Astron. Astrophys. Suppl. Ser. 144, 271-284

Differences in the dense gas between type 1 and type 2 Seyferts

S.J. Curran

Send offprint request: S. Curran

Onsala Space Observatory, Chalmers University of Technology, S-439 92 Onsala, Sweden
e-mail: sjc@oso.chalmers.se

Received December 1, 1999; accepted February 28, 2000

Abstract:

We have used observations of the CO  $J=1\rightarrow0$ transition in a sample of 20 Seyfert galaxies in order to test the result of Heckman et al. (1989), who find that type 2 Seyferts have a higher molecular gas content than type 1 Seyferts. From our observational results alone, we possibly find that $L_{\rm CO}/L_{\rm FIR}~({\rm Sy2})\approx L_{\rm CO}/L_{\rm FIR}~({\rm Sy1})$ for $L_{\rm FIR}\sim10^{11}L_{\odot}$ Seyferts, but $L_{\rm CO}/L_{\rm FIR}~({\rm Sy2})\approx3L_{\rm CO}/L_{\rm FIR}~({\rm Sy}1)$, when we use all of our sample ( $L_{\rm FIR}\sim10^{10}-10^{11}L_{\odot}$). The results suggest that star-burst activity is most dominant in the ( $L_{\rm FIR}\sim10^{10}L_{\odot}$) type 2 Seyferts, and the relative contribution of this activity to the FIR flux (compared to the AGN) may decrease with increasing $L_{\rm FIR}$.

Also, in order to test whether the Seyfert classes differ in their molecular gas distributions, we have used a simple model to estimate the inclination of the sub-kpc scale molecular ring, and, upon comparison with the observed spectra, conclude that the ring is generally aligned with the galactic disc, with the largest deviations from this being exhibited by the type 1 Seyferts. With regard to the nucleus, for most of the sample, the pc-scale obscuring torus appears to be approximately (within $\approx30^{\circ}$) aligned with the molecular ring for both Seyfert classes.

Key words: galaxies: Seyfert -- galaxies: abundances -- galaxies: structure -- galaxies: evolution