2011ApJ...736...37K

We present high angular resolution (0".5-2".0) observations of the millimeter continuum and the ¹²CO(J = 3-2), ¹³CO(J = 3-2), ¹³CO(J = 2-1), C¹⁸O(J = 2-1), HCN(J = 3-2), HCO⁺(J = 4-3), and HCO⁺(J = 3-2) line emission in the circumnuclear disk (r ≲ 100 pc) of the prototypical Seyfert 2 galaxy NGC 1068, carried out with the Submillimeter Array. We also include in our analysis new ¹³CO(J = 1-0) and improved ¹²CO(J = 2-1) observations of NGC 1068 at high angular resolution (1".0-2".0) and sensitivity, conducted with the Institute de Radioastronomie Millimetrique Plateau de Bure Interferometer. Based on the complex dynamics of the molecular gas emission indicating non-circular motions in the central ∼100 pc, we propose a scenario in which part of the molecular gas in the circumnuclear disk of NGC 1068 is blown radially outward as a result of shocks. This shock scenario is further supported by quite warm (T_kin ≥ 200 K) and dense (n(H₂) ≃ 10⁴/cm3) gas constrained from observed molecular line ratios. The HCN abundance in the circumnuclear disk is found to be [HCN]/[¹²CO] ~ 10^–3.5. This is slightly higher than the abundances derived for Galactic and extragalactic star-forming/starbursting regions. This result lends further support to X-ray-enhanced HCN formation in the circumnuclear disk of NGC 1068 as suggested by earlier studies. The HCO⁺ abundance ([HCO⁺]/[¹²CO] ~ 10^–5) appears to be somewhat lower than that of Galactic and extragalactic star-forming/starbursting regions. When trying to fit the centimeter-to-millimeter continuum emission by different thermal and non-thermal processes, it appears that electron-scattered synchrotron emission yields the best results while thermal free-free emission seems to overpredict the millimeter continuum emission.