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Examples from the ISSI Team’s public library of mock ICL images, which provides realistic test data for developing and benchmarking detection techniques across the community.

Using the masked-aperture photometry method now adopted by the Euclid Collaboration, Bellhouse et al. (2025) predicts that the Euclid Wide Survey will enable intracluster light (ICL) measurements in up to 80,000 galaxy clusters. Shown here are examples from the team’s public library of mock ICL images, which provides realistic test data for developing and benchmarking detection techniques across the community. 

Published: 25 September 2025

by Nina HatchFabio Crameri

Highlights

From the Teams

Intracluster light

Dark Matter

Star

Galaxies

Turning Ghostly Light into a Cosmic Tool

When you look at a galaxy cluster, most of the starlight comes from the galaxies themselves. But if you look deep enough, you will find a much fainter glow: the intracluster light (ICL). These are stars that no longer belong to any one galaxy, drifting freely in the space between them. Once seen as little more than an astronomical curiosity, ICL is now emerging as a powerful probe of how galaxy clusters grow and how dark matter is distributed.

The ISSI International Team 577, Exploiting Intracluster Light for Cosmology and Galaxy Evolution with Next Generation Facilities, set out to make this ghostly glow a reliable cosmological tool—ready for the upcoming flood of ultra-deep images from the telescopes Euclid, Roman, and Rubin.

But let us hear directly from the team around Prof. Nina Hatch, from the School of Physics and Astronomy at the University of Nottingham.

What we achieved

Our work tackled the biggest barriers to using ICL consistently across surveys and simulations. The team delivered four main advances:

1. Making simulations look real

We developed methods to turn simulated clusters into “mock telescope images” that include all the quirks of real observations—sky glow, instrument blur, and noise. This allows fair comparisons between techniques and helps identify biases.

2. Establishing measurement ground rules

We drew up practical guidelines for separating ICL from galaxies and background light, reporting uncertainties, and applying tests that any method must pass. These rules are already feeding into the construction of ICL catalogues from Euclid.

3. Building a community resource

We created a shared library of simulated clusters with both “ground truth” maps (stellar ages, chemistry, dynamics) and matching synthetic images. This library is openly available here for anyone to test and refine their own methods.

4. Linking ICL to cluster mass and history

We clarified when ICL can—and cannot—be used as a tracer of total cluster mass. Its reliability depends on how dynamically “settled” a cluster is and how deep the observations go.

$From Kimmig et al. (2025). The fraction of cluster stellar mass found in the intracluster light plus brightest cluster galaxy, f(ICL+BCG), is a reliable tracer of when a cluster assembled. In relaxed systems this fraction steadily increases by ~3–4% per Gyr, meaning it can be used to rank clusters by their dynamical state and formation time.$

From Kimmig et al. (2025). The fraction of cluster stellar mass found in the intracluster light plus brightest cluster galaxy, f(ICL+BCG), is a reliable tracer of when a cluster assembled. In relaxed systems this fraction steadily increases by ~3–4% per Gyr, meaning it can be used to rank clusters by their dynamical state and formation time.

From Butler et al. (2025). The intracluster light does not perfectly trace dark matter: its stars have ~25% lower mean specific energy and more radial orbits than the dark matter. This shows that while ICL is valuable for studying cluster histories, it cannot be used as an unbiased map of the dark matter distribution.

From Butler et al. (2025). The intracluster light does not perfectly trace dark matter: its stars have ~25% lower mean specific energy and more radial orbits than the dark matter. This shows that while ICL is valuable for studying cluster histories, it cannot be used as an unbiased map of the dark matter distribution.

Why it matters

Together, these advances transform ICL from a niche signal into a standardisable, cross-survey tool for mapping dark matter and tracing cluster assembly. In other words: what was once just a faint glow on the edges of galaxies may soon become one of our best rulers for cosmic structure.

Publications and resources

The team’s outputs already include peer-reviewed publications and practical resources for the community, with more discoveries expected as Euclid and other next-generation facilities deliver their data.

A Spiral Amongst Thousands

Credits ESA/Webb, NASA & CSA, A. Martel