MeerKAT observations of Abell 1775 and Abell 1795: the discovery of a hadronic radio halo?

TL;DR

This study uses MeerKAT and LOFAR radio observations to discover and analyze complex radio haloes in galaxy clusters Abell 1775 and Abell 1795, revealing large, flat-spectrum haloes in relaxed clusters and suggesting diverse origins.

Contribution

It presents the first detailed spectral analysis of radio haloes in Abell 1775 and Abell 1795, highlighting the presence of large, flat-spectrum haloes in relaxed clusters, challenging previous classifications.

Findings

Detection of diffuse radio haloes with inner and outer components

Identification of a large, flat-spectrum halo in Abell 1795

Observation of steep-spectrum emission linked to AGN activity

Abstract

Giant radio haloes are diffuse synchrotron sources typically found in merging galaxy clusters, while smaller mini-haloes occur in cool-core clusters. Both trace cosmic-ray electrons in the intracluster medium, though recent observations suggest their distinction is not always clear. We present new 903-1655 MHz MeerKAT observations of Abell 1775 and Abell 1795, both hosting cool cores and cold fronts. Combined with reprocessed 120-168 MHz LOFAR Two-metre Sky Survey data, we perform imaging and spectral analyses of their radio emission. In both clusters, we detect radio haloes with distinct inner and outer components. In Abell 1775, the halo appears diffuse at 1.3 GHz, while LOFAR images reveal steep-spectrum filaments. In Abell 1795, the inner component corresponds to a previously reported mini-halo candidate, but the full structure extends to $\sim$1 Mpc with a spectral index of $\alpha=-1.08\pm0.06$. The presence of such a large, flat-spectrum halo in a dynamically relaxed cluster makes Abell 1795 an outlier relative to typical merging systems. This suggests that some relaxed clusters may still retain sufficient turbulence to sustain particle re-acceleration, or that hadronic interactions producing secondary electrons play a significant role. Together with other recent discoveries in cool-core systems, our results indicate that some large radio haloes may have been overlooked in past studies due to limited dynamic range near bright central AGN. Finally, we detect steep-spectrum emission south of Abell 1795's central AGN, tracing a 45 kpc X-ray and optical filament that terminates in an X-ray cavity, likely linked to a past AGN outburst.