Submillimeter array/Plateau de bure interferometer multiple line observations of the nearby seyfert 2 galaxy NGC 1068: Shock-related gas kinematics and heating in the central 100 pc?

Abstract

We present high angular resolution (05-20) 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 ≲ 100pc) of the prototypical Seyfert 2 galaxy NGC1068, 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 NGC1068 at high angular resolution (10-20) 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 100pc, we propose a scenario in which part of the molecular gas in the circumnuclear disk of NGC1068 is blown radially outward as a result of shocks. This shock scenario is further supported by quite warm (Tkin ≥ 200K) and dense (n(H2)≃ 104cm-3) 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 NGC1068 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. © 2011. The American Astronomical Society. All rights reserved.