The relation between metallicity, stellar mass and star formation in galaxies: an analysis of observational and model data

TL;DR

This study investigates how metallicity relates to stellar mass and star formation in galaxies, revealing opposite trends in low and high mass galaxies and highlighting the role of gas accretion and mergers.

Contribution

It combines observational data and models to uncover the complex dependence of metallicity on star formation and mass, emphasizing the impact of gas-rich mergers and black hole growth.

Findings

Low-mass galaxies with high star formation are more metal-poor.

High-mass galaxies with low star formation have lower metallicities.

Massive low-metallicity galaxies often experienced gas-rich mergers.

Abstract

We study relations between stellar mass, star formation and gas-phase metallicity in a sample of 177,071 unique emission line galaxies from the SDSS-DR7, as well as in a sample of 43,767 star forming galaxies at z=0 from the cosmological semi-analytic model L-GALAXIES. We demonstrate that metallicity is dependent on star formation rate at fixed mass, but that the trend is opposite for low and for high mass galaxies. Low-mass galaxies that are actively forming stars are more metal-poor than quiescent low-mass galaxies. High-mass galaxies, on the other hand, have lower gas-phase metallicities if their star formation rates are small. Remarkably, the same trends are found for our sample of model galaxies. We find that massive model galaxies with low gas-phase metallicities have undergone a gas-rich merger in the past, inducing a starburst which exhausted their cold gas reservoirs and shut down star formation. This led to a gradual dilution in the gas-phase metallicities of these systems via accretion of gas. These model galaxies have lower-than-average gas-to-stellar mass ratios and higher-than-average central black hole masses. We confirm that massive galaxies with low gas-phase metallicities in our observational sample also have very massive black holes. We propose that accretion may therefore play a significant role in regulating the gas-phase metallicities of present-day massive galaxies.