Title: The LOFAR and JVLA view of the distant steep spectrum radio halo in MACS J1149.5+2223
Authors: L. Bruno, K. Rajpurohit, G. Brunetti, F. Gastaldello, A. Botteon, A. Ignesti, A. Bonafede, D. Dallacasa, R. Cassano, R. J. van Weeren, V. Cuciti, G. Di Gennaro, T. Shimwell, M. Brüggen
(Submitted on 18 Mar 2021)
Abstract: Radio halos and relics are Mpc-scale diffuse radio sources in galaxy clusters, with a steep spectral index $\alpha>1$ ($S\propto \nu^{-\alpha}$). 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 $\alpha=1.49\pm 0.12$ 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 ($\sim 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.
Comments: 15 pages, accepted for publication in A&A
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)
Journal reference: A&A 650, A44 (2021)
DOI: 10.1051/0004-6361/202039877
From: Luca Bruno [view email]
[v1] Thu, 18 Mar 2021 09:37:05 GMT (10933kb,D)