This work joins recent discoveries of diffuse radio emission in galaxy clusters: LOFAR and uGMRT have already found five megahalos, all at frequencies below 1 GHz. Neighboring studies in our archive also address radio halos in clusters (e.g., the radio halo in the Coma cluster, the radio glow of galaxies, the shadow of a cluster). This paper takes the next step—testing whether a megahalo can be seen at gigahertz frequencies, where steep spectra make such objects extremely faint.
We used archival MeerKAT L-band (1.28 GHz) observations with a total integration time of 12.9 hours. After standard calibration and self-calibration, we subtracted a model of compact sources from the data to isolate the diffuse emission. We then produced a map with 26 arcsecond resolution and measured the surface brightness profile in elliptical annuli, comparing it to an exponential decay model for the central halo.
At 1.28 GHz, we detected faint emission extending to ~1.96 Mpc, notably larger than the known radio halo of ~1.14 Mpc. The surface brightness profile deviates from the exponential model beyond a radius of ~162.5 arcseconds (about 697 kpc), corresponding to 0.55 R500. This flattening indicates an additional component—a megahalo candidate. The flux of the central halo is 6.53±0.33 mJy, and that of the outer component is 2.37±0.13 mJy (from direct detection); accounting for projection, the total megahalo flux is estimated at 4.48±0.25 mJy. The megahalo's volume is 3.83 times larger than that of the halo, while its volume emissivity is 5.6 times lower, consistent with known megahalos.
This is the first evidence of megahalo-like emission at gigahertz frequencies. Unlike neighboring works, which either confirmed known halos or searched for new ones at low frequencies, we show that deep MeerKAT observations can reveal extremely faint extended structures even where steep spectra make them nearly invisible. This expands the possibilities for finding megahalos and suggests that non-thermal components can exist in clusters without extreme merger signatures.
Confirming the candidate will require deep multi-frequency observations with matched angular resolution. The outer component is expected to have a steeper spectrum than the central halo. Future surveys with the Square Kilometre Array (SKA) will allow systematic detection and characterization of such faint sources, providing new insights into particle acceleration and the evolution of magnetic fields in the intracluster medium.
The result will impact observational radio astronomy of galaxy clusters and models of turbulent particle acceleration in diffuse plasma.
Conduct deep observations at 300–1400 MHz with identical uv-coverage to measure the spectral index of the outer component and confirm its nature.
For cluster radio astronomy, a key unresolved problem is the origin and prevalence of megahalos: are they a distinct class or simply the outer extensions of halos? This work provides the first hint that megahalos may exist at gigahertz frequencies, but without spectral data the question remains open.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This study uses archival MeerKAT observations at 1.28 GHz (12.9 h) to search for diffuse emission in the cluster RXC J0528.9−3927. After subtracting compact sources and constructing a map, the surface brightness profile was measured, showing a flattening beyond 0.55 R500, which indicates an additional component: a megahalo candidate. Fluxes and volume luminosity were estimated, consistent with known megahalos.
Where its strength is
The work demonstrates the possibility of detecting extremely faint extended structures at gigahertz frequencies thanks to deep MeerKAT observations, thereby expanding the search range for megahalos and showing the presence of non-thermal components in a cluster without obvious signs of merging.
Where it could go next
Worth trying
Why this may matter
Detecting a megahalo at gigahertz frequencies opens the door to systematic searches for such objects with MeerKAT and future instruments (SKA), which will help better understand particle acceleration and magnetic fields in the peripheral regions of clusters.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.57, against a usual 0.64. It looks like few have drilled nearby.
Megahalos occupy volumes up to 30 times larger than typical radio halos, yet their glow is 20 times fainter, like a giant ghostly bubble around a bright core.
In Stanisław Lem's novel 'Solaris', the planet's ocean creates giant structures invisible in ordinary light — somewhat like megahalos, which astronomers can only discern by their faint radio glow.