Gas accretion as fuel for residual star formation in Galaxy Zoo elliptical galaxies

TL;DR

This study analyzes the gas metallicity in elliptical galaxies to determine that external accretion, such as cosmological inflow and minor mergers, fuels their residual star formation, differing from spiral galaxies.

Contribution

It provides evidence that external gas accretion significantly contributes to star formation in elliptical galaxies, supported by metallicity measurements and chemical evolution modeling.

Findings

Star forming ellipticals have lower gas metallicity than their stars.

Approximately 7.4% of these galaxies have very low gas metallicity.

External accretion accounts for over 37% of the gas fueling star formation.

Abstract

In this letter we construct a large sample of early-type galaxies with measured gas-phase metallicities from the Sloan Digital Sky Survey and Galaxy Zoo in order to investigate the origin of the gas that fuels their residual star formation. We use this sample to show that star forming elliptical galaxies have a substantially different gas-phase metallicity distribution from spiral galaxies, with ~7.4% having a very low gas-phase metallicity for their mass. These systems typically have fewer metals in the gas phase than they do in their stellar photospheres, which strongly suggests that the material fuelling their recent star formation was accreted from an external source. We use a chemical evolution model to show that the enrichment timescale for low-metallicity gas is very short, and thus that cosmological accretion and minor mergers are likely to supply the gas in >37% of star-forming ETGs, in good agreement with estimates derived from other independent techniques.