M82, Starbursts, Star Clusters, and the Formation of Globular Clusters.pdf
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M82, Starbursts, Star Clusters, and the formation of Globular
Clusters
Eric Keto1, Luis C. Ho2, and K.-Y. Lo3
ABSTRACT
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 pc at
the target. We resolved the molecular gas into a large number of compact clouds,
with masses ranging from ～ 2 × 103 to 2 × 106 M⊙. The mass spectrum scales
as N(M) ∝ 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 HII
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 kms?1 con-
sistent with shock driven compression. Diffuse free-free radio emission and X-ray
emission around the clouds provides evidence for superheated ionized gas suffi-
cient 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 neighboring 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 chang-
ing 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.
Subject headings: galaxies: individual (M82) — galaxies: ISM — galaxies: star-
burst — galaxies: star clusters — ISM: clouds
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