On the Disappearance of a Cold Molecular Torus around the Low-luminosity Active Galactic Nucleus of NGC 1097
Abstract
We used the Atacama Large Millimeter/Submillimeter Array to map the CO(3-2) and the underlying continuum emissions around the type-1 low-luminosity active galactic nucleus (LLAGN; bolometric luminosity ≲ {10}42 erg s-1) of NGC 1097 at ∼10 pc resolution. These observations revealed a detailed cold gas distribution within a ∼100 pc of this LLAGN. In contrast to the luminous Seyfert galaxy NGC 1068, where a ∼7 pc cold molecular torus was recently revealed, a distinctively dense and compact torus is missing in our CO(3-2) integrated intensity map of NGC 1097. Based on the CO(3-2) flux, the gas mass of the torus of NGC 1097 would be a factor of ≳2-3 less than that found for NGC 1068 by using the same CO-to-H2 conversion factor, which implies less active nuclear star formation and/or inflows in NGC 1097. Our dynamical modeling of the CO(3-2) velocity field implies that the cold molecular gas is concentrated in a thin layer as compared to the hot gas traced by the 2.12 μm H2 emission in and around the torus. Furthermore, we suggest that NGC 1097 hosts a geometrically thinner torus than NGC 1068. Although the physical origin of the torus thickness remains unclear, our observations support a theoretical prediction that geometrically thick tori with high opacity will become deficient as AGNs evolve from luminous Seyferts to LLAGNs.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- August 2017
- DOI:
- 10.3847/2041-8213/aa808f
- arXiv:
- arXiv:1707.05806
- Bibcode:
- 2017ApJ...845L...5I
- Keywords:
-
- galaxies: active;
- galaxies: individual: NGC 1097;
- galaxies: ISM;
- galaxies: Seyfert;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 9 pages, 5 figures. Accepted for publication in ApJL