2025ApJ...989L...2E

Intracluster light (ICL), the diffuse glow of stars stripped from galaxies during a cluster's formation, is an established tracer of a cluster's dynamical history. The upcoming Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) is set to revolutionize studies of ICL by imaging the entire southern sky down to a limiting surface brightness μ ≳ 30 mag arcsec^–2 by year 10. In this Letter, we create a precursor LSST data set (reaching the equivalent of year 8 depth) using Dark Energy Camera observations of Abell 3667 (A3667) and study its ICL. We have discovered a low surface brightness (μ ≳ 26 mag arcsec^–2) optical bridge extending over ∼400 kpc, which connects the two brightest galaxies (BCG1 and BCG2) in the cluster; the color and surface brightness of the bridge is consistent with formation via a major merger. The inner regions of BCG1 (r < 200 kpc) and BCG2 (r < 50 kpc) are consistent with formation via gradual stripping of satellite galaxies, but BCG2's outer profile appears disrupted by a recent merger. We hypothesize that the bridge is a relic of a recent first pass between the two brightest galaxies and is composed of stars being stripped from BCG2. Future studies of ICL with LSST will discover new features such as the bridge in local clusters while enabling detailed studies of the stellar populations of these features with its six photometric bands.