Non-thermal emission from galaxy clusters: a Pandoras vase for astrophysics

Deep radio observations of the sky have revealed the presence of extended (∼ 1 Mpc) radio sources in about 50 merging galaxy clusters (see 1,2 and references therein). This diffuse radio emission is not related to unresolved radio-galaxies, but rather to the presence of relativistic particles (γ >>1000) and magnetic fields of the order of µGauss in the intracluster volume. The physical mechanisms responsible for the origin of this non-thermal intracluster component are matter of debate (e.g. 3,4 ), as well as the effects of intracluster cosmic rays (CRs) and magnetic fields on the thermodynamical evolution and mass estimate of galaxy clusters (e.g. 5,6 ). A deep understanding of the evolutionary physics of all the different cluster components (dark matter, galaxies, thermal and non-thermal intracluster medium -ICM) and of their mutual interactions is indeed essential for high-precision cosmology with galaxy clusters 7 .

In the following, I will give an overview of our current knowledge of the non-thermal component of galaxy clusters. I will also stress the importance of a new generation of multi-wavelength telescopes -such as the Low Frequency Array (LOFAR), and the Gamma-and hard X-ray (HXR) satellites Fermi and NuSTAR -for a deep understanding of the non-thermal cluster physics. The ΛCDM model with H 0 =70 km s -1 Mpc -1 , Ω m = 0.3 and Ω Λ = 0.7 has been adopted.

The presence of intracluster CR electrons (CRes) and magnetic fields was pointed out in 1970 by Willson 8 , whose detailed radio analysis of the Coma cluster followed the first detection in 1959 of a noticeably diffuse cluster radio source -Coma C -by Large et al. 9 .

Diffuse cluster radio sources are very elusive. On the one hand their low-surface brightness (∼ µJy/arcsec 2 at 1.4 GHz) requires low angular resolution observations in order to achieve the necessary signal-to-noise ratio. On the other hand complementary high-resolution observations are needed in order to identify and remove emission from point sources. Samples of clusters hosting diffuse radio sources started to be available from the 90’s (e.g. 10 ), with the advent 15,16,18 ). A2163 is the hottest Abell cluster and it hosts one of the most luminous radio halos 15 . A radio-mini halo is at the center of the most X-ray luminous cluster RX J1347.5-1145 16 . Double radio relics have been discovered in ZwCl 2341.1+0000 18 . Diffuse radio emission has also been detected in this cluster along the optical filament of galaxies shown here by dashed contours 19 .

of continuum radio surveys such as the NVSS 11 . It emerged that the non-thermal plasma emitting at radio wavelengths could be not a common property of galaxy clusters (see 12 and references therein). It was also found that a common feature of intracluster radio sources is a steep synchrotron spectral slope (α 1 a , 13 ). Based on the observed difference in other physical properties (e.g. position in the host cluster, size and morphology) a working classification with three main classes of intracluster radio sources was soon adopted 14 :

• radio halos are extended ( 1 Mpc) sources that have been detected at the centre of merging clusters. Their morphology is similar to the X-ray morphology of the cluster (Fig. 1, left panel);

• radio mini-halos are smaller sources ( 500 kpc) located at the centre of cool-core clusters. They surround a powerful radio galaxy (Fig. 1, middle panel);

• radio relics have extensions similar to halos and are also detected in merging clusters, but they are usually located in the cluster outskirts and have an elongated morphology (Fig. 1, right panel). In some clusters double relics have been detected (see 17,18 and references therein).

The discovery of a non-thermal intracluster component through radio observations has opened a number of astrophysical questions: How do cosmic rays and magnetic fields originate within the intracluster volume? Are all the clusters hosting a non-thermal component? How does it affect the thermodynamical evolution and the mass estimates of galaxy clusters? As detailed in the following sections, new observational facilities will allow us to address most of these open questions in the next few years.

3 Non-thermal component of galaxy clusters: the known and unknown

The intensity of intracluster magnetic fields can be measured 20,4 :

• through Faraday rotation measures (RM) of polarized radio sources within / behind clusters (current measurements: ∼1-10 µGauss);

• by comparing synchrotron radiation from diffuse radio sources with non-thermal HXR emission due to Inverse Compton (IC) scattering of CMB photons by relativistic electrons (current measurements: ∼0.1-0.3 µGauss);

• by assuming energy equipartition between intracluster CRs and magnetic fields (current measurements: ∼0.1-1.0 µGauss);

• through the study of cold fronts in merging galaxy clusters (current measurements: ∼10 µGauss).

The discrepancy between these different measurements can indeed be related to

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