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

Abstract

ABSTRACT In this paper, we present the measurements of the [N ii] 205 μm line for a flux-limited sample of 122 (ultra-)luminous infrared galaxies [(U)LIRGs] and 20 additional normal galaxies, obtained with the Herschel Space Observatory (Herschel). We explore the far-infrared (FIR) color dependence of the [N ii] 205 μm (L [N ii]205 μm) to the total infrared (L IR) luminosity ratio, and find that L [N ii]205 μm/L IR only depends modestly on the 70–160 μm flux density ratio ( f 70 / f 160 ?> ) when f 70 / f 160 ≲ 0.6 ?> , whereas such dependence becomes much steeper for f 70 / f 160 > 0.6 ?> . We also investigate the relation between L [N ii]205 μm and star formation rate (SFR), and show that L [N ii]205 μm has a nearly linear correlation with SFR, albeit the intercept of such a relation varies somewhat with f 60 / f 100 ?> , consistent with our previous conclusion that [N ii] 205 μm emission can serve as an SFR indicator with an accuracy of ∼0.4 dex, or ∼0.2 dex if f 60 / f 100 ?> is known independently. Furthermore, together with the Infrared Space Observatory measurements of [N ii], we use a total of ∼200 galaxies to derive the local [N ii] 205 μm 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 ( &rgr; ˙ SFR ?> ) using the newly derived SFR calibrator and LF. The resulting log &rgr; ˙ SFR = − 1.96 ± 0.11 ?> M ⊙ ?> yr−1 Mpc−3 agrees well with previous studies. Finally, we determine the electron densities (n e ) of the ionized medium for a subsample of 12 (U)LIRGs with both [N ii] 205 μm and [N ii] 122 μm data, and find that n e is in the range of ∼1–100 cm−3, with a median value of 22 cm−3.