HUBBLE-SPACE-TELESCOPE ULTRAVIOLET IMAGES OF 5 CIRCUMNUCLEAR STAR-FORMING RINGS

We present UV (similar to 2300 Angstrom) images, obtained with the Hub ble Space Telescope (HST) Faint Object Camera, of the central 20'' of five galaxies containing circumnuclear star-forming rings. The five ga laxies are from a well-defined sample of 103 normal, nearby galaxies w e have observed with HST. At the HST resolution (0.05''), the rings br eak up into discrete star-forming clumps. Each clump is composed of ma ny 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 cl usters. These objects are similar to those that have been recently rep orted in colliding and starburst galaxies, and in several other circum nuclear rings. A large fraction, 15%-50%, of the UV emission originate s in these compact clusters. Compact clusters therefore may be the pre ferred mode of star formation in starburst environments. For one galax y, NGC 2997, we measure the UV-optical colors of the individual cluste rs using an archival HST WFPC2 image at similar to 6000 Angstrom. Comp aring the colors and luminosities to starburst population synthesis mo dels, we show that the clusters are less than 100 Myr old and have mas ses 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 masse s and the ages of the young clusters indicate that these objects will remain bound and evolve into globular clusters. However, data in addit ional wavebands are needed to critically test this hypothesis. The lum inosity function of the clusters in the rings is similar in shape to t hose 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 h as an active nucleus at its center, arguing against a direct correspon dence between circumnuclear star formation and nuclear activity. (C) 1 996 American Astronomical Society.