Searching for Embedded Super-Star Clusters in IC 4662, NGC 1705, and NGC 5398

Abstract

Radio observations of IC 4662, NGC 5398, and NGC 1705 are used in combination with optical images to search for extremely young star clusters that are still embedded in their birth material. Radio data were obtained at 3 and 6 cm using the Australia Telescope Compact Array, and optical data were retrieved from the Hubble Space Telescope (HST) archive. In IC 4662, three inverted spectrum thermal radio sources and one nonthermal source were detected. The thermal sources have Lyman continuum fluxes ranging from ~20–200 × 1049 s-1 (the equivalent of ~20–200 O7.5-type stars), modeled core radii of r 1 pc, and densities ne 103–104 cm-3. The HST images of this galaxy indicate a number of optical star clusters that are 10 Myr and have masses of ~103–104 M⊙. The young optical clusters and radio clusters in IC 4662 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 NGC 1705 down to a 3 σ level of ~5 × 1024 ergs s-1 Hz-1 (0.17 mJy). At the distance of NGC 1705, the Galactic ultracompact H II region complex W49A would have been a ~5 σ detection; therefore, the lack of detections of any thermal sources down to this detection threshold implies that star formation in NGC 1705 has very nearly ceased. NGC 5398 hosts an impressive thermal radio source that is associated with the giant H II region Tol 89. This source has a Lyman continuum flux of ~4500 × 1049 s-1 (the equivalent of ~4500 O7.5-type stars). At the distance to Tol 89 and the resolution of these radio data, we are only able to set an upper limit on its radius of r 90 pc and a lower limit on its mean density of ne > 50 cm-3.