Abstract  

   Far-infrared (FIR) emission lines, such as the [C II] 158 祄, [N II] 122 and 205 祄, mid-J CO lines, are important for cooling the interstellar medium (ISM) and for providing critical diagnostic tools for the study of the star-forming ISM. The first part of this talk is about the [NII] 205 micron emission ([NII]205), which is a major coolant in ionized ISM, and is expected to be a good star formation rate indicator. Here, for the first time, we present the flux measurements of the [NII]205 line for a flux-limited sample of 125 (ultra-)luminous infrared galaxies [(U)LIRGs] from the GOALS survey and 20 additional normal galaxies, obtained with the SPIRE FTS instrument of Herschel. We find that LNII/LIR only depends modestly on the herschel-measured 70-to-160 micron flux density ratio (f70/f160) when f70/f160 <~ 0.6, whereas such dependence becomes much steeper for f70/f160> 0.6. We also show that LNII has a nearly linear correlation with SFR, albeit the intercept of such relation varies somewhat with f60/f100, suggesting that [NII]205 emission can serve as a useful SFR indicator with an accuracy of ~0.4 dex, or ~0.2 dex if f60/f100 is known independently. Furthermore, we derive the local [NII]205 luminosity function (LF) by tying it to the known IR LF with a bivariate method. As a practical application, we compute the local SFR volume density using the newly derived SFR calibrator and LF. The resulting SFR volume density agrees well with previous studies. Finally, we determine the electron densities (n_e) of the ionized medium for a subsample of 12 (U)LIRGs, and find that n_e is in the range of ~1-100 cm^{-3}, with a median value of 22 cm^{-3}.