A Herschel Survey of the [N <scp>ii</scp>] 205 μm Emission in Local Infrared Luminous Galaxies

The [N ii] line is a major coolant in ionized interstellar medium, and is expected to be a good star formation rate indicator. Here we present a statistical study of [N ii] line emission for a large sample of local luminous infrared galaxies (LIRGs) using Herschel SPIRE FTS data (Lu et al. 2012; Zhao et al. 2012, in preparation). For our sample of galaxies, the [N ii] to the total infrared luminosity ratio (L[Nii]/LIR) varies from ∼ 10−5 to ∼ 10−4. We investigate the correlation between L[Nii] and LIR, as well as the dependence of L[Nii]/LIR on LIR, infrared colors (f60/f100) and the OIII 88 μm to [N ii] luminosity ratio. We find that L[Nii] strongly, and almost linearly correlates with LIR for star-forming galaxies, namely log LIR = (4.23 &#177; 0.33) + (0.99 &#177; 0.05) log L[NII] (see Fig. 1). The scatter in this relation is mainly due to the variation of hardness, and/or high ionization parameter, of the background UV field.