Model-dependent metallicities of young stellar populations in TIMER galaxies

Abstract Stellar populations in the central regions of disc galaxies provide key constraints on their formation and secular evolution. We investigate the properties of young stellar populations in the centres of nearby barred galaxies from the TIMER survey, using four stellar population models, two based on empirical stellar libraries (MILES, X-Shooter) and two on theoretical libraries (Starburst99, HR-pyPopStar), complemented by independent gas-phase metallicities from emission-line diagnostics. The inferred stellar metallicities depend strongly on the adopted model. Measured as the 5th-to-95th percentile spread across the three galaxies analysed in detail, the empirical libraries recover variations of 1.11 and 0.98 dex, whereas the theoretical ones return 0.40 and 0.49 dex; the gas in the same regions spans 0.49 dex. Gas metallicities are indeed lower in the central star-forming regions, but the amplitude of that depression (about 0.3 dex in NGC 1097) exceeds what bar-driven inflow of disc gas can account for, given typical abundance gradients in literature. The age–metallicity relations consistently reveal two distinct components: an older population consistent with a self-enrichment scenario, and a younger population formed from gas inflows with a different chemical enrichment history. The empirical libraries produce a monotonic decline of metallicities from old to young populations, which would require a sustained inflow of metal-poor gas outpacing its own enrichment. The theoretical libraries instead recover a young component whose metallicity departs from the older populations only at the most recent ages and by a few tenths of a dex, compatible with a single dilution episode. Our results demonstrate that model choice critically affects inferred metallicities, with models based on empirical stellar libraries recovering age–metallicity relations that demand physical conditions the gas measurements cannot sustain.