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$\begin{figure} \par\includegraphics[width=16.6cm]{5964f1CMJN.eps}\end{figure}$ Figure 1: Radio contours of the A2255 cluster of galaxies obtained at 20 cm (Govoni et al. 2005) overlaid on the ROSAT X-ray image. The white symbols + and $\times$ indicate the position of the cluster X-ray centroid and peak respectively. The radio image at 20 cm has a FWHM of 15'' $\times$ 15''. The radio contours are: 0.4, 0.8, 1.6, 3.2, 6.4, 12.8, 25.6, and 51.2 mJy/beam. The sensitivity (1 $\sigma$ ) is 0.016 mJy/beam. For graphical reasons the first contour is at 25 $\sigma$ therefore we see only the radio galaxies of the cluster while the low brightness, diffuse emissions (radio halo and relic) are not visible here. Pointed high resolution observations at 6 cm of four cluster radio galaxies are inset. The radio contours at 6 cm are: 0.06, 0.12, 0.24, 0.48, 0.96, 1.92, 3.84, and 7.68 mJy/beam. See Table 2 for more information (e.g. resolution, sensitivity, peak brightness) of these observations.
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In the text

$\begin{figure} \par\includegraphics[width=15.25cm,clip]{5964f2.eps}\end{figure}$ Figure 2: Radio contours at 3.6 cm of four cluster radio galaxies overlaid on the DSS2 red plate (J1712.4+6401, J1713.3+6347, J1713.5+6402 and J1715.1+6402, from left to right). The radio contours are: 0.04, 0.08, 0.16, 0.32, 0.64, 1.28, 2.56, and 5.12 mJy/beam. See Table 2 for more information (e.g. resolution, sensitivity, peak brightness) of these observations.
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{5964f3.eps}\end{figure}$ Figure 3: Source J1712.4+6401: Left: total intensity contours and polarization vectors at 6 cm (4535 MHz). Right: total intensity contours and polarization vectors at 3.6 cm (8465 MHz). The angular resolution is 2.0'' $\times$ 2.0''. Contour levels are drawn at: -0.2, 0.2, 0.4, 0.8, 1, 2, 3, and 3.5 mJy/beam. The lines give the orientation of the electric vector position angle (E-field) and are proportional in length to the fractional polarization ( $1'' \simeq 16.7$ %).
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{5964f4.eps}\end{figure}$ Figure 4: Source J1713.3+6347: Left: total intensity contours and polarization vectors at 6 cm (4535 MHz). Right: total intensity contours and polarization vectors at 3.6 cm (8465 MHz). The angular resolution is 2.0'' $\times$ 2.0''. Contour levels are drawn at: -0.1, 0.1, 0.15, 0.3, 0.6, 1, and 2 mJy/beam. The lines give the orientation of the electric vector position angle (E-field) and are proportional in length to the fractional polarization ( $1'' \simeq 16.7$ %).
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{5964f5.eps}\end{figure}$ Figure 5: Source J1713.5+6402: Left: total intensity contours and polarization vectors at 6 cm (4535 MHz). Right: total intensity contours and polarization vectors at 3.6 cm (8465 MHz). The angular resolution is 2.0'' $\times$ 2.0''. Contour levels are drawn at: -0.1, 0.1, 0.5, 1, 2, 4, and 10 mJy/beam. The lines give the orientation of the electric vector position angle (E-field) and are proportional in length to the fractional polarization ( $1'' \simeq 16.7$ %).
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In the text

$\begin{figure} \par\includegraphics[width=8cm,clip]{5964f6.eps}\end{figure}$ Figure 6: Source J1715.1+6402: Left: total intensity contours and polarization vectors at 6 cm (4535 MHz). Right: total intensity contours and polarization vectors at 3.6 cm (8465 MHz). The angular resolution is 2.0'' $\times$ 2.0''. Contour levels are drawn at: -0.06, 0.06, 0.1, 0.2, 0.5, 1, 2, and 3 mJy/beam. The lines give the orientation of the electric vector position angle (E-field) and are proportional in length to the fractional polarization ( $1'' \simeq 16.7$ %).
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In the text

$\begin{figure} \par\includegraphics[width=13cm]{5964f7CMJN.eps}\end{figure}$ Figure 7: Images of the rotation measure computed using the polarization angle maps at the frequencies 4535, 4885, 8085 and 8465 MHz with a resolution of 2''. Contours refer to the total intensity images at 3.6 cm. Contour levels are: 0.06, 0.15, 0.5, 1, and 2 mJy/beam.
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In the text

$\begin{figure} \par\includegraphics[width=7.1cm]{5964f8CMJN.eps}\end{figure}$ Figure 8: Histograms of the rotation measure images for all significant pixels.
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In the text

$\begin{figure} \par\includegraphics[width=16.3cm,clip]{5964f9.eps}\end{figure}$ Figure 9: Simulated RM images for different values of the magnetic field power spectrum spectral index n. The three power spectra are normalized to have the same total magnetic field energy which is distributed over the range of spatial scales from 4 kpc up to 512 kpc. In these simulations the average field at the cluster center is $\langle\vec B\rangle$ $_{\rm0}$ = $2~\mu$ G and its energy density decreases from the cluster center according to $B^{2}\propto n_{\rm e}(r)$ . Each RM image shows a field of view of about 2 $\times$ 2 Mpc. We performed the RM integration from the cluster center up to 4 Mpc along the line-of-sight. The circles represent the cluster core radius.
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In the text

