High-J CO SLEDs in nearby infrared bright galaxies observed by Herschel/PACS

N. Mashian, E. Sturm, A. Sternberg, A. Janssen, S. Hailey-Dunsheath, J. Fischer, A. Contursi, E. González-Alfonso, J. Graciá-Carpio, A. Poglitsch, S. Veilleux, R. Davies, R. Genzel, D. Lutz, L. Tacconi, A. Verma, A. Weiss, E. Polisensky, T. Nikola

Research output: Contribution to journalArticlepeer-review


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 Jupp = 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-H2 conversion factor, αCO, for each respective source. For the ULIRGs we find α values in the canonical range 0.4-5M (K km s-1 pc2)-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.

Original languageEnglish
Article number81
JournalAstrophysical Journal
Issue number2
StatePublished - 1 Apr 2015


  • Galaxies: Active
  • Galaxies: Ism
  • Galaxies: Starburst

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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