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'' or 17 pc at the target. We resolved the molecular gas into a large number of compact clouds, with masses ranging from similar to 2 x 10(3) to 2 x 10(6) M-circle dot. The mass spectrum scales as N(M) proportional to M-1.5 +/- 0.1, similar to the mass spectra of young massive star clusters, suggesting that individual molecular clouds are transformed in the starburst into individual star clusters. The larger clouds are surrounded by supernovae and H (II) regions, suggesting that star formation proceeds from the outside of the clouds and progresses inward, consistent with triggering by a sudden increase in external pressure. The clouds with internal star formation have velocity gradients and inverse P Cygni spectral line profiles indicating inward motions of 35 km s(-1), consistent with shock-driven compression. Diffuse free-free radio emission and X-ray emission around the clouds provide evidence for superheated ionized gas sufficient to drive the compression. Clouds with spectral lines indicating expansion show little internal star formation, suggesting that the dynamics precedes and is responsible for the star formation rather than the inverse. M82 is known to be in interaction with the neighboring galaxy M81. The overall picture is consistent with the formation of massive star clusters from individual giant molecular clouds crushed by a sudden galactic-scale increase in external pressure generated by the changing dynamics that result from a near-collision with a neighboring galaxy. Present-day globular clusters may have formed in a similar fashion in primordial galaxies.