Nuclear rings in active galaxies

Abstract

We investigate two current problems in active galactic nuclei-the mode of fueling the putative black hole, and the question of whether the circumnuclear regions have experienced unusual chemical processing-by studying the kinematics and chemical abundance of the gas in the inner regions of active galaxies with circumnuclear star formation. We discuss results for three galaxies with nuclear rings: NGC 1097 and NGC 1672, both of which contain a LINER nucleus, and NGC 5248, a galaxy with a ring but no nuclear activity. Narrowband images have been obtained in Hα + [N II], to map out the ionized gas. Medium-dispersion Iong-slit spectroscopy at severa! positions over the nuclear region has been used to obtain the gaseous velocity field and the distribution of excitation. The gaseous kinematics indicate Iarge central mass concentrations in the three galaxies. The two galaxies with LINER nuclei appear to have two inner Lindblad resonances, with the star-forming rings Iocated between them. The high sensitivity of the data has aliowed us to detect faint ionized gas with high [N II]/Hα ratios-a warm ionized medium-up to several kiloparsecs from the nuclei. We find a correlation between the [N II]/Hα and [S II]/Hα ratios and the FWHM of the [N II] λ6584 emission Iine. This correlation is strongest for the circumnuclear gas but is also present in the warm ionized medium of the two galaxies with LINER nuclei, suggesting that shocks contribute to the ionization of the gas. From Iong-slit spectroscopy at Iow dispersion, we have obtained the chemical composition of the HII regions in the ring and far beyond. The highest central metaliicities (0/H≈2 times solar and N/0≈3 times solar) were found around the two LINER nuclei, confirming previous results based on spectroscopy of the narrow-Iine region that LINER nuclei have abundances considerably in excess of solar. Nevertheless, these abundances are similar to those of the nuclei of non-active galaxies with the same morphological type and absolute mag­ nitude.