Dilution in elliptical galaxies: Implications for the relation between metallicity, stellar mass and star formation rate

Abstract

We investigate whether dilution in some elliptical galaxies is the cause of a positive correlation between specific star formation rate (sSFR) and gas-phase metallicity (Zg) at high stellar mass in the local Universe. In the Munich semi-analytic model of galaxy formation, LGALAXIES, massive, low-sSFR, elliptical galaxies are seen to undergo a gradual dilution of their interstellar medium, via accretion of metal-poor gas in cold-gas clumps and low-mass satellites. This occurs after a merger-induced starburst and the associated supernova feedback have quenched most of the original gas reservoir. Signatures of this evolution are present in these model galaxies at z = 0, including low gas fractions, large central black holes, old ages, and importantly, low (Zg-Z*). Remarkably, all of these properties are also found in massive, low-sSFR, elliptical galaxies in the sloan digital sky survey data release 7 (SDSS-DR7). This provides strong, indirect evidence that gradual dilution is also occurring in nearby ellipticals in the real Universe. This scenario provides an explanation for the positive correlation between SFR and Zg measured in high-M* galaxies, and therefore has consequences for the local fundamental metallicity relation, which assumes a weak anticorrelation above ~1010.5M⊙. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.