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

Abstract

We report ALMA observations of NGC 1052 to search for mass accretion in a gas-poor active galactic nucleus. We detected CO emission representing a rotating ring-like circumnuclear disk (CND) seen edge-on with a gas mass of 5.3 × 10⁵ 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, SO₂, CS, CN, and H₂O, with H¹³CN and HC¹⁵N and vibrationally excited (V₂ = 1) HCN and HCO⁺. Broader absorption line widths than CND emission-line widths imply the 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 obtain an H₂ column density of (3.3 ± 0.7) × 10²⁵ cm^-2 using H¹²CN, H¹³CN, and HCO⁺ absorption features and adopting abundance ratios of ¹²C to ¹³C and HCO⁺ to H₂, and we derived a torus gas mass of (1.3 ± 0.3) × 10⁷ M_⊙, which is ∼9% of the central black hole mass. The molecular gas in the torus is clumpy, with an estimated covering factor of ${0.17}_{-0.03}^{+0.06}$ . The gas density of the clumps inside the torus is inferred to be (6.4 ± 1.3) × 10⁷ cm^-3, which meets the excitation conditions for an H₂O maser. The specific angular momentum in the torus exceeds the flat rotation curve extrapolated from that of the CND, indicating a Keplerian rotation inside a 14.4 pc sphere of influence.