Nuclear Activity in Circumnuclear Ring Galaxies

Abstract

We have analyzed the frequency and properties of the nuclear activity in a sample of galaxies with circumnuclear rings and spirals (CNRs), compiled from published data. From the properties of this sample a typical circumnuclear ring can be characterized as having a median radius of 0.7 kpc (mean 0.8 kpc, rms 0.4 kpc), located at a spiral Sa/Sb galaxy (75% of the hosts), with a bar (44% weak, 37% strong bars). The sample includes 73 emission line rings, 12 dust rings and 9 stellar rings. The sample was compared with a carefully matched control sample of galaxies with very similar global properties but without detected circumnuclear rings. We discuss the relevance of the results in regard to the AGN feeding processes and present the following results: 1) bright companion galaxies seem not to be important for the appearance of CNRs, which appear to be more related to intrinsic properties of the host galaxies or to minor merger processes; 2) the proportion of weak bars in galaxies with a CNR is higher than expected; 3) the incidence of Seyfert (Sy) activity coeval with CNRs is significantly larger than the rate expected from the morphological distribution of the host galaxies; 4) the rate of Sy 2 to Sy 1 type galaxies with CNRs is about three times larger than the expected ratio for galaxies without CNRs and is opposite to that predicted by the geometric paradigm of the classical unified model for AGNs, although it does support the hypothesis that Sy 2 activity is linked to circumnuclear star formation. The possible selection effects of the sample are discussed, and we conclude that the detected trends are strong enough to justify high quality observations of as large as possible set of galaxies with circumnuclear rings and their matched control samples.