The Oxygen Abundances of Luminous and Ultraluminous Infrared Galaxies

Luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs) dominate the star formation rate budget of the universe at z > 1, yet no local measurements of their heavy element abundances exist. We measure nuclear or near-nuclear oxygen abundances in a sample of 100 star-forming LIRGs and ULIRGs using new, previously published, and archival spectroscopy of strong emission lines (including [O II] 3727, 3729 A) in galaxies with redshifts <z> ~ 0.1. When compared to local emission-line galaxies of similar luminosity and mass (using the near-infrared luminosity-metallicity and mass-metallicity relations), we find that LIRGs and ULIRGs are under-abundant by a factor of two on average. As a corollary, LIRGs and ULIRGs also have smaller effective yields. We conclude that the observed under-abundance results from the combination of a decrease of abundance with increasing radius in the progenitor galaxies and strong, interaction- or merger-induced gas inflow into the galaxy nucleus. This conclusion demonstrates that local abundance scaling relations are not universal, a fact that must be accounted for when interpreting abundances earlier in the universe's history when merger-induced star formation was the dominant mode. We use our local sample to compare to high-redshift samples and assess abundance evolution in LIRGs and ULIRGs. We find that abundances in these systems increased by ~0.2 dex from z ~ 0.6 to z ~ 0.1. Evolution from z ~ 2 submillimeter galaxies to z ~ 0.1 ULIRGs also appears to be present, though uncertainty due to spectroscopic limitations is large.