First Full‑Scale Map of a Galaxy‑Cluster’s Magnetic Field Revealed

When you think of a galaxy cluster, you probably picture a massive collection of galaxies bound together by gravity, a sort of cosmic city. What you don’t see, because it’s invisible to the naked eye, is the tangled magnetic field that threads the hot plasma filling the space between those galaxies. Until now, astronomers could only guess at the field’s strength near the core or in a few bright spots.

That changed on 21 July 2026, when a team led by A. Botteon (INAF) published a breakthrough study in Astronomy & Astrophysics. Using an unprecedented 56 hours of sub‑arcsecond LOFAR‑VLBI observations at 144 MHz, they produced a high‑resolution radio map that covers the entire Abell 2255 cluster – from the bustling centre out to roughly one megaparsec, essentially the cluster’s virial radius.

The map is not a pretty picture of stars or galaxies; it’s a colour‑coded rendering of the magnetic‑field strength and orientation, inferred from the way low‑frequency radio waves are polarized as they travel through the intracluster medium. In the core the field hovers around a few microgauss, while farther out it tapers to a fraction of that, yet it remains coherent enough to shape the diffuse synchrotron emission that fills the cluster.

Why does this matter? Magnetic fields influence how cosmic‑ray electrons move, how heat is conducted, and even how clusters merge over billions of years. Prior to this work, we only had fragmentary snapshots – think of trying to understand a city’s traffic flow from a handful of street‑level photos. Now we have a city‑wide aerial view.

Other recent studies, such as a Science Advances paper showing that relativistic electrons and magnetic fields permeate the whole cluster volume, support the idea that Abell 2255 is a textbook example of a merging system where turbulence constantly stirs the magnetic brew. The new LOFAR‑VLBI map gives those qualitative hints a quantitative backbone.

Looking ahead, the technique demonstrated here can be applied to other nearby clusters, especially as next‑generation radio arrays (like the Square Kilometre Array) come online. For now, the magnetic skeleton of Abell 2255 stands as the first complete reconstruction of its kind – a milestone that turns the “invisible” into something we can actually study and model.