Metallicity and Stellar Ages in Galaxy Mergers and Remnants

Major galaxy mergers reshape morphologies and trigger bursts of star formation. Because stellar metallicity and age preserve chemical enrichment and star formation histories, they provide diagnostics of merger-driven evolution. I analyse 28,445 galaxies from the Sloan Digital Sky Survey (SDSS) with stellar population measurements, of which 854 are major mergers and 116 are post-merger remnants identified by the UMKC Galaxy Evolution Group. Stellar ages and metallicities are derived from absorption-line diagnostics following Gallazzi et al. (2005). Post-merger remnants are systematically younger than either ongoing mergers or non-interacting galaxies. They are also metal-poor, differing from the field at confidence 0.9986 and being twice as likely to fall below [Z/H] = −0.2. The deficit is carried by a strongly rejuvenated sub-population rather than by a uniform shift of the class. Two-sample, two-dimensional Kolmogorov–Smirnov (KS) tests in the mass–metallicity and mass–age planes separate every merger class from the field population at > 99.99% confidence, with one-dimensional tests applied as a supplement. Comparing definite mergers with remnants directly, their metallicity distributions alone are not separable (0.9445) while their ages differ at confidence 1.0000, identifying lightweighted stellar age as the primary diagnostic of post-coalescence status. The results support a scenario in which low-metallicity gas is funneled to galaxy centers during coalescence, diluting abundances and fueling rejuvenated starbursts. Both signatures scale with progenitor gas content. Blue, star-forming mergers are offset by −0.14 dex in metallicity and −0.28 dex in age from mass-matched controls, while red, quenched mergers show no offset in either quantity; the same ordering appears when the sample is split by progenitor morphology. Rejuvenation and dilution are further coupled within individual galaxies, as expected if both arise from a single burst forming stars out of inflowing metal-poor gas. These results demonstrate that major mergers leave clear stellar population signatures in both metallicity and age, and that those signatures trace gas inflow.