Dynamic cluster outskirts

Dynamic cluster outskirts

Composite image of the galaxy cluster Abell 2255.

Image credit: ROSAT/LOFAR/SDSS/Botteon, et al. 2022

Radio observations of galaxy clusters so far have shown that diffuse emission can extend up to 1 Mpc from their centers. Cosmological simulations, on the other hand, predicted that turbulence and shocks driven by large-scale accretion should be able to produce synchrotron radiation at much larger scales as well, even though whether this emission would be detectable is still unclear. To clarify the matter and explore the implications on our understanding of the role of relativistic particles and magnetic fields, Andrea Botteon and collaborators (Sci. Adv. 8, 44; 2022) carried out deep radio observations of the cluster Abell 2255.

The authors used the Low-Frequency Array (LOFAR) at 145 and 49 MHz, which allowed them to reach significantly better resolution and sensitivity than previously available observations of the same cluster. The data shows that the diffuse emission is widespread, covering a region that extends as far as 5 Mpc from the center, therefore providing support to the theoretical predictions.  Furthermore, an analysis of the distribution of the spectral index of the radio emission confirms the simultaneous presence of shocks and turbulence. 

The presence of magnetic fields of about 1 microgauss and relativistic electrons between 1- to 10-GeV are required to explain the intensity of the emission. However, compression of a primordial magnetic field is not able to account for the observed field intensity, therefore suggesting that turbulent dynamo amplification plays a key role in the process. Furthermore, the findings demonstrate that the efficient kinetic energy conversion by shocks and turbulence, generated by the intense dynamical activity during the structure formation process, produces a significant non-thermal component of relativistic particles.

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