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

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We report the detection of far-IR CO rotational emission from the prototypical Seyfert 2 galaxy NGC 1068. Using Herschel-PACS, we have detected 11 transitions in the Jupper 14-30 EupperkappaB 580-2565 Kappa range, all of which are consistent with arising from within the central 10 700 pc. The detected transitions are modeled as arising from two different components a moderate-excitation ME component close to the galaxy systemic velocity and a high-excitation HE component that is blueshifted by approx. 80 km s-1. We employ a large velocity gradient model and derive EtaH2 approx. 105.6 cm-3, Tkin approx. 170 K, and MH2 approx. 106.7 M for the ME component and etaH2 approx. 106.4 cm-3, Tkin approx. 570 K, and MH2 approx. 105.6 M for the HE component, although for both components the uncertainties in the density and mass are plus or minus 0.6-0.9 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 approx. 200 pc diameter ring of material traced by H2 1-0 S1 observations. The blueshift of the HE lines may also be consistent with the bluest regions of this H2 ring, but a better kinematic match is found with a clump of infalling gas approx. 40 pc north of the active galactic nucleus 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.

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  • Astronomy
  • Physical Chemistry
  • Atomic and Molecular Physics and Spectroscopy
  • Nuclear Physics and Elementary Particle Physics

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