High-J CO Sleds in Nearby Infrared Bright Galaxies Observed By Herschel/PACS
We report the detection of far-infrared (FIR) CO rotational emission from nearby active galactic nuclei (AGNs) and starburst galaxies, as well as several merging systems and Ultra-Luminous Infrared Galaxies (ULIRGs). Using the Herschel Photodetector Array Camera and Spectrometer (PACS), we have detected transitions in the J_(upp) = 14–30 range. The PACS CO data obtained here provide the first reference of well-sampled FIR extragalactic CO spectral line energy distributions (SLEDs) for this range. We find a large range in the overall SLED shape, even among galaxies of similar type, demonstrating the uncertainties in relying solely on high-J CO diagnostics to characterize the excitation source of a galaxy. Combining our data with low-J line intensities taken from the literature, we present a CO ratio–ratio diagram and discuss its value in distinguishing excitation sources and physical properties of the molecular gas. The position of a galaxy on such a diagram is less a signature of its excitation mechanism, than an indicator of the presence of warm, dense molecular gas. We then quantitatively analyze the CO emission from a subset of the detected sources with single-component and two-component large velocity gradient (LVG) radiative transfer models to fit the CO SLEDs. From these fits we derive the molecular gas mass and the corresponding CO-to-H_2 conversion factor, ɑ_(CO), for each respective source. For the ULIRGs we find α values in the canonical range 0.4– 5M_⊙ (K km s^(−1) pc^2)^(−1), while for the other objects, α varies between 0.2 and 14. Finally, we compare our best-fit LVG model results with previous studies of the same galaxies and comment on any differences.
© 2015 American Astronomical Society. Received 2014 September 4; accepted 2015 January 20; published 2015 March 25. N.M. is supported by the Raymond and Beverly Sackler Tel Aviv University-Harvard Astronomy Program. We thank the DFG for support via German-Israeli Project Cooperation grant STE1869/1-1.GE625/15-1. Basic research in IR 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.V. thanks the Leverhulme Trust for a Research Fellowship. S.V. also acknowledges partial support from NASA through Herschel grants 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); 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). This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under grant No. DGE1144152. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
Published - 0004-637X_802_2_81.pdf
Submitted - 1410.0006v2.pdf