The beautiful mess in Abell 2255
  We present LOFAR observations of one of the most spectacular objects in the
radio sky: Abell 2255. This is a nearby ($z = 0.0806$) merging galaxy cluster
hosting one of the first radio halos ever detected in the intra-cluster medium
(ICM). The deep LOFAR images at 144 MHz of the central $\sim10$ Mpc$^2$ region
show a plethora of emission on different scales, from tens of kpc to above Mpc
sizes. In this work, we focus on the innermost region of the cluster. Among the
numerous interesting features observed, we discover remarkable bright and
filamentary structures embedded in the radio halo. We incorporate archival WSRT
1.2 GHz data to study the spectral properties of the diffuse synchrotron
emission and find a very complex spectral index distribution in the halo
spanning a wide range of values. We combine the radio data with Chandra
observations to investigate the connection between the thermal and non-thermal
components by quantitatively comparing the radio and X-ray surface brightness
and the spectral index of the radio emission with the thermodynamical
quantities of the ICM. Despite the multitude of structures observed in the
radio halo, we find that the X-ray and radio emission are overall well
correlated. The fact that the steepest spectrum emission is located in the
cluster center and traces regions with high entropy possibly suggests the
presence of seed particles injected by radio galaxies that are spread in the
ICM by turbulence generating the extended radio halo.
