2021/03/18 by L. Bruno, K. Rajpurohit, G. Brunetti +11 · 1 voice
Physics and Astronomy · #astro-ph.CO
paper · pdf · doi:10.1051/0004-6361/202039877
published as A&A 650, A44 (2021) · 15 pages, accepted for publication in A&A
arxiv created 2021/03/18 · arxiv published 2021/03/18 · arxiv updated 2021/06/09
Radio halos and relics are Mpc-scale diffuse radio sources in galaxy clusters, with a steep spectral index α>1 (S∝ ν-α). It has been proposed that they arise from particle acceleration induced by turbulence and weak shocks, injected in the intracluster medium (ICM) during mergers. MACS J1149.5+2223 (MACS J1149) is a high redshift (z=0.544) galaxy cluster possibly hosting a radio halo and a relic. We analysed LOFAR, GMRT, and JVLA radio data at 144, 323, 1500 MHz, and Chandra X-ray data to characterise the thermal and non-thermal properties of the cluster. We obtained radio images at different frequencies to investigate the spectral properties of the radio halo. We used Chandra X-ray images to constrain the thermal properties of the cluster. We measured a steep spectrum of the halo, with α=1.49± 0.12 between 144 and 1500 MHz. The radio surface brightness distribution across the halo is found to correlate with the X-ray brightness of the ICM, with a sub-linear slope in the range 0.4 to 0.6. We also report two possible cold fronts in north-east and north-west, but deeper X-ray observations are required to firmly constrain the properties of the upstream emission. We show that the combination of high redshift, steep radio spectrum, and sub-linear radio-X scaling of the halo rules out hadronic models. An old (∼ 1 Gyr ago) major merger likely induced the formation of the halo through stochastic re-acceleration of relativistic electrons. We suggest that the two possible X-ray discontinuities may actually be part of the same cold front. In this case, the coolest gas pushed towards the north-west might be associated with the cool core of a sub-cluster involved in the major merger. The peculiar orientation of the south-east relic might indicate a different nature of this source and requires further investigation.