1202.5746 (S. Hailey-Dunsheath et al.)

Herschel-PACS Observations of Far-IR CO Line Emission in NGC 1068: Highly Excited Molecular Gas in the Circumnuclear Disk    [PDF]

S. Hailey-Dunsheath, E. Sturm, J. Fischer, A. Sternberg, J. Graciá-Carpio, R. Davies, E. González-Alfonso, D. Mark, A. Poglitsch, A. Contursi, R. Genzel, D. Lutz, L. Tacconi, S. Veilleux, A. Verma, J. A. de Jong

We report the detection of far-IR CO rotational emission from the prototypical Seyfert 2 galaxy NGC 1068. Using Herschel-PACS, we have detected 10 transitions in the J_upper=14-24 (E_upper/k_B = 580-1656 K) range, all of which are consistent with arising from within the central 10" (700 pc). The detected transitions are modeled as arising from 2 different components: a moderate excitation (ME) component close to the galaxy systemic velocity, and a high excitation (HE) component that is blueshifted by ~70 km s^{-1}. We employ a large velocity gradient (LVG) model and derive n_H2~10^{5.7} cm^{-3}, T_kin~150 K, and M_H2~10^{6.9} M_sun for the ME component, and n_H2~10^{6.3} cm^{-3}, T_kin~440 K, and M_H2~10^{5.8} M_sun for the HE component, although for both components the uncertainties in the density and mass are ~plus/minus 1 dex. Both components arise from denser and possibly warmer gas than traced by low-J CO transitions, and the ME component likely makes a significant contribution to the mass budget in the nuclear region. We compare the CO line profiles with those of other molecular tracers observed at higher spatial and spectral resolution, and find that the ME transitions are consistent with these lines arising in the ~200 pc diameter ring of material traced by H_2 1-0 S(1) observations. The blueshift of the HE lines may also be consistent with the bluest regions of this H_2 ring, but a better kinematic match is found with a clump of infalling gas ~40 pc north of the AGN. We consider potential heating mechanisms, and conclude that X-ray or shock heating of both components is viable, while far-UV heating is unlikely. We also report sensitive upper limits extending up to J_upper=50, which place constraints on the emission from the X-ray obscuring medium.

View original: http://arxiv.org/abs/1202.5746