Herrera-Camus, R.Bolatto, A.Smith, J. D.Draine, B.Pellegrini, E.Wolfire, M.Croxall, K.Looze, I. deCalzetti, D.Kennicutt, R.Crocker, A.Armus, L.van der Werf, P.Sandstrom, K.Galametz, M.Brandl, B.Groves, BrentRigopoulou, D.Walter, F.Leroy, A.Boquien, M.Tabatabaei, F. S.Beirao, P.2016-09-062016-09-061538-4357http://hdl.handle.net/1885/108636The [N II] 122 and 205 μm transitions are powerful tracers of the ionized gas in the interstellar medium. By combining data from 21 galaxies selected from the Herschel KINGFISH and Beyond the Peak surveys, we have compiled 141 spatially resolved regions with a typical size of ∼1 kpc, with observations of both [N II] far-infrared lines. We measure [N II] 122/205 line ratios in the ∼0.6–6 range, which corresponds to electron gas densities of ne ∼ 1–300 cm−3, with a median value of ne = 30 cm−3. Variations in the electron density within individual galaxies can be as high as a factor of ∼50, frequently with strong radial gradients. We find that ne increases as a function of infrared color, dust-weighted mean starlight intensity, and star-formation rate (SFR) surface density (ΣSFR). As the intensity of the [N II] transitions is related to the ionizing photon flux, we investigate their reliability as tracers of the SFR. We derive relations between the [N II] emission and SFR in the low-density limit and in the case of a lognormal distribution of densities. The scatter in the correlation between [N II] surface brightness and ΣSFR can be understood as a property of the ne distribution. For regions with ne close to or higher than the [N II] line critical densities, the low-density limit [N II]-based SFR calibration systematically underestimates the SFR because the [N II] emission is collisionally quenched. Finally, we investigate the relation between [N II] emission, SFR, and ne by comparing our observations to predictions from the MAPPINGS-III code.RHC acknowledges support from a Fulbright-CONICYT grant. ADB acknowledges partial support from a CAREER grant NSF-AST0955836, from NASA-JPL 1373858, NSF-AST 1412419, and from a Research Corporation for Science Advancement Cottrell Scholar award. Beyond the Peak research has been supported by a NASA/JPL grant (RSA 1427378). FST acknowledges financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) under grant number AYA2013-41243-P. 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). 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. SPIRE has been developed by a consortium of institutes led by Cardiff University (UK) and including Univ. Lethbridge (Canada); NAOC (China); CEA, LAM(France); IFSI, Univ. Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, Univ. Sussex (UK); and Caltech, JPL, NHSC, Univ. Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC, UKSA (UK); and NASA (USA).17 pageshttp://www.sherpa.ac.uk/romeo/issn/0004-637X/ Publisher's version/PDF may be used on any website or authors' institutional repository (Sherpa/Romeo as of 6/9/2016)galaxiesISMstar formationstructure2016-07-2910.3847/0004-637X/826/2/175