A Massive Molecular Torus inside a Gas-Poor Cirnumnuclear Disk in the Radio Galaxy NGC 1052 Discovered with ALMA

Abstract

We report ALMA observations of NGC 1052 to quest mass accretion in a gas-poor active galactic nucleus (AGN). We detected CO emission representing a rotating ring-like circumnuclear disk (CND) seen edge-on with the gas mass of $5.3\times 10^5$ M${}_{\odot }$. The CND has smaller gas mass than that in typical Seyfert galaxies with circumnuclear star formation and is too gas-poor to drive mass accretion onto the central engine. The continuum emission casts molecular absorption features of CO, HCN, HCO${}^{+}$, SO, SO${}_2$, CS, CN, and H${}_2$O, with H${}^{13}$CN and HC${}^{15}$N and vibrationally-excited (v${}_2=1$) HCN and HCO${}^{+}$. Broader absorption line widths than CND emission line widths imply presence of a geometrically thick molecular torus with a radius of $2.4\pm 1.3$ pc and a thickness ratio of $0.7\pm 0.3$. We estimate the H${}_2$ column density of $(3.3\pm 0.7)\times 10^{25}$ cm${}^{-2}$ using H${}^{12}$CN, H${}^{13}$CN, and HCO${}^{+}$ absorption features and adopting abundance ratio of ${}^{12}$C-to-${}^{13}$C and a HCO${}^{+}$-to-H${}_2$, and derived the torus gas mass of $(1.3\pm 0.3)\times 10^7$ M${}_{\odot }$, which is $\sim 9$\% of the central black-hole mass. The molecular gas in the torus is clumpy with the estimated covering factor of $0.17_{-0.03}^{+0.06}$. The gas density of clumps inside the torus is inferred to be $(6.4\pm 1.3)\times 10^7$ cm${}^{-3}$, which meets the excitation conditions of H${}_2$O maser. Specific angular momentum in the torus exceeds a flat-rotation curve extrapolated from that of the CND, indicating a Keplerian rotation inside a 14.4-pc sphere of influence.

Accepted for publication in ApJ, 27 pages, 13 figures, 7 tables