A Herschel Survey of the [N II] 205 micron Line in Local Luminous
  Infrared Galaxies --- The [N II] 205 micron Emission as a Star Formation Rate
  Indicator
  We present, for the first time, a statistical study of [N II] 205 mciron 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 (LNII) and the total infrared luminosity (LIR), as well
as the dependence of LNII/LIR ratio on LIR, far infrared colors (IRAS
$f_{60}/f_{100}$) and the [O III] 88 micron to [N II] luminosity ratio. We find
that LNII correlates almost linearly with LIR for non AGN galaxies (all having
$L_{IR} < 10^{12} L_solar$) in our sample, which implies that LNII can serve as
a SFR tracer which is particularly useful for high redshift galaxies which will
be observed with forthcoming submm spectroscopic facilities such as the Atacama
Large Millimeter/submillimeter Array. Our analysis shows that the deviation
from the mean LNII-LIR relation correlates with tracers of the ionization
parameter, which suggests 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.
