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K.E. Johnson (U. Wisconsin/NRAO), R. Indebetouw (U.Wisconsin), D.J. Pisano (ATNF)
Radio observations of IC4662, NGC5398, and NGC1705 are used in combination with optical images to search for extremely young star clusters which are still embedded in their birth material. Radio data were obtained at 3cm and 6cm using ATCA, and optical data were obtained at SSO and retrieved from the HST archive. In IC4662, three inverted spectrum thermal radio sources and one non-thermal source were detected. These sources have Lyman continuum fluxes ranging from ~20-200 \times 1049s-1 (the equivalent of ~20-200 O7-type stars), modeled core radii of r < 1pc, and densities ne > 103 - 104cm-3. The HST images of this galaxy indicate a number of optical star clusters which are < 10Myr which have masses of ~103 - 104 M\odot. The young optical clusters and radio clusters in IC4662 are spatially exclusive --- supporting either large amounts of extinction in the vicinity of the radio clusters or sequential star formation. No thermal sources are detected in NGC1705 down to a 3\sigma level of ~5 \times 1024erg s-1 Hz-1. At the distance of NGC1705, the Galactic ultracompact HII region complex W49A would have been a >5\sigma detection; therefore the lack of detections of any thermal sources implies that star formation in NGC1705 has very nearly shut off. NGC5398 hosts an extremely impressive thermal radio source which is associated with the giant HII region Tol~89. This source has a Lyman continuum flux of ~4500 \times 1049s-1 (the equivalent of ~4500 O7-type stars). At the distance to Tololo 89 and the resolution of these radio data, we are only able to put an upper limit on its radius of r ~90pc and a lower limit on its density of ne > 50cm-3. We gratefully acknowledge support for this work from the National Science Foundation.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.