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The Properties of Super Star Clusters in A Sample of Starburst Galaxies

TitleThe Properties of Super Star Clusters in A Sample of Starburst Galaxies
Publication TypeThesis
Year of Publication2001
AuthorsJohnson, KE

“Super star clusters” are the most massive extreme in the continuum of young star clusters. In this thesis, I examine the properties of such super star clusters in a sample of starburst galaxies with space and ground-based observations and in the optical, mid-infrared, and radio regimes. Using optical photometry, I estimate the ages and masses, as well as construct luminosity functions for the super star cluster systems. Additional Hα observations allow me to place tighter constraints on the burst ages and trace very recent star formation. The super star clusters detected in these galaxies typically have estimated ages ≲ 103-6 M⊙, and luminosity functions consistent with other super star cluster systems with a slope of α ≈ -2 (φ(L)∝L α)).

Next I discuss an even earlier stage of massive star cluster evolution, when super star clusters are still embedded in their birth material. I overview the discovery of “ultra dense HII regions” (UDH IIs) with radio and mid-infrared observations. From the radio observations, I calculate the electron densities, radii, and number of ionizing photons (and therefore number of embedded massive stars). The mid-infrared observations confirm the presence of hot dust cocoons surrounding these objects. These embedded clusters account for at least ~60% of the mid- to far-infrared flux of He 2-10. I also discuss the impact of UDHIIs on the radio to far-infrared flux ratio.

Finally, I present a sample of 35 embedded star formation regions (ranging from the size of small OB-associations to super star clusters) serendipitously detected in nearby galaxies. This sample of objects begins to fill in the continuum of cluster masses between individual UCHII regions and the embedded massive clusters.