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The [NII] 122 and 205 \mu m transitions are powerful tracers of the ionizedgas in the interstellar medium. By combining data from 21 galaxies selectedfrom the Herschel KINGFISH and Beyond the Peak surveys, we have compiled 141spatially resolved regions with a typical size of ~1 kiloparsec, withobservations of both [NII] far-infrared lines. We measure [NII] 122/205 lineratios in the ~0.6-6 range, which corresponds to electron gas densities$n_e$~1-300 cm$^{-3}$, with a median value of $n_e$=30 cm$^{-3}$. Variations inthe electron density within individual galaxies can be as a high as a factor of~50, frequently with strong radial gradients. We find that $n_e$ increases as afunction of infrared color, dust-weighted mean starlight intensity, and starformation rate surface density ($\Sigma_{SFR}$). As the intensity of the [NII]transitions is related to the ionizing photon flux, we investigate theirreliability as tracers of the star formation rate (SFR). We derive relationsbetween the [NII] emission and SFR in the low-density limit and in the case ofa log-normal distribution of densities. The scatter in the correlation between[NII] surface brightness and $\Sigma_{SFR}$ can be understood as a property ofthe $n_e$ distribution. For regions with $n_e$ close to or higher than the[NII] line critical densities, the low-density limit [NII]-based SFRcalibration systematically underestimates the SFR since [NII] emission iscollisionally quenched. Finally, we investigate the relation between [NII]emission, SFR, and $n_e$ by comparing our observations to predictions from theMAPPINGS-III code.

The Ionized Gas in Nearby Galaxies as Traced by the [NII] 122 and 205 \mu m Transitions