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3.1. Introduction 23
Figure 3.1: Example of a relic. Left: Westerbork Synthesis Radio Telescope (WSRT) image at
1.4 GHz overlaid on the X-ray emission from ROSAT showing the hot ICM (red contours). Right:
Polarization electric field vectors at 4.9 GHz (corrected for the effects of Faraday rotation) and
with lengths proportional to the fractional polarization at the same frequency. A reference vector
for 100% polarization is drawn in the top left corner. Both figures are taken from van Weeren et
al. (2010).
(e.g. Giovannini & Feretti 2002; Clarke & Enßlin 2006; Giacintucci et al. 2009. Both radio halos
and relics show steep radio spectra (α≥1) and low surface brightness (∼ 10 −6 Jy arcsec −2 at
1.4 GHz).
The most widely accepted scenario for the origin of radio relics is that (primary) relativistic
electrons are injected into the ICM from AGN activity and/or from star formation in galaxies. They
are then accelerated by shocks or mergers (Enßlin et al. 1998; Röttiger, Burns & Stone 1999) or
by adiabatic compression of fossil radio plasma (Ensslin & Gopal-Krishna 2001). This picture is
supported by the typical high degree of polarization and the orientation of magnetic field vectors,
which appear to be oriented along the major axis of elongated relics with coherence scales of
several hundreds of kpc (Right panel of Fig. 3.1). Indeed, compression and/or shocks can locally
amplify and align the field with their surfaces.
In contrast, the formation of radio halos is still debated. Since primary relativistic electrons
lose energy on short timescales (∼ 10 7−8 yrs), they cannot diffuse any significant distance in
the cluster before ceasing to radiate. To explain the large extension, up to Mpc scales, of radio
halos, continuous injection processes and/or particle re-acceleration are required. These might
be due to gas turbulence, efficient in energetic mergers (primary models). Another possibility
is that secondary electrons are continuously injected in the ICM by hadronic collisions between
relativistic protons and ICM thermal protons (secondary models). Because of their higher mass,
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