Hubble Space Telescope ultraviolet images of five circumnuclear star-forming rings

We present UV (similar to 2300 Angstrom) images, obtained with the Hubble Space Telescope (HST) Faint Object Camera, of the central 20'' of five galaxies containing circumnuclear star-forming rings. The five galaxies are from a well-defined sample of 103 normal, nearby galaxies we have observed with HST. At the HST resolution (0.05''), the rings break up into discrete star-forming clumps. Each clump is composed of many luminous (L(lambda)(2300 Angstrom)approximate to 10(35)-10(37) erg s(-1) Angstrom(-1)) and compact (R less than or equal to 5 pc) star clusters. These objects are similar to those that have been recently reported in colliding and starburst galaxies, and in several other circumnuclear rings. A large fraction, 15%-50%, of the UV emission originates in these compact clusters. Compact clusters therefore may be the preferred mode of star formation in starburst environments. For one galaxy, NGC 2997, we measure the UV-optical colors of the individual clusters using an archival HST WFPC2 image at similar to 6000 Angstrom. Comparing the colors and luminosities to starburst population synthesis models, we show that the clusters are less than 100 Myr old and have masses of at least a few 10(3)M(circle dot), with some as high as 10(5)M(circle dot). The UV extinction to those clusters that are detected in the UV is at most a factor of 10. In NGC 2997, the limits on the masses and the ages of the young clusters indicate that these objects will remain bound and evolve into globular clusters. However, data in additional wavebands are needed to critically test this hypothesis. The luminosity function of the clusters in the rings is similar in shape to those measured for super stat clusters in other star-forming galaxies, and extends to luminosities lower by several orders of magnitude. All five of the UV-detected circumnuclear rings occur in barred or weakly barred spiral galaxies of type Sc or earlier. None of the five rings has an active nucleus at its center, arguing against a direct correspondence between circumnuclear star formation and nuclear activity. (C) 1996 American Astronomical Society.