A faint radio glow extends approximately 2 megaparsecs around a galaxy cluster, far beyond its already known halo.
The cluster is called RXC J0528.9−3927. A cluster contains many galaxies, but also large quantities of very hot gas and dark matter. Researchers observed it with MeerKAT, a network of radio telescopes installed in South Africa. These antennas detect radio waves invisible to the human eye.
Megahalo candidate and radio halo
At the center of the cluster, astronomers already knew of a diffuse radio emission approximately 1.14 megaparsecs wide. The new data reveal a second, much fainter component surrounding it. It reaches approximately 2 megaparsecs in diameter, or nearly 6.5 million light-years.
This emission does not simply come from individual galaxies. It is diffuse and occupies the space between them. Radio waves of this type are produced by very fast electrons moving through magnetic fields. Their presence therefore makes it possible to probe what is happening in the cluster’s extended gas.
To distinguish the new structure from the central halo, the researchers measured how radio brightness decreases with distance. The decline changes markedly in the outer regions. This change indicates that the peripheral emission could form a distinct component, rather than merely being an extension of the already known halo.
Aerial view of several antennas belonging to the MeerKAT radio telescope in the Karoo, South Africa.
Source: SARAO
Astronomers call these extremely extended radio emissions “megahalos.” This family was identified only recently thanks to the European LOFAR network, which observes at relatively low frequencies. RXC J0528.9−3927 could become the first example detected around 1.28 GHz, a significantly higher frequency.
This difference matters for understanding the origin of the electrons observed far from the center. These particles gradually lose their energy and cannot simply travel several million light-years without being reaccelerated. Such a vast emission can therefore provide information about gas movements and magnetic fields extending into the cluster’s outer regions.
However, the authors remain cautious: the object is still considered a megahalo candidate. Deeper observations conducted at several frequencies must confirm that the outer emission has the expected properties. They will also make it possible to measure its radio spectrum and better determine the mechanism that sustains these energetic particles.