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

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 (Z_g) at high stellar mass in the local Universe. In the Munich semi-analytic model of galaxy formation, L-GALAXIES, 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 (Z_g-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 Z_g measured in high-M_* galaxies, and therefore has consequences for the local fundamental metallicity relation, which assumes a weak anticorrelation above ∼10^10.5 M_⊙.