Deep multi-frequency rotation measure tomography of the galaxy cluster
  A2255
  We aim to unveil their 3-dimensional geometry of Abell 2255 through WSRT
observations at 18, 21, 25, 85, and 200 cm. The polarization images of the
cluster were processed through rotation measure (RM) synthesis, producing three
final RM cubes. The radio galaxies and the filaments at the edges of the halo
are detected in the high-frequency RM cube, obtained by combining the data at
18, 21, and 25 cm. Their Faraday spectra show different levels of complexity.
The radio galaxies lying near by the cluster center have Faraday spectra with
multiple peaks, while those at large distances show only one peak, as do the
filaments. Similar RM distributions are observed for the external radio
galaxies and for the filaments, with much lower average RM values and RM
variance than those found in previous works for the central radio galaxies. The
85 cm RM cube is dominated by the Galactic foreground emission, but it also
shows features associated with the cluster. At 2 m, no polarized emission from
A2255 nor our Galaxy is detected. The radial trend observed in the RM
distributions of the radio galaxies and in the complexity of their Faraday
spectra favors the interpretation that the external Faraday screen for all the
sources in A2255 is the ICM. Its differential contribution depends on the
amount of medium that the radio signal crosses along the line of sight. The
filaments should therefore be located at the periphery of the cluster, and
their apparent central location comes from projection effects. Their high
fractional polarization and morphology suggest that they are relics rather than
part of a genuine radio halo. Their inferred large distance from the cluster
center and their geometry could argue for an association with large-scale
structure (LSS) shocks.