$\begin{figure} \par\includegraphics[width=7.1cm,clip]{5964f10.eps}\end{figure}$ Figure 10: Spatial distribution of member galaxies in A2255 taken from Yuan et al. (2003). The solid line represents the fit of the King model $n_{\rm gal}=n_{\rm 0~gal}(1+r^2/r_{\rm c~gal}^2)^{-\beta_{\rm gal}}$ to the data. We derived a central density of galaxies $n_{\rm 0~gal}=1.1$ $\pm$ 0.7 $\times$ 10^-7 kpc^-3 and a core radius $r_{\rm c~gal}=300$ $\pm$ 50 kpc. The $\beta _{\rm gal}$ parameter have been fixed to 3/2. The fit, has been extended only to those members characterized by a red magnitude brighter than $M_{\rm R} <-21$ .
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In the text

$\begin{figure} \par\includegraphics[width=10.2cm,clip]{5964f11.eps}\end{figure}$ Figure 11: Comparison of $\sigma _{\rm RM}$ ( left column panels) and $\arrowvert \langle {\rm RM} \rangle \arrowvert$ ( right column panels) data of the three cluster radio galaxies (dots) with the simulated profiles (dark lines). The dark gray regions show the dispersion of the profiles calculated in the simulations shown in Fig. 9. The light gray regions represent the increase of the scatter of simulated RM in the case in which the Faraday depth of the simulated RM images is extracted randomly from the 3 dimensional distribution of galaxies in A2255. The horizontal dashed lines represent the lower limits of $\sigma _{\rm RM}$ and $\langle {\rm RM} \rangle$ caused by the presence of fitting errors and the RM galactic contribution. In the right panels the value of the reduced $\chi ^{2}$ for two degree of freedom is also indicated.
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In the text

$\begin{figure} \par\includegraphics[width=16.4cm]{5964f12CMJN.eps}\end{figure}$ Figure 12: Simulated radio halo brightness (contours) and fractional polarization (color) for the three different magnetic field spectral indices n = 2, 3 and 4. For a direct comparison with the data (Fig. 15; bottom right panel) these images have been convolved with a beam of 25 $^{\prime \prime }$ (37.5 kpc) and the lowest contour level has been chosen to match the dynamic range of 10 as in the observations.
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In the text

$\begin{figure} \par\includegraphics[width=15cm,clip]{5964f13.eps}\end{figure}$ Figure 13: Left: magnetic field power spectra as a function of the distance from the cluster center for the variable magnetic field power spectrum slope model. The dashed and dotted reference lines have a slope of 2 and 4, respectively. Right: radial profile of the magnetic field strength. The solid line represents the average magnetic field strength while the gray region indicates the root mean square of the magnetic field fluctuations.
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In the text

$\begin{figure} \par\includegraphics[width=10.5cm,clip]{5964f14.eps}\end{figure}$ Figure 14: Top: simulated RM image as appears for a variable value of the magnetic field power spectrum spectral index n=2-4. In this simulation the average field at the cluster center is $\langle\vec B\rangle$ $_{\rm0}$ = $2.5~\mu$ G and its energy density decreases from the cluster center according to $B^{2}\propto n_{\rm e}(r)$ . The RM image shows a field of view of about 2 $\times$ 2 Mpc. We performed the RM integration from the cluster center up to 4 Mpc along the line-of-sight. The circle represent the cluster core radius. Bottom: comparison of $\sigma _{\rm RM}$ ( left column panel) and $\arrowvert \langle {\rm RM} \rangle \arrowvert$ ( right column panel) data of the three cluster radio galaxies (dots) with the simulated profiles (dark lines). The dark gray regions show the dispersion of the profiles calculated in the simulations. The light gray regions represent the increase of the scatter of simulated RM in the case in which the Faraday depth of the simulated RM images is extracted randomly from the 3 dimensional distribution of galaxies in A2255. The horizontal dashed lines represent the lower limits of $\sigma _{\rm RM}$ and $\langle {\rm RM} \rangle$ caused by the presence of fitting errors and the RM galactic contribution. In the right panel the value of the reduced $\chi ^{2}$ for two degree of freedom is also indicated.
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In the text

$\begin{figure} \par\includegraphics[width=14.5cm]{5964f15CMJN.eps}\end{figure}$ Figure 15: Top: simulated ( left panel) radio halo brightness for a variable magnetic field spectral index n = 2-4. For a direct comparison with the data ( right panel) this image has been convolved with a beam of 25 $^{\prime \prime }$ (37.5 kpc) and the lowest contour level has been chosen to match the dynamic range of 10 as in the observations. Bottom: simulated ( left panel) radio halo brightness (contours) and fractional polarization (color) for the variable magnetic field spectral index n = 2-4. For a direct comparison with the data see the right panel. Note that in the observed image of A2255 the radio galaxies have been subtracted and the emission of the peripheral radio relic is not in the field of view.
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In the text

$\begin{figure} \par\includegraphics[width=15.25cm,clip]{5964f2.eps}\end{figure}$	Figure 2: Radio contours at 3.6 cm of four cluster radio galaxies overlaid on the DSS2 red plate (J1712.4+6401, J1713.3+6347, J1713.5+6402 and J1715.1+6402, from left to right). The radio contours are: 0.04, 0.08, 0.16, 0.32, 0.64, 1.28, 2.56, and 5.12 mJy/beam. See Table 2 for more information (e.g. resolution, sensitivity, peak brightness) of these observations.
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$\begin{figure} \par\includegraphics[width=13cm]{5964f7CMJN.eps}\end{figure}$	Figure 7: Images of the rotation measure computed using the polarization angle maps at the frequencies 4535, 4885, 8085 and 8465 MHz with a resolution of 2''. Contours refer to the total intensity images at 3.6 cm. Contour levels are: 0.06, 0.15, 0.5, 1, and 2 mJy/beam.
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$\begin{figure} \par\includegraphics[width=7.1cm]{5964f8CMJN.eps}\end{figure}$	Figure 8: Histograms of the rotation measure images for all significant pixels.
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