Star formation in starburst galaxies is concentrated in dense, massive knots of stars called super star clusters (SSCs). These clusters may represent the building blocks of galaxies, and determining their properties is key to understanding the formation, enrichment and evolution of galaxies. SSCs are coeval stellar populations with sufficient stars to sample extensively 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.
The nuclear starburst in M82 contains roughly two dozen SSCs that are prominent in the near-infrared, with typical ages of ∼10 7 years. The light of these young SSCs is dominated by red supergiant stars. I use high-spectral-resolution near-infrared Keck/NIRSPEC spectroscopy to measure the stellar velocity dispersions of these clusters, and find line-of-sight dispersions of σ ∼ 7--35 km s -1 . From King model fits to light profiles in HST/NICMOS images, I determine that the clusters are typically 1--3 pc in radius, and measure the photometry and luminosities of the SSCs. I apply the virial theorem to derive masses for 15 SSCs in the nuclear starburst; all lie in the range of 10 5 to 10 7 solar masses. This dataset represents the largest study to date of SSC virial masses in a single galaxy.
The nature of the ultraluminous x-ray (ULX) sources that are closely associated with star formation and SSCs is the cause of much current debate. One possibility is that ULXs are accreting intermediate-mass black holes (IMBHs). Formation theories for IMBHs in SSCs focus on the role of stellar collisions and mergers in the dense environs of a cluster core. The key question is whether a runaway star of sufficient mass can form via collisions before the population of massive stars is depleted by supernova explosions. I identify several M82 clusters with dynamical friction timescales of less than 5 Myr; high-spatial-resolution x-ray observations of these clusters may uncover additional compact x-ray sources. (Abstract shortened by UMI.)
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