Filter Design for Estimating the Stellar Metallicity of Metal-poor Stars from Gaia XP Spectra

Abstract The estimation of stellar atmospheric parameters for large-scale samples, particularly metal-poor stars, is a cornerstone of Galactic archaeology. In this work, we optimized a photometric filter design tailored to measuring stellar metallicities for very metal-poor stars with Fe/H < −1. The optimal configurations consist of a central wavelength λ c = 3960 Å with a bandwidth Δ λ = 80 Å for giant stars, and λ c = 3920 Å with Δ λ = 80 Å for dwarf stars. By applying these optimized filters to synthetic photometry derived from Gaia XP spectra, we inferred metallicities for both populations. Both internal and external validations demonstrate high precision across a wide metallicity range: 0.18–0.19 dex for −2 ≤ Fe/H ≤ −1, 0.23–0.33 dex for −3 ≤ Fe/H ≤ −2, and approximately 0.39 dex for the most metal-poor regime, successfully extending down to Fe/H ≈ −4 for giant stars and Fe/H ≈ −3.3 for dwarf stars. Finally, we present a catalog of approximately 14.5 million metal-poor stars with robust Fe/H measurements, along with more than 10,000 red giant ultra-metal-poor candidates with Fe/H < −4.0, providing a valuable resource for exploring the early formation and chemical evolution of the Milky Way.