thatresult from a near-collision with a neighboring galaxy. Present day globularclusters may have formed in a similar fashion in primordial galaxies.Comment: Submitted to Ap