4.3. Mini-halos

There are a few clusters where the relativistic electrons can be traced out quite far from the central galaxy, forming what is called mini-halo. Mini-halos are diffuse steep-spectrum radio sources, extended on a moderate scale (up to 500 kpc), surrounding a dominant radio galaxy at the cluster center. Unlike radio halos and relics, mini-halos are not tied to on-going merger events in clusters, as they are typically found at the center of cooling core, i.e relaxed, clusters. The prototype example of a mini-halo is at the center of the Perseus cluster. The size is ~ 450 kpc, with no significant polarization [94, 95]. The strong polarized emission, detected through the entire cluster at 92 cm, at a Faraday depth (~ 25 - 90 rad m^-2) higher than the galactic contribution seems not to be related to the mini-halo [96].

Other examples of mini-halos are in PKS 0745-191 [97], Virgo [98], and possibly A2390 [43]. The mini-halo in A2390 is polarized at levels of 10 - 20%.

Gitti et al. [99] suggested that the electrons of the Perseus mini-halo cannot be supplied by the central radio galaxy, but are continuously undergoing reacceleration due to the MHD turbulence associated with the cooling flow region. They show that an isotropic magnetic field compression [100] appears to well reproduce the observed surface brightness profile and total synchrotron spectrum along with the radial spectral steepening. On the other hand the radial compression of the magnetic field [101] does not appear to be applicable to the mini-halo in the Perseus cluster. The above model was successfully applied also to the mini-halo in A2626 [102]. Pfrommer and Enßlin [103], on the other hand, discussed the possibility that relativistic electrons in mini-halos are of secondary origin and thus produced from the interaction of cosmic ray protons with the ambient thermal protons.