| Description |
LIRGs and ULIRGs are found to deviate strongly from the mass-metallicity relation. More specifically, at a given stellar mass, the metallicity of LIRGs and ULIRGs, as inferred from optical spectroscopy, is much lower than the value expected from the mass-metallicity relation observed in normal star-forming galaxies. This metallicity anomaly of LIRGs and ULIRGs has been ascribed, by many models, to the inflow of metal poor gas, from the galaxy outskirts, induced by the tidal forces associated with these merging systems. However, an alternative possible scenario is that in these dusty systems optical spectra only probe the outer, metal poor regions, while the bulk of the gas is actually metal rich, but it is heavily absorbed by dust. We propose a definitive test of the scenarios discussed above by obtaining an independent, extinction-free measurement of the gas metallicity in a sample of LIRGs-ULIRGs, by exploiting newly developed far-IR metallicity diagnostics. In particular, we have recently shown that by measuring the relative intensities of the fine structure lines [OIII]52um, [OIII]88um, [NIII]57um and [NII]122um, it is possible to constrain the gas metallicity with high accuracy and in a wavelength range nearly unaffected by dust extinction. If these far-IR diagnostics confirm the low metallicities observed in the optical spectra, this would support models envisaging inflow of metal poor gas in interacting systems. If, instead, the far-IR diagnostics measure much higher metallicities, this would favor the scenario where the bulk of the gas is metal rich, but heavily absorbed, and it has been elusive to optical observations. |
