| Abstract: |
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]/L IR) varies from ˜ 10-5 to ˜ 10-4. We investigate the correlation between L [Nii] and L IR, as well as the dependence of L [Nii]/L IR on L IR, infrared colors (f60/f100) and the O III 88 μm to [N ii] luminosity ratio. We find that L [Nii] strongly, and almost linearly correlates with L IR for star-forming galaxies, namely log L IR = (4.23 ± 0.33) + (0.99 ± 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.
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