J-ATLAS: Javalambre Andromeda and Triangulum Legacy Astrophysical Survey

The outskirts of nearby galaxies preserve long-lived signatures of hierarchical assembly and provide a fossil record of galaxy formation. J-ATLAS, the Javalambre Andromeda and Triangulum Legacy Astrophysical Survey, is a proposed wide-field optical photospectroscopic survey of the M31--M33 system with JST/T250 and JPCam. Using the dense J-PAS filter system, it will deliver homogeneous low-resolution optical SEDs for resolved stars, compact stellar systems, emission-line sources, and background objects across the nearest massive spiral-galaxy environment visible from the northern sky. The survey will map disks, halos, streams, satellites, and the intergalactic region, constraining stellar populations, photometric metallicities, carbon-rich evolved stars, star clusters, planetary nebulae, and low-surface-brightness substructure. We also present the ongoing pilot survey, centred on the Giant Southern Stream and Andromeda I, whose 92 complete pointings are being analysed to validate crowded-field photometry, calibration, source classification, and photometric metallicity diagnostics before extending the methodology to the full J-ATLAS footprint survey.