Merging galaxies produce outliers from the fundamental metallicity relation

Abstract

From a large sample of ≈170 000 local SDSS (Sloan Digital Sky Survey) galaxies, we find that the fundamental metallicity relation (FMR) has an overabundance of outliers, compared to what would be expected from a Gaussian distribution of residuals, with significantly lower metallicities than predicted from their stellar mass and star formation rate (SFR). This lowmetallicity population has lower stellar masses, bimodial specific SFRs with enhanced star formation within the aperture and smaller half-light radii than the general sample and is hence a physically distinct population. We show that they are consistent with being galaxies that are merging or have recently merged with a satellite galaxy. In this scenario, low-metallicity gas flows in from large radii, diluting the metallicity of star-forming regions and enhancing the specific SFR until the inflowing gas is processed and the metallicity has recovered. We introduce a simple model in which mergers with a mass ratio larger than a minimum dilute the central galaxy's metallicity by an amount that is proportional to the stellar mass ratio for a constant time, and show that it provides an excellent fit to the distribution of FMR residuals. We find the dilution time-scale to be τ = 1.568+0.029-0.027 Gyr, the average metallicity depression caused by a 1:1 merger to be α = 0.2480+0.0017-0.0020 dex and the minimum mass ratio merger that can be discerned from the intrinsic Gaussian scatter in the FMR to be ξmin = 0.2030+0.0127-0.0095 (these are statistical errors only). From this we derive that the average metallicity depression caused by a merger with mass ratio between 1:5 and 1:1 is 0.114 dex.