Astrophys. J., 802, 81 (2015/April-1)
High-J CO sleds in nearby infrared bright galaxies observed by Herschel/PACS.
MASHIAN N., STURM E., STERNBERG A., JANSSEN A., HAILEY-DUNSHEATH S., FISCHER J., CONTURSI A., GONZALEZ-ALFONSO E., GRACIA-CARPIO J., POGLITSCH A., VEILLEUX S., DAVIES R., GENZEL R., LUTZ D., TACCONI L., VERMA A., WEISS A., POLISENSKY E. and NIKOLA T.
Abstract (from CDS):
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 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.
galaxies: active - galaxies: ISM - galaxies: starburst
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