- 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 pc at the target.
We resolved the molecular gas into a large number of compact clouds,
with masses ranging from 2 x 10^3 to 2 x 10^6 solar masses.
The mass spectrum N(M) scales as 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.
with internal star formation have velocity gradients and inverse P-Cygni spectral line
profiles indicating inward motions of 35 kms consistent
with shock driven compression.
Diffuse free-free radio emission
and X-ray emission around the clouds provides evidence for
superheated ionized gas sufficient to drive the compression.
Clouds with spectral lines indicating expansion
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
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
Eric Keto, Luis C. Ho & K.-Y. Lo, 2005, ApJ, 635, 1062
- The Mid-Infrared Fine Structure Lines of Neon as an Indicator of Star Formation Rate in Galaxies
The fine-structure lines of singly (NeII 12.8 micron) and doubly (NeIII
15.6 micron) ionized neon are among the most prominent features in the
mid-infrared spectra of star-forming regions, and have the potential to be
a powerful new indicator of the star formation rate in galaxies. Using a
sample of star-forming galaxies with measurements of the fine-structure lines
available from the literature, we show that the sum of the NeII and NeIII
luminosities obeys a tight, linear correlation with the total infrared
luminosity, over 5 orders of magnitude in luminosity. We discuss the
formation of the lines and their relation with the Lyman continuum
luminosity. A simple calibration between star formation rate and the
NeII + NeIII luminosity is presented.
Luis C. Ho & Eric Keto, 2007, ApJ, 658, 314