The Stellar Content of Henize 2‐10 from Space Telescope Imaging Spectrograph Ultraviolet Spectroscopy

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Abstract
We use new Space Telescope Imaging Spectrograph long-slit ultraviolet spectroscopy of region A in the nearby starbursting galaxy He 2-10 to examine properties of both compact individual super-star clusters and diffuse intracluster regions. The four most luminous clusters in our slit formed coevally 4-5 Myr ago and have lower mass estimates of several × 104 to a few × 105 M☉. A fifth cluster is located ~200 pc away, and appears to be several megayears older. Extracted spectra of the diffuse light appear to be remarkably similar to those of the young clusters, showing P Cygni profiles from massive stars, and are very well fitted by instantaneous burst models of 4-5 Myr. Various mechanisms are considered to determine the nature of the underlying diffuse light in He 2-10. We rule out scattering of cluster light as the dominant source of the observed field. A scenario including the formation of a large number of lower mass, undetectable but coeval (compact) clusters can explain the field spectra, assuming that the clusters in the center of region A follow a power-law luminosity distribution, L-α, with α = -2.0. This is somewhat steeper than the power-law slope of -1.7 measured by Johnson et al. for clusters in a larger portion of He 2-10 and implies very high projected (compact) cluster densities within region A. Rather, we favor a scenario where the dominant contribution to the field stellar population comes from more diffuse "scaled OB associations," which are not lacking in the most massive stars. Our observations establish that the far-UV field light in He 2-10 originates in a different stellar population than found for NGC 5253 and other nearby starbursts, where older, dissolving clusters appear to be the main mechanism responsible for creating the diffuse UV light.

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