Particle acceleration in a nearby galaxy cluster pair: the role of cluster dynamics

Abstract

Context. Diffuse radio emission associated with the intra-cluster medium (ICM) is observed in a number of merging galaxy clusters. It is currently believed that in mergers a fraction of the kinetic energy is channeled into non-thermal components, such as turbulence, cosmic rays and magnetic fields, that may lead to the formation of giant synchrotron sources in the ICM. Aims. Studying merging galaxy clusters in different evolutionary phases is fundamental to understanding the origin of radio emission in the ICM. Methods. We observed the nearby galaxy cluster pair RXC J1825.3+3026 (z ∼ 0.065) and CIZA J1824.1+3029 (z ∼ 0.071) at 120 − 168 MHz with the LOw Frequency ARray (LOFAR) and made use of a deep (240 ks) XMM-Newton dataset to study the nonthermal and thermal properties of the system. RXC J1825.3+3026 is in a complex dynamical state, with a primary on-going merger in the E-W direction and a secondary later stage merger with a group of galaxies in the SW, while CIZA J1824.1+3029 is dynamically relaxed. These two clusters are in a pre-merger phase. Results. We report the discovery of a Mpc-scale radio halo with a low surface brightness extension in RXC J1825.3+3026 that follows the X-ray emission from the cluster center to the remnant of a galaxy group in the SW. This is among the least massive systems and the faintest giant radio halo known to date. Contrary to this, no diffuse radio emission is observed in CIZA J1824.1+3029 nor in the region between the pre-merger cluster pair. The power spectra of the X-ray surface brightness fluctuations of RXC J1825.3+3026 and CIZA J1824.1+3029 are in agreement with the findings for clusters exhibiting a radio halo and the ones where no radio emission has been detected, respectively. Conclusions. We provide quantitative support to the idea that cluster mergers play a crucial role in the generation of non-thermal components in the ICM.

The velocity field of the Lyra complex

“…Rather, it suggests the presence of an additional population of high velocity galaxies. This supports the suggestion of Clavico et al (2019) that SG was the central galaxy of a group just disrupted by its interaction with RXCJ1825, an interaction causing the elongation of Xray isophotes toward the SW direction, the same direction of the extension of the radio halo (Botteon et al 2019).…”

“…They suggest that this is related to a recent merger where a group, now traced by its central galaxy SG, has interacted with RXCJ1825 and is in an advanced state of disruption. This picture is also well in agreement with the presence of an elongation of the radio halo just in the same direction (Botteon et al 2019). We find that the South-West region of the velocity field is very peculiar for its high velocity and the SG galaxy, there embedded, is the fifth brightest galaxy of the Lyra field.…”

“…RXCJ1825 is found to be not relaxed and we have evidence that the likely merger related to the two dominant galaxies, a very past merger according to Clavico et al (2019), lies in the plane of the sky along the East-West, slightly ENE-WSW, direction. The merger is also supported by the detection of a radio halo (Botteon et al 2019). 5.…”

“…Herebelow we discuss two possible alternative scenarios. The direction suggested by the tails points slightly at West of RXCJ1825 (see Botteon et al 2019), thus RG might be related to this cluster. In this case it is in front of RXCJ1825 running toward it.…”

“…Finally, we discuss RG, the tailed radio-galaxy lying South of CIZAJ1824 and discovered by Botteon et al (2019). RG is characterized by a velocity similar to that of BCG-CC and results well bound to the Lyra complex.…”