AAS 205th Meeting, 9-13 January 2005
Session 85 Star Clusters and Associations
Oral, Tuesday, January 11, 2005, 2:00-3:30pm, Royal Palm 1-3

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[85.05] The Super Star Cluster Population of the M82 Nuclear Starburst: Masses, Luminosities and the IMF

N. McCrady (UC Berkeley)

Star formation in starburst galaxies is concentrated in dense, massive knots of stars called super star clusters (SSCs). These clusters are thought to be the building blocks of galaxies, and determining their properties is crucial to understanding the formation, enrichment and evolution of galaxies. SSCs are coeval stellar populations with sufficient stars to sample the cluster initial mass function (IMF). Critical to characterization of the IMF is the detection of low-mass stars, the light of which is swamped by high luminosity supergiants. Measurement of the kinematic mass of a cluster represents the only means of detecting and quantifying the contribution of low-mass stars.

The nuclear starburst in M82 is host to over 20 infrared-bright SSCs. I use high-resolution near-infrared Keck/NIRSPEC echelle spectroscopy to measure the stellar velocity dispersions of these clusters. The SSCs are resolved in HST NICMOS and ACS images, from which I measure halflight radii and integrated luminosities. I calculate virial masses for the SSCs, typically 105 to 106 solar masses. Comparison of the observed light-to-mass ratios and population synthesis models enables me to constrain the IMF for individual clusters and demonstrates apparent variations within a single starburst galaxy. I present evidence for mass segregation despite the young cluster ages (typically 10-50 Myr), and discuss implications for interpretation of the IMF. The mass and luminosity distribution functions for the population bear on the question of whether the clusters represent a young globular cluster population. With a sample of over 20 clusters, this dataset represents the largest study of SSCs in a single galactic environment, providing a strong test of environmental dependency of the IMF. This work has been supported by NSF Grant AST--0205999.

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