Metallicity Dilution Without a Detectable Star Formation Difference in Morphologically Disturbed versus Quiet Early-Type Galaxies

Abstract

We identify a population of diluted early-type galaxies (ETGs) with gas-phase metallicities significantly below the local mass-metallicity relation. After matching to a sample of undiluted galaxies matched in stellar mass and redshift, we show that the metallicity deficit remains highly significant. Morphological classification does not provide statistically significant evidence for different star-formation responses: disturbed systems exhibit both metallicity dilution and enhanced star formation, while quiet diluted ETGs show substantial metallicity deficits but the sample too small to detect a similar enhancement at high significance. A simple gas-mixing model shows that the observed metallicity deficit can be reproduced through the accretion of metal-poor gas, requiring an accreted gas mass of only ~7% of the stellar mass. We suggest that metal-poor gas accretion is a common origin of diluted ETGs, while whether their subsequent star-formation response depends on the dynamical state of the host galaxy remains an open question.