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

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 × 10^5 M_⊙. 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_2O, with H^13CN and HC^15N 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 ± 1.3 pc and a thickness ratio of 0.7 ± 0.3. We estimate the H_2 column density of (3.3 ± 0.7) × 10^25 cm^-2 using H^12CN, H^13CN, and HCO^+ absorption features and adopting abundance ratio of ^12C-to-^13C and a HCO^+-to-H_2, and derived the torus gas mass of (1.3 ± 0.3) × 10^7 M_⊙, which is ∼ 9% of the central black-hole mass. The molecular gas in the torus is clumpy with the estimated covering factor of 0.17^+0.06_-0.03. The gas density of clumps inside the torus is inferred to be (6.4 ± 1.3) × 10^7 cm^-3, which meets the excitation conditions of H_2O 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.