First Light And Reionization Epoch Simulations (FLARES) XXI: The UV Indices of Galaxies in the Early Universe
UV absorption line indices trace chemical enrichment and star formation histories in high-redshift galaxies, yet their reliability as quantitative stellar metallicity, Z*, diagnostics remains uncertain. In this work, we combine synthetic spectral modelling with cosmological simulations to assess the behaviour of rest-frame UV indices in the early Universe. Using the forward-modelling package Synthesizer, we compute equivalent widths for UV indices based on BPASS stellar population synthesis models and examine their sensitivity to metallicity, star formation history, nebular emission, and model assumptions. We first investigate idealised stellar populations to establish the metallicity dependence of each index. Most indices show increasing equivalent width with metallicity, although the strength and linearity of this relation varies between features. The 1719 Angstrom index exhibits one of the most stable correlations with stellar metallicity, while the 1460 Angstrom index shows stronger sensitivity to nebular emission, bursty star formation, and model-dependent effects at high metallicity. We then apply these models to galaxies from the First Light and Reionization Epoch Simulations, FLARES, which provide realistic star formation and chemical enrichment histories. The simulated galaxy populations reproduce a stellar mass-metallicity relation and allow the UV indices to be tested in composite spectra. Although scatter increases due to population mixing and stochastic enrichment, the overall metallicity trends remain largely preserved. These predictions provide a theoretical framework for interpreting rest-frame UV spectra from JWST and future surveys, supporting the use of UV absorption indices as complementary tracers of stellar metallicity in early galaxies.