First Light And Reionization Epoch Simulations (FLARES) XXI: The UV Indices of Galaxies in the Early Universe

UV absorption line indices trace both chemical enrichment and star formation histories in high-redshift galaxies, yet their reliability as tracers of stellar metallicity (Z*) remains uncertain. In this study, we combine synthetic spectral modelling and cosmological simulations to establish a theoretical framework for interpreting these features in the early Universe. Using the forward modelling package Synthesizer, we compute equivalent widths for a suite of UV indices based on BPASS stellar population synthesis models and investigate their sensitivity to metallicity, star formation history (SFH), and model assumptions. Certain indices, particularly the 1719 Å feature, exhibit strong and consistent correlations with stellar metallicity, while others display increased sensitivity to SFH. To assess the impact of realistic galaxy assembly histories, we apply these models to galaxies drawn from the First Light and Reionization Epoch Simulations (FLARES). The simulations provide diverse stellar populations with realistic metallicity distributions and SFHs, enabling an investigation of UV index behaviour within complex enrichment environments. We examine the relationship between galaxy properties and metallicity in FLARES and reproduce a synthetic mass-metallicity relation (MZR). Across most indices, equivalent widths increase monotonically with metallicity, consistent with predictions from simple stellar population models. The 1719 Å index emerges as one of the most reliable tracers of stellar metallicity, while the 1460 Å feature shows enhanced sensitivity to nebular emission and bursty star formation. These results provide a theoretical benchmark for interpreting rest-frame UV spectra of high-redshift galaxies observed with JWST.