Kavli Affiliate: Bradford A. Benson
| First 5 Authors: Isaac S. Magolego, Isaac S. Magolego, , ,
| Summary:
Radio halos are diffuse synchrotron sources that trace the turbulent
intracluster medium (ICM) of galaxy clusters. However, their origin remains
unknown. Two main formation models have been proposed: the hadronic model, in
which relativistic electrons are continuously injected by cosmic-ray protons;
and the leptonic turbulent re-acceleration model, where cluster mergers
re-energise electrons in situ. A key discriminant between the two models would
be the existence of ultra-steep spectrum radio halos (USSRHs), which can only
be produced through turbulent re-acceleration. Here we report the discovery of
an USSRH in the galaxy cluster SPT-CLJ2337$-$5942 at redshift $z = 0.78$ in the
MeerKAT-South Pole Telescope 100 deg$^2$ survey. This discovery is noteworthy
for two primary reasons: it is the highest redshift USSRH system to date; and
the close correspondence of the radio emission with the thermal ICM as traced
by $mathitChandra$ X-ray observations, further supporting the leptonic
re-acceleration model. The halo is under-luminous for its mass, consistent with
a minor merger origin, which produces steep-spectrum, lower luminosity halos.
This result demonstrates the power of wide-field, high-fidelity, $lesssim1$
GHz surveys like the MeerKAT-SPT 100 deg$^2$ programme to probe the origin and
evolution of radio halos over cosmic time, ahead of the Square Kilometre Array.
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