A Herschel Survey of the [NII] 205 μm Line in Local Luminous Infrared Galaxies -- The [NII] 205 μm Emission as a Star formation Rate Indicator

Zhao et al. 2013, ApJ, 765, L13 (ADS, arXiv)

Abstract

We present, for the first time, a statistical study of [NII] 205 μm 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 [NII] luminosity (L[NII]) and the total infrared luminosity (LIR), as well as the dependence of L[NII]/LIR ratio on LIR, far-infrared colors (IRAS f60/f100), and the [OIII] 88 μm to [NII] luminosity ratio. We find that L[NII] correlates almost linearly with LIR for non-active galactic nucleus galaxies (all having LIR < 1012 L) in our sample, which implies that L[NII] can serve as a star formation rate tracer which is particularly useful for high-redshift galaxies that will be observed with forthcoming submillimeter spectroscopic facilities such as the Atacama Large Millimeter/submillimeter Array. Our analysis shows that the deviation from the mean L[NII]-LIR relation correlates with tracers of the ionization parameter, which suggests that 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.


Key Figures

Correlation between IR and [NII] Luminosities.

Figure 2: [NII] 205 μm plotted against total infrared luminosities for normal and luminous infrared galaxies. The circles (red) are galaxies having Herschel observations. The diamonds (blue) are galaxies from Brauher et al. (2008), for which an additional uncertainty of ~0.3dex should be taken into account besides the plotted error bar. The superposed line represents a geometrical mean, least-squares linear fit to all data points except for those solid symbols.

[NII] to IR luminosity ratio plotted against different parameters.

Figure 3: Ratio of [NII] 205 μm to IR luminosity is plotted against ((a) and (d)) IR luminosity, (b) IRAS 60 μm/100 μm color, and (c) [OIII] 88 μm/[NII] 205 μm flux ratio. The symbols are the same as in Figure 2 except that here larger circles represent ULIRGs. The symbols enclosed by an additional red circle in panels (a) and (b) are the same sources as shown in panels (c) and (d). Equation (1) is shown by the dashed line in panel (a). The ordinate of panel (d) is a nominal value after the correction for the hardness effect (using the equation shown in this panel). Mrk 231 is outside the plotted range of panel (d).