Oral Presentation

ALMA reveals a rotating dense molecular torus in NGC 1068

Author(s): Masatoshi Imanishi

Presenter: Masatoshi Imanishi (NAOJ)

The unification paradigm of AGNs postulates that a dusty molecular torus is present around a mass accreting supermassive black hole (SMBH), in order to naturally explain the presence of type-1 AGNs (which show broad optical emission lines) and type-2 AGNs (which do not). This paradigm was proposed from the observation of the nearby type-2 AGN, NGC 1068. However, our observational understanding of the torus of AGNs has been incomplete, even for the prototypical AGN NGC 1068, simply because the torus is spatially compact, say <10 pc in physical scale or <0.15" at the distance of >10 Mpc. We present the results of our very high-spatial-resolution (0.04 x 0.07") ALMA observations of the NGC 1068 nucleus in the HCN J=3-2 and HCO+ J=3-2 emission lines, which are expected to probe dense molecular gas in the torus. In NGC 1068, the optical [OIII] emission and radio jet are elongated along the north-south direction from the mass-accreting SMBH. Thus, the putative torus in NGC 1068 is expected to be aligned roughly along the east-west direction. Previous ALMA high spatial resolution CO J=6-5 observations by two independent groups found north-south oriented dynamical properties, which are difficult to interpret within the torus paradigm and are possibly contaminated by outflowing molecular gas along the polar direction of the torus. In the HCN J=3-2 and HCO+ J=3-2 lines, we have for the first time clearly detected the dense molecular gas emission oriented along the east-west direction both morphologically and dynamically. Our observational results conform to the classical torus picture in NGC 1068.

We also found that the torus dense molecular gas properties are more complicated than those predicted by the simple classical torus picture. Namely, (1) molecular emission from the torus is not axi-symmetric, (2) the dense molecular gas in the torus is counter-rotating with respect to that outside the torus in the host galaxy, and (3) the rotation speed is significantly slower than that expected from a pure Keplerian motion governed by the central SMBH with an estimated mass of ten million solar mass. We present these observational results and future prospect to better understand the nature of the torus in NGC 1068.