3.6. Magnetic fields in Abell clusters
Already the short summary of the main experimental techniques used for the detection of large scales magnetic fields shows that there may be problems in the determination of magnetic fields right outside galaxies. There magnetic fields are assumed to be often of nG strength. However, due to the lack of sources for the determination of the column density of electrons, it is hard to turn the assumption into an experimental evidence. Magnetic fields in clusters have been recently reviewed by Giovannini  and, more extensively, by Carilli and Taylor .
Since various theoretical speculations suggest that also clusters are magnetized, it would be interesting to know if regular Abell clusters posses large scale magnetic fields. Different results in this direction have been reported [70, 71, 72] (see also ). Some studies during the past decade [70, 71] dealt mainly with the case of a single cluster (more specifically the Coma cluster). The idea was to target (with Faraday rotation measurements) radio sources inside the cluster. However, it was also soon realized that the study of many radio sources inside different clusters may lead to experimental problems due to the sensitivity limitations of radio-astronomical facilities. The strategy is currently to study a sample of clusters each with one or two bright radio-sources inside.
In the past it was shown that regular clusters have cores with a detectable component of RM [79, 72]. Recent results suggest that µ Gauss magnetic fields are indeed detected inside regular clusters . Inside the cluster means in the intra-cluster medium. Therefore, these magnetic fields cannot be associated with individual galaxies.
Regular Abell clusters with strong x-ray emission were studied using a twofold technique [73, 74]. From the ROSAT full sky survey, the electron density has been determined . Faraday RM (for the same set of 16 Abell clusters) has been estimated through observations at the VLA radio-telescope.
The amusing result (confirming previous claims based only on one cluster [70, 71]) is that x-ray bright Abell clusters possess a magnetic field of µ Gauss strength. The clusters have been selected in order to show similar morphological features. All the 16 clusters monitored with this technique are at low red-shift (z < 0.1) and at high galactic latitude (| b| > 200).
These recent developments are rather promising and establish a clear connection between radio-astronomical techniques and the improvements in the knowledge of x-ray sky. There are various satellite missions mapping the x-ray sky at low energies (ASCA, CHANDRA, NEWTON (10)). There is the hope that a more precise knowledge of the surface brightness of regular clusters will help in the experimental determination of large scale magnetic fields between galaxies. It should be however mentioned that evidence for the presence of relativistic electrons and magnetic fields in clusters was directly available even before, from measurements of extended regions of radio synchrotron emission (for frequencies 10-2 < < 1 GHz) .
It is interesting to notice that intra-cluster magnetic fields of µ G strength can induce Faraday rotation on CMB polarization. By combining informations from Sunyaev-Zeldovich effect and X-ray emission from the same clusters, it has been recently suggested that a richer information concerning electron column density can be obtained . In Fig. 4 the results reported in  are summarized. In Fig. 4 the RM of the sample of x-ray bright Abell clusters is reported after the subtraction of the RM of the galaxy. At high galactic latitude (where all the observed clusters are) the galactic contribution is rather small and of the order of 9.5 rad / m2.
Figure 4. From Ref.  the RM deduced from a sample of 16 X-ray bright Abell clusters is reported as a function of the source impact parameter.
The results reported in . In Fig. 4 the open points represent sources viewed through the thermal cluster gas, whereas the full points represent control sources at impact parameters larger than the cluster gas. The excess in RM attributed to clusters sources is clearly visible.
Using the described techniques large scale magnetic fields can be observed and studied in external galaxies, in clusters and also in our own galaxy. While the study of external galaxies and clusters may provide a global picture of magnetic fields, the galactic observations performed within the Milky Way are more sensitive to the local spatial variations of the magnetic field. For this reasons local and global observations are complementary. The flipped side of the coin, as we will see in the second part of the present Section, is that the global structure of the magnetic field of our galaxy is not known directly and to high precision but it is deduced from (or corroborated by) the global knowledge of other spiral galaxies.
10 ASCA is operating between 0.4 AND 10 keV and it is flying since February 1993. CHANDRA (NASA mission) and NEWTON (ESA mission) have an energy range comparable with the one of ASCA and were launched, almost simultaneously, in 1999. Back.