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arXiv · 2506.19901

Discovery of Diffuse Radio Emission in a Massive z=1.709 Cool Core Cluster: A Candidate Radio Mini-Halo

Julie Hlavacek-Larrondo·Roland Timmerman·Christoph Pfrommer·Erik Osinga·Larissa Tevlin·Tracy M. A. Webb·Natalia Martorella·Xiaoyuan Zhang·Reinout van Weeren·Hyunseop Choi·Gabriella Di Gennaro·Marie-Lou Gendron-Marsolais·Carter Rhea
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Abstract

Clusters of galaxies host spectacular diffuse radio sources, extending over scales from 100 kpc to several Mpcs. These sources, with extremely faint surface brightness ($μ$Jy/arcsec$^2$ level), are not tied to individual galaxies but trace synchrotron emission from large-scale magnetic fields and relativistic particles within the intracluster environment. Here, we report the discovery of a candidate radio mini-halo in SpARCS104922.6+564032.5, the most distant cool-core galaxy cluster identified to date at $z=1.709$, using deep LOFAR 120-168 MHz observations. We show that this emission originates from diffuse cluster-associated processes rather than unresolved AGN or star-forming galaxies. The diffuse radio emission coincides spatially with the X-ray emission of the hot intracluster medium and has a radio power of $P_{\rm 150~MHz}=49.8^{+14.7}_{-11.7} \times10^{24}$ W Hz$^{-1}$, exhibiting striking similarities to low-redshift radio mini-halos. This discovery doubles the redshift of previously known mini-halos, challenging models of inverse Compton losses and indicating the presence of strong magnetic fields, enhanced turbulence in high-redshift clusters, or active hadronic processes that require a cosmic ray-to-thermal energy ratio of 0.07 within 200 kpc, assuming a clumped distribution with spatial correlations among the gas, cosmic rays, and magnetic field that partially compensate for cosmological redshift dimming. It further implies that magnetic fields are efficiently amplified to $\sim$$10~μ$G levels within a Mpc$^3$ volume during the epoch of cluster formation before $z\sim2$. These findings provide critical insights into high-redshift cluster physics and emphasize the transformative potential of next-generation radio surveys, such as those with the SKA and ngVLA, in exploring the early evolution of galaxy clusters.

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Julie Hlavacek-Larrondo, Roland Timmerman, Christoph Pfrommer, Erik Osinga, Larissa Tevlin, Tracy M. A. Webb, Natalia Martorella, Xiaoyuan Zhang, Reinout van Weeren, Hyunseop Choi, Gabriella Di Gennaro, Marie-Lou Gendron-Marsolais, Carter Rhea. 2025-06-24. Discovery of Diffuse Radio Emission in a Massive z=1.709 Cool Core Cluster: A Candidate Radio Mini-Halo. https://arxiv.org/abs/2506.19901

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