MeerKAT maps a larger, steep-spectrum radio halo in Abell 370

What keeps a vast cloud of radio-emitting particles glowing between galaxies? In Abell 370, a massive, merging galaxy cluster, sharper measurements of a faint radio halo offer evidence for one leading explanation: turbulence that repeatedly boosts the energy of existing electrons.

Graham D. Lawrie and Roger P. Deane report that the halo is larger than earlier observations indicated and has an unusually steep radio spectrum. Their study is available as an arXiv preprint; the manuscript also records acceptance by Monthly Notices of the Royal Astronomical Society on 5 November 2026.

Separating the halo from brighter sources

Abell 370 lies at a redshift of 0.375 and has a mass roughly a million billion times that of the Sun. It is not a settled system: its disturbed structure and two prominent central galaxies point to a merger. Such environments can host giant radio halos, diffuse emission spread through the hot gas between galaxies rather than confined to individual galaxies.

The radio light is synchrotron radiation, produced by energetic electrons moving through magnetic fields. Earlier observations had detected faint diffuse emission in Abell 370, but its extent and spectral properties remained uncertain.

The researchers used four MeerKAT observing sessions of about five hours each, taken in December 2024, covering approximately 580–1015 megahertz. Extracting the halo required more than collecting a deep image. Bright sources elsewhere in the field introduced imaging distortions, while compact sources within the halo contaminated its faint glow.

The team corrected for troublesome off-centre sources, modelled and subtracted compact sources, and made lower-resolution images sensitive to extended emission. They also masked residual contamination in ten regions, estimating the underlying diffuse brightness from surrounding clean emission.

The resulting maps reveal a halo about 1.1 megaparsecs across, with extensions toward the southeast and southwest. Earlier estimates ranged from about 500–700 kiloparsecs to roughly 950 kiloparsecs. The new measurement therefore extends an already detected halo rather than discovering the emission for the first time.

A spectrum that fades unusually quickly

To measure how the halo’s brightness changes with frequency, the authors divided the MeerKAT data into seven frequency bands and smoothed their images to the same resolution. They measured the emission within a common region and fitted a power-law spectrum using statistical sampling.

The resulting spectral index is −1.54 ± 0.05. A negative index means the emission becomes fainter at higher radio frequencies; a more negative value means a faster decline. Typical radio halos have indices around −1.1 to −1.4, while values around −1.5 or steeper fall into the ultra-steep-spectrum category.

This measurement supports that classification for Abell 370. It is steeper than an earlier estimate of −1.10 ± 0.09 obtained across a different frequency range. The authors suggest their larger measurement region and differences in earlier image quality could contribute to the discrepancy, rather than establishing a single definitive explanation.

Connecting radio emission to hot gas

The researchers also compared the radio map with Chandra X-ray observations, which trace the cluster’s hot gas. Both show a broadly north–south elongated structure. A region-by-region analysis found a positive relationship between their surface brightnesses, with a fitted slope of 0.60 ± 0.08.

That slope is sub-linear: brighter X-ray regions tend to be brighter in radio emission too, but the radio brightness rises less rapidly. This correspondence supports the interpretation that the diffuse emission belongs to the cluster-wide gas environment.

The halo is also relatively faint for the cluster’s mass compared with a published relation for ordinary radio halos, placing it among known ultra-steep-spectrum systems. The strength of that comparison depends partly on which cluster mass estimate is adopted.

Supporting a mechanism, not proving it

Ultra-steep spectra favour models in which merger-driven turbulence re-accelerates existing electrons. They are harder to reproduce with models relying solely on electrons generated in collisions involving cosmic-ray protons. These observations support turbulent re-acceleration, but do not directly measure that process or exclude mixed mechanisms.

Important limits remain. The spectrum is measured within one observing band, and the spatial spectral map reliably covers the core rather than the entire halo. Its mean pixel index, −1.40, is not identical to the integrated measurement. Deeper observations are needed to map the faint outskirts and examine the radio–X-ray connection in greater detail.