Abstract
Abstract We report a discovery of a double radio relic in the cluster merger ZwCl1447.2+2619 (z = 0.376) with uGMRT observations at 420 MHz and 700 MHz. The linear sizes of the northern and southern relics are ∼0.3 Mpc and ∼1.2 Mpc, respectively, which is consistent with the theoretical expectation that a larger relic is produced in the less massive subcluster side. However, ZwCl1447.2+2619 is unlike other known double radio relic systems, where the larger relics are much more luminous by several factors. In this merger, the higher surface brightness of the smaller northern relic makes its total radio luminosity comparable to that of the much larger southern relic. The surface brightness ratio ∼0.1 between the two radio relics differs significantly from the relation observed in other double radio relic systems. From our radio spectral analysis, we find that both relics signify similar weak shocks with Mach numbers of 2.9 ± 0.8 and 2.0 ± 0.7 for the northern and southern relics, respectively. Moreover, the northern relic is connected to a discrete radio source with an optical counterpart, which indicates the possible presence of cosmic-ray injection and reacceleration. Therefore, we propose that this atypical surface brightness ratio can be explained with the particle acceleration efficiency precipitously dropping in the weak shock regime and/or with reacceleration of fossil cosmic rays. Our multi-wavelength analysis and numerical simulation suggest that ZwCl1447.2+2619 is a postmerger, which has experienced a near head-on collision ∼0.7 Gyr ago.
Highlights
Mpc-scale diffuse radio emissions from merging galaxy clusters are broadly classified into two main categories: radio halos and radio relics
We report the discovery of a double radio relic in the galaxy cluster ZwCl1447.2+2619 at z = 0.376 (Golovich et al 2019a)
The vector connecting the centers of these two radio emissions is well aligned with that defined by the aforementioned two substructures revealed by the WL and X-ray data, which is consistent with the expectation that the merger happened in the NE–SW direction
Summary
Mpc-scale diffuse radio emissions from merging galaxy clusters are broadly classified into two main categories: radio halos and radio relics. Radio halos are diffuse structures roughly following the distribution of the intracluster medium (ICM, e.g., Brunetti & Jones 2014). Radio relics are characterized by arc-like morphologies in the cluster outskirts. As their name implies, radio relics have no distinct counterparts in other wavelengths. Radio relics have no distinct counterparts in other wavelengths Their low spectral ages rule out the possibility that the relics originate from point-source diffusion. Radio relics have been interpreted as indicating the locations of cosmic-ray formation (i.e., particle acceleration) (e.g., Ensslin et al 1998; van Weeren et al 2010, 2019)
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