Molecular Gas in Seyfert Galaxies
Doctoral thesis, 2000
In this report we present the results of a single dish study of the molecular gas properties in Seyfert galaxies. From the observations of the closest lying example, the Circinus galaxy, we find that about half of the molecular gas is distributed in a circumnuclear ring orbiting the central super-massive black hole, believed to be powering this type 2 Seyfert. The 100 pc-scale ring orbits with non-Keplerian rotation at a maximum velocity of > 180 km s-1. From this we deduce that the NH2/ICO Galactic conversion ratio causes us to overestimate the gas mass fraction by a factor of > 5. From further studies, the remainder of the gas appears to be in a 90° wide outflow which extends to ±500 pc along the rotation axis of the ring. The maximum outflow velocity of < ±190 km s-1 gives a mechanical luminosity of >107Lsun and the orientation of the outflow axis suggests that the molecular ring is very nearly coplanar with the central body obscuring the active galactic nucleus.
By observing a sample of 20 Seyfert galaxies, we find that the HCN to CO luminosity ratio, LHCN/ LCO > 1/6, is considerably higher than for normal spiral galaxies and comparable to the ratios found in ultra-luminous infra-red galaxies (ULIRGs). Our observations suggest that there may be differences in the molecular gas distributions between low (LFIR ~ 1010 Lsun) and high (LFIR ~ 1011 Lsun) far infra-red luminous (FIR) Seyferts. By combining our results with the FIR luminosities, we find that LFIR/ LCO (LFIR ~ 1011 Lsun Seyferts) > 10LFIR/ LCO (normal spirals) and LFIR/ LHCN (Seyferts) > LFIR/ LHCN (ULIRGs) > LFIR/ LHCN (normal spirals). From this we cannot ascertain the relative radiation contribution of the star-burst and the active nucleus, although between NGC 4945 and Circinus, in which we have performed multi-transitional observations, these seem to differ. This may have the effect of lowering current estimates of the star formation rates in Seyfert galaxies, although both galaxies do appear to have relatively high populations (c.f. the Galaxy) of massive (>10Msun) stars. Finally, estimates of the gas inclinations in these 20 Seyferts appear to suggest that the various gas disk structures on all scales are usually aligned in each galaxy, supporting the notion that the obscuration may be but a dense component of the large-scale gas structure.
interstellar medium molecules