On the disappearance of a cold molecular torus around the low-luminosity active galactic nucleus of NGC 1097

TL;DR

This study used ALMA to map cold gas around the low-luminosity AGN in NGC 1097, revealing the absence of a dense, thick molecular torus unlike in more luminous Seyfert galaxies, supporting theories of torus evolution.

Contribution

First detailed ALMA observations of cold gas in a LLAGN showing the disappearance of a dense molecular torus compared to luminous Seyferts.

Findings

No dense, compact torus detected in NGC 1097.

The gas mass of the torus is significantly less than in NGC 1068.

NGC 1097 hosts a geometrically thinner torus.

Abstract

We used the Atacama Large Millimeter/submillimeter Array (ALMA) to map the CO(3-2) and the underlying continuum emissions around the type 1 low-luminosity active galactic nucleus (LLAGN; bolometric luminosity $\lesssim 10^{42}$ erg~s$^{-1}$) of NGC 1097 at $\sim 10$ pc resolution. These observations revealed a detailed cold gas distribution within a $\sim 100$ pc of this LLAGN. In contrast to the luminous Seyfert galaxy NGC 1068, where a $\sim 7$ 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 $\gtrsim 2-3$ less than that found for NGC 1068 by using the same CO-to-H$_2$ 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 $\mu$m H$_2$ 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.