Croxall, Kevin V. and Armus, L. and Beirão, P. and Helou, G. and Appleton, P. N. (2012) Resolving the Far-IR Line Deficit: Photoelectric Heating and Far-IR Line Cooling in NGC 1097 and NGC 4559. Astrophysical Journal, 747 (1). Art. No. 81. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20120327-074624584
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The physical state of interstellar gas and dust is dependent on the processes which heat and cool this medium. To probe heating and cooling of the interstellar medium over a large range of infrared surface brightness, on sub-kiloparsec scales, we employ line maps of [C II] 158 μm, [O I] 63 μm, and [N II] 122 μm in NGC 1097 and NGC 4559, obtained with the Photodetector Array Camera & Spectrometer on board Herschel. We matched new observations to existing Spitzer Infrared Spectrograph data that trace the total emission of polycyclic aromatic hydrocarbons (PAHs). We confirm at small scales in these galaxies that the canonical measure of photoelectric heating efficiency, ([C II] + [O I])/TIR, decreases as the far-infrared (far-IR) color, ν_f ν(70 μm) νf_ν(100 μm), increases. In contrast, the ratio of far-IR cooling to total PAH emission, ([C II] + [O I])/PAH, is a near constant ~6% over a wide range of far-IR color, 0.5 < νf_ν(70 μm) νf_ν(100 μm) ≾ 0.95. In the warmest regions, where νf_ν(70 μm) νf_ν(100 μm) ≳ 0.95, the ratio ([C II] + [OI])/PAH drops rapidly to 4%. We derived representative values of the local ultraviolet radiation density, G_0, and the gas density, n_H, by comparing our observations to models of photodissociation regions. The ratio G_0/n_H, derived from fine-structure lines, is found to correlate with the mean dust-weighted starlight intensity, langUrang, derived from models of the IR spectral energy distribution. Emission from regions that exhibit a line deficit is characterized by an intense radiation field, indicating that small grains are susceptible to ionization effects. We note that there is a shift in the 7.7/11.3 μm PAH ratio in regions that exhibit a deficit in ([C II] + [O I])/PAH, suggesting that small grains are ionized in these environments.
|Additional Information:||© 2012 American Astronomical Society. Received 2011 November 11; accepted 2012 January 2; published 2012 February 15. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. 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). HIPE is a joint development by the Herschel Science Ground Segment Consortium, consisting of ESA, the NASA Herschel Science Center, and the HIFI, PACS, and SPIRE consortia. This work is based (in part) on observations made with Herschel, a European Space Agency Cornerstone Mission with significant participation by NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.|
|Subject Keywords:||galaxies: individual (NGC 1097, NGC 4559); galaxies: ISM; ISM: lines and bands|
|Official Citation:||Resolving the Far-IR Line Deficit: Photoelectric Heating and Far-IR Line Cooling in NGC 1097 and NGC 4559 Kevin V. Croxall et al. 2012 ApJ 747 81|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||27 Mar 2012 15:33|
|Last Modified:||26 Dec 2012 14:59|
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