Gas Kinematics of Super Stellar Clusters from Brackett Line Emission

Many galaxies, including our own Milky Way, are surrounded by large collections of stars called globular clusters. These gravitationally-bound star clusters have very high stellar densities, ~10⁶ stars within a light year's radius, a million times higher than in our own solar neighborhood. How did these massive star clusters form? Unfortunately, the globular clusters of the Milky Way are all more than 10 billion years of age, so the evidence of their formation is long gone. However, it is possible that extreme star forming regions seen in other nearby galaxies consist of young super star clusters (SSCs) that may be the precursors to globular clusters. Presented here is high spectral resolution Brackett line spectroscopy of several galaxies containing suspected SSCs. Echelle spectra data was collected using the NIRSPEC (Near Infrared Spectrograph) instrument on the Keck Telescope for the hydrogen recombination lines, Brackett α and γ at λ = 4.05 μm, 2.17 μm respectively. Using custom IDL (Interactive Data Language) procedures, Gaussian profiles are fit to the spectra to determine the velocity dispersion and thus the bulk motions of the hot, ionized gas surrounding these young clusters. We find that extreme star forming regions are diverse, occurring in both compact and extended regions, as well as in a variety of galaxy types, and that the Brackett photon fluxes imply large numbers of massive young stars. The line profiles suggest that we are seeing clusters at different stages in the process of the dispersal of their natal gas.