NUCLEAR STAR-FORMING STRUCTURES AND THE STARBURST-ACTIVE GALACTIC NUCLEUS CONNECTION IN BARRED SPIRALS - NGC-3351 AND NGC-4303

A high-resolution Hubble Space Telescope WFPC2 F218W UV image of the b arred spiral NGC 4303 (classified as a LINER-type active galactic nucl eus [AGN]) reveals for the first time the existence of a nuclear spira l structure of massive star-forming regions all the way down to the UV -bright unresolved core (size less than or equal to 8 pc) of an active galaxy. The spiral structure, as traced by the UV-bright star-forming regions, has an outer radius of 225 pc and widens as the distance fro m the core increases. The UV luminosity of NGC 4303 is dominated by th e massive star-forming regions, and the unresolved LINER-type core con tributes only 16% of the integrated UV luminosity. The nature of the U V-bright LINER-type core-stellar cluster or pure AGN-is still unknown. In contrast to NGC 4303, the UV F218W image of the non-AGN barred gal axy NGC 3351 shows a nuclear star-forming ring of 315 pc (semimajor ax is) with a faint core. In the ring, the star formation is arranged in clumps of about 60-85 pc in diameter. Each clump consists of a few com pact UV-bright clusters embedded in a more diffuse component. The inte grated IUE spectrum of NGC 3351 shows the presence of Si IV 1400 Angst rom and C IV 1550 Angstrom absorption lines, typical features of young , 4-5 Myr old, massive star clusters. The presence of ring and spiral star-forming structures in the nuclear regions of these two barred spi rals supports the bar-induced gas-fueling scenario by which bars accum ulate gas in the nuclear regions of galaxies, produce nuclear star-for ming rings (NGC 3351), and might eventually generate or feed an AGN (N GC 4303).