July
1989
•
1989ApJ...342..735H
Authors
•
Heckman, T. M.
•
Blitz, L.
•
Wilson, A. S.
•
Armus, L.
•
Miley, G. K.
Abstract
•
We have used the NRAO 12 m telescope to search for emission in the 115 GHz 1-0 line of CO in a sample of 43 Seyfert galaxies and have detected 18. These galaxies were selected to be bright in the optical, the far-infrared, or both. Together with previously published results, CO data are now available for 55 Seyfert galaxies (26 detections). The CO properties of an optically selected subset of the Seyfert galaxies, namely those 29 Seyferts in the Revised Shapley Ames Catalog (RSA) having Hubble types from S0/a to Sbc, are compared with the CO properties of normal galaxies of the same Hubble type. We find that these RSA type 2 Seyferts have an average ratio of CO-to-blue luminosity that is about twice as large as that of the normal galaxies, but the RSA type 1 Seyferts have normal CO luminosities. We also find that the RSA type 2 Seyfert galaxies (but not the RSA type 1 Seyferts) have an unusually large average ratio of CO luminosity-to-H I mass compared to normal disk galaxies (by a factor >~ 5). The 55 Seyfert galaxies with CO data--as a class--follow the same relationship between CO and far-IR luminosities as do normal and starburst galaxies, suggesting that the far-IR radiation in this sample of Seyferts has the same origin as in normal and starburst galaxies (dust reradiation of starlight). We have then compared the far-infrared luminosities of all the 42 known Seyfert galaxies in the RSA to those of a sample of non-Seyfert galaxies from the RSA, carefully chosen to match the Seyferts in absolute magnitude, distance, and Hubble type. The RSA type 2 Seyferts have an average far-infrared luminosity that is ~4 times larger than the non-Seyfert comparison sample, while the RSA type 1 Seyferts are not significantly more luminous than the non-Seyferts. Thus, type 2 Seyferts may have abnormally large dust and molecular gas contents and high rates of star-formation, while the Seyfert 1 galaxies appear normal. This molecular gas may be related in some indirect way to the nuclear material hypothesized to obscure the broad line region in type 2 Seyferts. The differences in CO and far-IR properties also imply that the two classes of Seyfert galaxies are intrinsically different from one another and that the Seyfert 1 population cannot evolve into the Seyfert 2 population in a time scale less than a few million years (or vice versa). We have also searched for any very broad CO emission lines associated with the high-velocity optical emission-line gas in six Seyfert nuclei. We have been able to put limits on this component that are in some cases a small fraction of the narrower CO emission detected from the galaxy. We find that while Seyfert galaxies exhibit a statistically significant correlation between CO luminosity and nonthermal radio power, they are systematically stronger nonthermal radio sources than non-Seyfert galaxies having the same CO luminosity. We argue that these results support the idea that the radio continuum in Seyferts has a composite origin with both starburst and active galactic nucleus components. We have also compared the CO line widths to the widths of the global H I λ = 21 cm lines, the nuclear [O III] λ5007 lines, and the central stellar velocity dispersions. The CO line widths correlate best with the H I line widths, and the correlation between the CO and [O III] widths improves markedly when the CO (but not the [O III]) line widths are corrected for galaxy inclination. This suggests that the CO is coplanar with the galaxy disk (like the H I), but that the nuclear [O III]-emitting region (the classical narrow line region) is not. The CO versus [O III] line width correlation provides additional evidence for a gravitational component to the dynamics of the narrow line region.
Links
