Merging galaxies produce outliers from the Fundamental Metallicity Relation

TL;DR

This study analyzes a large galaxy sample and finds that merging galaxies create outliers in the Fundamental Metallicity Relation by diluting metallicity through gas inflows, which can be modeled to understand galaxy interactions.

Contribution

The paper introduces a simple model linking galaxy mergers to metallicity dilution and star formation enhancement, providing quantitative fits to observed outliers in the FMR.

Findings

Merging galaxies cause significant outliers with lower metallicities.

A model with a metallicity dilution timescale of ~1.57 Gyr fits the data.

Minimum discernible merger mass ratio is approximately 0.2.

Abstract

From a large sample of $\approx 170,000$ local SDSS 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 low-metallicity 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 $\tau=1.568_{-0.027}^{+0.029}$ Gyr, the average metallicity depression caused by a 1:1 merger to be $\alpha=0.2480_{-0.0020}^{+0.0017}$ dex and the minimum mass ratio merger that can be discerned from the intrinsic Gaussian scatter in the FMR to be $\xi_\text{min}=0.2030_{-0.0095}^{+0.0127}$ (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.