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Published October 1, 2015 | Published + Submitted
Journal Article Open

A Deep Herschel/Pacs Observation of CO(40-39) in NGC 1068: A Search for the Molecular Torus


Emission from high-J CO lines in galaxies has long been proposed as a tracer of X-ray dominated regions (XDRs) produced by active galactic nuclei (AGNs). Of particular interest is the question of whether the obscuring torus, which is required by AGN unification models, can be observed via high-J CO cooling lines. Here we report on the analysis of a deep Herschel/PACS observation of an extremely high-J CO transition (40-39) in the Seyfert 2 galaxy NGC 1068. The line was not detected, with a derived 3σ upper limit of 2 x 10^(-17) W m^(-2. We apply an XDR model in order to investigate whether the upper limit constrains the properties of a molecular torus in NGC 1068. The XDR model predicts the CO spectral line energy distributions for various gas densities and illuminating X-ray fluxes. In our model, the CO(40-39) upper limit is matched by gas with densities of ~10^6-10^7 cm^(-3), located at 1.6–5 pc from the AGN, with column densities of at least 10^(25) cm^(-2). At such high column densities, however, dust absorbs most of the CO(40-39) line emission at λ = 65.69 µm. Therefore, even if NGC 1068 has a molecular torus that radiates in the CO(40-39) line, the dust can attenuate the line emission to below the PACS detection limit. The upper limit is thus consistent with the existence of a molecular torus in NGC 1068. In general, we expect that the CO(40-39) is observable in only a few AGN nuclei (if at all), because of the required high gas column density, and absorption by dust.

Additional Information

© 2015 American Astronomical Society. Received 2015 April 8; accepted 2015 August 12; published 2015 September 24. We thank the anonymous referee for helpful comments that improved the paper. We thank Rowin Meijerink for providing us with an extended XDR grid. Basic research in infrared astronomy at NRL is funded by the US ONR; J.F. also acknowledges support from the NHSC. E.G.A is a Research Associate at the Harvard-Smithsonian Center for Astrophysics. A.S. thanks the DFG for support via German-Israeli Project Cooperation grant STE1869/1-1.GE625/15-1. S.V. acknowledges support by NASA through Herschel contracts 1427277 and 1454738. PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAF-IFSI/OAA/OAP/OAT, LENS, SISSA (Italy); and IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI/INAF (Italy), and CICYT/MCYT (Spain). Herschel is an ESA space observatory with science instruments provided by European-led principal investigator consortia and with important participation from NASA.

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Submitted - 1508.07165v1.pdf

Published - Janssen_2015p74.pdf


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August 22, 2023
August 22, 2023