Circumnuclear Star Formation in Active Galaxies

Abstract

We study the gas kinematics and chemical abundances in the inner regions of six galaxies with active galactic nuclei. The galaxies were selected for having star-forming regions in rings or spirals within a few kiloparsecs of the nucleus. The goal of the project is to search for any connections between the gas-dynamics, the chemical abundances, and the nuclear activity. Narrowband images have been obtained to map out the ionized gas. Medium-dispersion long-slit spectroscopy at several positions over the nuclear region has been used to obtain the gaseous velocity field and distribution of excitation. The H ii regions in the rings are located near the turnover point of the rotation curves, suggesting association with the inner Lindblad resonance. The nuclear ring in the LINER NGC 1326 may be located between the two inner Lindblad resonances, and the inner rings in the Seyfert 2 galaxy NGC 3081 and the LINER/ Seyfert 1 NGC 7213 may be associated with the Ω – κ/4 resonance. A warm ionized medium (WIM) is found to pervade the inner region in all galaxies. There is a correlation between the [N II]/Hα and [S ii]/ Hα ratios and the FWHM of the [N ii] λ6584 emission line in this WIM, indicating that shocks contribute to the ionization of the gas. We show that contamination by this diffuse emission increases the measured [N ii]/Hα and [S ii]/Hα ratios at H ii regions close to the nuclei and thus may account, in part, for the systematically higher [N ii]/Hα and [S ii]/Hα found in the present work and by previous authors for near-nuclear H ii regions compared with H ii regions farther out in the disk of the galaxy. In the present paper, we correct for this contamination by interpolating the contribution of the WIM at the H ii regions and subtracting it off. Oxygen and nitrogen abundances are obtained for 18 H ii regions and are extrapolated to obtain the abundance of the nuclear gas, which is found to range from solar to 2 times solar for oxygen and from 2 to 4 times solar for nitrogen. Such values are similar to those observed in nonactive galaxies with the same luminosity and morphological type and do not support the idea that the central regions of active galaxies have undergone unusual chemical processing.