2013ApJ...765L..13Z

We present, for the first time, a statistical study of [N II] 205 µm line emission for a large sample of local luminous infrared galaxies using Herschel Spectral and Photometric Imaging Receiver Fourier Transform Spectrometer (SPIRE FTS) data. For our sample of galaxies, we investigate the correlation between the [N II] luminosity (L_[N II]_) and the total infrared luminosity (L_IR), as well as the dependence of L_[N II]_/LIR_ ratio on L_IR, far-infrared colors (IRAS f₆₀/f₁₀₀), and the [O III] 88 µm to [N II] luminosity ratio. We find that L_[N II]_ correlates almost linearly with L_IR for non-active galactic nucleus galaxies (all having L_IR< 10¹² L_☉) in our sample, which implies that L_[N II]_ can serve as a star formation rate tracer which is particularly useful for high-redshift galaxies that will be observed with forthcoming submillimeter spectroscopic facilities such as the Atacama Large Millimeter/submillimeter Array. Our analysis shows that the deviation from the mean L_[N II]_–LIR_relation correlates with tracers of the ionization parameter, which suggests that the scatter in this relation is mainly due to the variations in the hardness, and/or ionization parameter, of the ambient galactic UV field among the sources in our sample.