Metallicities and N/O abundances in star forming galaxies measured with infrared nebular lines

We present a new method to derive heavy element abundances in star forming galaxies using the unique suite of nebular lines in the mid- to far-infrared (IR) range. Using grids of photo-ionisation models that cover a wide range in O/H and N/O, and ionisation parameter, HII-CHI-MISTRY-IR (HCm-IR) provides abundances based on extinction-free and temperature-insensitive tracers, two significant advantages over optical diagnostics when these are applied to dust obscured regions or unresolved (stratified) nebulae. The code performance is probed using three different galaxy samples covering a wide range in metallicity, 7.2 < 12 + log(O/H) < 8.9, with available IR spectroscopy from Spitzer, Herschel and SOFIA, including: 28 low-metallicity dwarf galaxies, 19 nearby starbursts, and 9 luminous IR galaxies. The IR-based abundances are robust even when the hydrogen recombination lines, typically faint in this range, are not available. When compared to various optical-based methods, HCm- IR is consistent within an average difference of 0.1 dex and a scatter of 0.2 dex. Additionally, HCm-IR provides an independent N/O measurement when [OIII]52,88textmum and [NIII]57textmum are measured, probing the secondary production of nitrogen. We find systematically lower N/O IR abundances when compared to the optical determinations, suggesting that IR lines may trace higher excitation gas located closer to massive stars. Finally, we applied the code to a sample of 8 distant galaxies located at 1.8 < z < 7.5 with ground-based detections of the far-IR lines redshifted in the submm range, revealing sub-solar N/O and O/H abundances. The script to derive chemical abundances with HCm-IR is publicly available.