We studied “intracluster light” (ICL)—the ultra-faint glow from stars that drift between galaxies inside a galaxy cluster. Our goal was to turn this glow into a dependable ruler for how clusters grow over time and a way to test ideas about the Universe. With new wide, deep images from the Euclid, Roman, and Rubin telescopes on the way, we focused on building the tools and ground rules needed to measure ICL consistently.
We delivered four main results.
- We created a method to make computer-made clusters look like real telescope pictures, including sky glow and camera blur. That lets us compare techniques fairly and see where they might go wrong.
- We discussed guidelines for measuring ICL—how to separate it from bright galaxies and background light, how to report uncertainties, and a set of tests that any method should pass. These are being used to construct ICL measurements from the Euclid data.
- We built a shared library of realistic mock clusters with “ground truth” maps (e.g. star ages and chemistry) plus matching synthetic images, so others can develop and repeat analyses. These are available here: mock images
- We set out when ICL does—and does not—trace the total mass of a cluster, showing how this depends on whether a cluster is calm or recently merged and on how deep the images are.
Together, these outputs will standardise analyses across surveys, and elevate ICL from a niche signal to a practical tool for mapping dark matter and charting how galaxy clusters assemble.