THE [N ii] 205 μm EMISSION IN LOCAL LUMINOUS INFRARED GALAXIES*

<jats:title>ABSTRACT</jats:title> <jats:p>In this paper, we present the measurements of the [N <jats:sc>ii</jats:sc>] 205 <jats:italic>μ</jats:italic>m line for a flux-limited sample of 122 (ultra-)luminous infrared galaxies [(U)LIRGs] and 20 additional normal galaxies, obtained with the <jats:italic>Herschel Space Observatory</jats:italic> (<jats:italic>Herschel</jats:italic>). We explore the far-infrared (FIR) color dependence of the [N <jats:sc>ii</jats:sc>] 205 <jats:italic>μ</jats:italic>m (<jats:italic>L</jats:italic> <jats:sub>[N <jats:sc>ii</jats:sc>]205 <jats:italic>μ</jats:italic>m</jats:sub>) to the total infrared (<jats:italic>L</jats:italic> <jats:sub>IR</jats:sub>) luminosity ratio, and find that <jats:italic>L</jats:italic> <jats:sub>[N <jats:sc>ii</jats:sc>]205 <jats:italic>μ</jats:italic>m</jats:sub>/<jats:italic>L</jats:italic> <jats:sub>IR</jats:sub> only depends modestly on the 70–160 <jats:italic>μ</jats:italic>m flux density ratio (<jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj522527ieqn1.gif" xlink:type="simple"/> </jats:inline-formula>) when <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj522527ieqn2.gif" xlink:type="simple"/> </jats:inline-formula>, whereas such dependence becomes much steeper for <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj522527ieqn3.gif" xlink:type="simple"/> </jats:inline-formula>. We also investigate the relation between <jats:italic>L</jats:italic> <jats:sub>[N <jats:sc>ii</jats:sc>]205 <jats:italic>μ</jats:italic>m</jats:sub> and star formation rate (SFR), and show that <jats:italic>L</jats:italic> <jats:sub>[N <jats:sc>ii</jats:sc>]205 <jats:italic>μ</jats:italic>m</jats:sub> has a nearly linear correlation with SFR, albeit the intercept of such a relation varies somewhat with <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj522527ieqn4.gif" xlink:type="simple"/> </jats:inline-formula>, consistent with our previous conclusion that [N <jats:sc>ii</jats:sc>] 205 <jats:italic>μ</jats:italic>m emission can serve as an SFR indicator with an accuracy of ∼0.4 dex, or ∼0.2 dex if <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj522527ieqn5.gif" xlink:type="simple"/> </jats:inline-formula> is known independently. Furthermore, together with the <jats:italic>Infrared Space Observatory</jats:italic> measurements of [N <jats:sc>ii</jats:sc>], we use a total of ∼200 galaxies to derive the local [N <jats:sc>ii</jats:sc>] 205 <jats:italic>μ</jats:italic>m luminosity function (LF) by tying it to the known IR LF with a bivariate method. As a practical application, we also compute the local SFR volume density (<jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj522527ieqn6.gif" xlink:type="simple"/> </jats:inline-formula>) using the newly derived SFR calibrator and LF. The resulting <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj522527ieqn7.gif" xlink:type="simple"/> </jats:inline-formula> <jats:inline-formula> <jats:tex-math> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apj522527ieqn8.gif" xlink:type="simple"/> </jats:inline-formula> yr<jats:sup>−1</jats:sup> Mpc<jats:sup>−3</jats:sup> agrees well with previous studies. Finally, we determine the electron densities (<jats:italic>n</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub>) of the ionized medium for a subsample of 12 (U)LIRGs with both [N <jats:sc>ii</jats:sc>] 205 <jats:italic>μ</jats:italic>m and [N <jats:sc>ii</jats:sc>] 122 <jats:italic>μ</jats:italic>m data, and find that <jats:italic>n</jats:italic> ...