Radio spectral study of the cluster of galaxies Abell 2255
  Spectral index studies of halos, relics, and radio galaxies provide useful
information on their origin and connection with merger processes. We present
WSRT multi-wavelength observations of the galaxy cluster Abell 2255 at 25 cm,
85 cm, and 2 m. The spectral index images allowed us to study the integrated
spectrum of halo and relic and to investigate the physical properties of the
Beaver head-tail radio galaxy belonging to the cluster. In the radio halo, the
spectral index is steeper at the center and flatter at the locations of the
radio filaments, clearly detected at 25 cm. In the relics, the spectral index
flattens, moving away from the cluster center. For the Beaver radio galaxy, the
spectrum severely steepens from the head towards the end of the tail, because
of the energy losses suffered by the relativistic particles. In the 2 m map,
which is the first high-sensitivity image presented in the literature at such a
long wavelength, a new Mpc-size emission region is detected between the known
radio halo and the NW relic. Not detecting this feature in the more sensitive
85 cm observations implies that it must have a very steep spectrum (alpha <=
-2.6). The observational properties of the radio halo suggest that either we
are looking at a superposition of different structures (filaments in the
foreground plus real halo in the background) seen in projection across the
cluster center or that the halo is intrinsically peculiar. The newly detected
extended region to the NW of the halo could be considered as an asymmetric
extension of the halo itself. However, since radio halos are known in the
literature as structures showing a regular morphology, the new feature could
represent the first example of steep Mpc-size diffuse structures (MDS),
detected around clusters at very low frequencies.
