Metallicity diagnostics with infrared fine-structure lines

Abstract

Although measuring the gas metallicity in galaxies at various redshifts is crucial to constrain galaxy evolutionary scenarios, only rest-frame optical emission lines have been generally used to measure the metallicity. This has prevented us to accurately measure the metallicity of dust-obscured galaxies, and accordingly to understand the chemical evolution of dusty populations, such as ultraluminous infrared galaxies. Here we propose diagnostics of the gas metallicity based on infrared fine-structure emission lines, which are nearly unaffected by dust extinction even the most obscured systems. Specifically, we focus on fine-structure lines arising mostly from Hii regions, not in photo-dissociation regions, to minimize the dependence and uncertainties of the metallicity diagnostics from various physical parameters. Based on photoionization models, we show that the emission-line flux ratio of ([Oiii]51.80+[Oiii]88.33)/[Niii]57.21 is an excellent tracer of the gas metallicity. The individual line ratios [Oiii]51.80/[Niii]57.21 or [Oiii]88.33/[Niii]57.21 can also be used as diagnostics of the metallicity, but they are more strongly dependent on the gas density. The line ratios [Oiii]88.33/[Oiii]51.80 and [Nii]121.7/[Niii]57.21 can be used to measure and, therefore, account for the dependences on the gas density and ionization parameter, respectively. We show that these diagnostic fine-structure lines are detectable with Herschel in luminous infrared galaxies out z ~ 0.4. Metallicity measurements with these fine-structure lines will be feasible at relatively high redshift (z ~ 1 or more) with SPICA, the future infrared space observatory. © 2011 ESO.