Hubble Space Telescope Observations of Circumnuclear Star-Forming Rings in NGC 1097 and NGC 6951

We have obtained new and archival Hubble Space Telescope V-band images of kiloparsec-sized circumnuclear star-forming rings in the barred spiral galaxies NGC 1097 and NGC 6951, both of which contain active nuclei. The images resolve the rings into two-armed spirals composed of bright knots located on the outer edges of prominent dust lanes. The two-armed ring morphology seen in these two galaxies appears to be common in barred spirals and is in accord with the results of simulations of bar-driven inflow. In both galaxies, circumnuclear star formation is tightly confined to the ring: in the region between the ring and the nucleus there is an intricate spiral pattern of narrow dust lanes, but no bright knots are visible. Miniature gaseous or stellar bars interior to nuclear rings have been suggested as mechanisms for transporting gas to active nuclei, but it is not clear from the optical morphology whether either of these mechanisms may be at work in these two galaxies. The young star clusters in the rings are extremely compact, with mean effective radii of 2.5 pc in NGC 1097 and <= 4 pc in NGC 6951. Without correcting for extinction, the clusters have absolute V magnitudes of up to - 12.6 mag; with a highly uncertain extinction correction the brightest clusters have luminosities of M_v_ = - 14 to - 15 mag. Such high luminosities and small radii indicate that these objects are examples of "super star clusters" which may be newly formed globular clusters like those found recently in merging galaxies. The centers of barred galaxies may therefore be common sites for the present-day formation of globular clusters. The image of NGC 1097 also contains the type II supernova 1992bd 12 days prior to its discovery in ground-based observations.