The [NII] 205 µm Emission in Local Luminous Infrared Galaxies

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\gt 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 ({\dot{ρ }}_{{SFR}}) using the newly derived SFR calibrator and LF. The resulting {log} {\dot{ρ }}_{{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.

Based on Herschel observations. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.