4.4. Magnetic fields beyond clusters

Recent attempts to detect intergalactic magnetic fields beyond clusters, i.e. in even more rarefied regions of the intergalactic space, have shown recent promise in imaging diffuse synchrotron radiation of very low level. A very faint emission has been detected at 327 MHz with the WSRT in the Coma cluster region between the radio halo Coma C and the relic 1253+275 [104]. The surface brightness of this diffuse emission is very low and it is only enhanced at low frequency and low resolution, so it is only visible as a positive noise in Fig. 3. The existence of this feature is confirmed by the asymmetric extension of the central halo Coma C imaged at 1.4 GHz with the Effelsberg single dish [35], and by the VLA data at 74 MHz [36]. The equipartition magnetic field in this region [105] is 10^-7G. These data may indicate the existence of a more widespread and somewhat lower intergalactic magnetic field than in the ICM. It could be possibly associated with large-scale shocks related to the formation of the large-scale structure in the universe.

A possible evidence of magnetic field in the intergalactic medium is found [106] in the filament of galaxies ZwCl 2341.1+0000 at z ~ 0.3.

The study of the RM of distant quasars could provide an independent information on the magnetic field in the intergalactic medium. An upper limit of 10^-9 G for a cosmic magnetic field outside clusters of galaxies has been derived in the literature [1, 107]. However, this limit relies on several assumptions. New generation instruments will shed light on this point.