HERSCHEL-PACS OBSERVATIONS OF FAR-IR CO LINE EMISSION IN NGC 1068: HIGHLY EXCITED MOLECULAR GAS IN THE CIRCUMNUCLEAR DISK
- Max-Planck-Institut fuer extraterrestrische Physik, Postfach 1312, D-85741 Garching (Germany)
- Naval Research Laboratory, Remote Sensing Division, 4555 Overlook Ave SW, Washington, DC 20375 (United States)
- Sackler School of Physics and Astronomy, Tel Aviv University, Ramat Aviv 69978 (Israel)
- Departamento de Fisica, Universidad de Alcala de Henares, 28871 Alcala de Henares, Madrid (Spain)
- Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)
- Department of Astrophysics, Oxford University, Oxford OX1 3RH (United Kingdom)
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 J{sub upper} = 14-30 (E{sub upper}/k{sub B} = 580-2565 K) 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{sup -1}. We employ a large velocity gradient model and derive n{sub H2} {approx} 10{sup 5.6} cm{sup -3}, T{sub kin} {approx} 170 K, and M{sub H2} {approx} 10{sup 6.7} M{sub Sun} for the ME component and n{sub H2} {approx} 10{sup 6.4} cm{sup -3}, T{sub kin} {approx} 570 K, and M{sub H2} {approx} 10{sup 5.6} M{sub Sun} for the HE component, although for both components the uncertainties in the density and mass are {+-}(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 H{sub 2} 1-0 S(1) observations. The blueshift of the HE lines may also be consistent with the bluest regions of this H{sub 2} 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. We discuss the prospects of placing the HE component near the AGN and conclude that while the moderate thermal pressure precludes an association with the {approx}1 pc radius H{sub 2}O maser disk, the HE component could potentially be located only a few parsecs more distant from the AGN and might then provide the N{sub H} {approx} 10{sup 25} cm{sup -2} column obscuring the nuclear hard X-rays. Finally, we also report sensitive upper limits extending up to J{sub upper} = 50, which place constraints on a previous model prediction for the CO emission from the X-ray obscuring torus.
- OSTI ID:
- 22039128
- Journal Information:
- Astrophysical Journal, Vol. 755, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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