Molecular Gas, Morphology, and Seyfert Galaxy Activity

Abstract

We probe the cause of the elevated star formation in host galaxies of Seyfert 2 nuclei compared with Seyfert 1 hosts and with field galaxies.12CO (1-0) observations of a large sample of Seyfert galaxies indicate no significant difference in the total amount of molecular gas as a function of the Seyfert nuclear type, nor are Seyfert galaxies significantly different in this regard from a sample of field galaxies once selection effects are accounted for. Therefore, the total amount of molecular gas is not responsible for the enhanced star-forming activity in Seyfert 2 hosts. To probe how this gas is being converted more efficiently into stars in Seyfert 2 hosts than in the other galaxies, we investigate the occurrence of bars, interactions, and distorted morphologies among Seyfert galaxies. We find a significantly higher rate of asymmetric morphologies for Seyfert 2 galaxies with respect to Seyfert 1 galaxies and field galaxies. Relative to field galaxies, the effect is at a greater than 99.9% confidence level. The presence of asymmetric morphologies in individual Seyfert galaxies is correlated with their tendency to exhibit enhanced star-forming activity. These results suggest that asymmetric morphologies are an important cause for the link between Seyfert type and star-forming activity: bars and distortions in Seyfert 2 hosts are likely both to enhance star-forming activity and to funnel gas into the nuclear region, thus obscuring and possibly contributing to the feeding of the active nucleus.