NUCLEAR RINGS IN ACTIVE GALAXIES

We investigate two current problems in active galactic nuclei-the mode of fueling the putative black hole, and the question of whether the c ircumnuclear regions have experienced unusual chemical processing-by s tudying the kinematics and chemical abundance of the gas in the inner regions of active galaxies with circumnuclear star formation. We discu ss results for three galaxies with nuclear rings: NGC 1097 and NGC 167 2, both of which contain a LINER nucleus, and NGC 5248, a galaxy with a ring but no nuclear activity. Narrowband images have been obtained i n H alpha + [N II], to map out the ionized gas. Medium-dispersion long -slit spectroscopy at several positions over the nuclear region has be en used to obtain the gaseous velocity field and the distribution of e xcitation. The gaseous kinematics indicate large central mass concentr ations in the three galaxies. The two galaxies with LINER nuclei appea r to have two inner Lindblad resonances, with the star-forming rings l ocated between them. The high sensitivity of the data has allowed us t o detect faint ionized gas with high [N II]/H alpha ratios-a warm ioni zed medium-up to several kiloparsecs from the nuclei. We find a correl ation between the [NII]/H alpha and [S II]/H alpha ratios and the FWHM of the [N II] lambda 6584 emission line. This correlation is stronges t for the circumnuclear gas but is also present in the warm ionized me dium of the two galaxies with LINER nuclei, suggesting that shocks con tribute to the ionization of the gas. From long-slit spectroscopy at l ow dispersion, we have obtained the chemical composition of the H II r egions in the ring and far beyond. The highest central metallicities ( O/H approximate to 2 times solar and N/O approximate to 3 times solar) were found around the two LINER nuclei, confirming previous results b ased on spectroscopy of the narrow-line region that LINER nuclei have abundances considerably in excess of solar. Nevertheless, these abunda nces are similar to those of the nuclei of non-active galaxies with th e same morphological type and absolute magnitude.