5.2. Clusters of galaxies as the largest magnets ( ~ 5 to 50 Mpc)
5.2.1. Several sources per individual cluster of galaxies
The technique of choosing selected celestial zones (see Sect. 2.1.4), originally used to study extended objects in our Milky Way, has been applied to study the intracluster gas within an entire cluster of galaxies, using more distant background galaxies and quasars as probes. Thus Broten et al. (1986) first studied in this way the Abell 2319 cluster of galaxies at z = 0.053 ( 320 Mpc), with a canonical Abell cluster diameter of 6 Mpc.
A large scale magnetic feature was found at a level of 120 radians/m2 in the A 2319 cluster of galaxies (Table 2 in Vallée et al., 1986). This is compatible with a model of a magnetic field located in a few uniform cells, each cell being randomly oriented. This allowed the first detection of a magnetic field strength in the intracluster gas between the galaxies, with values of 2 µGauss in cells of about 20 kpc in A 2319 (Vallée et al., 1987).
This technique involved the transformation of the X-ray emission profile of the intracluster gas into a thermal electron density distribution, following the inversion method first adapted for X-ray astronomy in Vallée (1981).
In a similar search for magnetic features in the Great Attractor supercluster of galaxies, no excess of rotation measure could be found across it, with an upper limit of 8 radians/m2 (Vallée, 1989). In turn, this implied an upper limit on the supercluster magnetic field of at most 0.1 µGauss for the ordered component Breg.
Using this technique, a second example of a detected excess rotation measure from a cluster of galaxies was found in the Coma cluster, at a level of 40 radians/m2 (Kim et al., 1990), implying a cluster magnetic field Breg of 2 µGauss. Also, Vallée (1990a) found an excess rotation measure of 10 radians/m2 and a possible large scale magnetic field in the Virgo supercluster of galaxies, with Breg of about 2 µGauss.
The nearest large scale structure or arrangement of galaxies found is the Local Virgo Supercluster of galaxies, centered at z = 0.004 ( 24 Mpc) and extending all the way to us (overall size ~ 50 Mpc). To study the Virgo supercluster, a Virgo-centered supercluster coordinate system has been proposed (see Appendix in Vallée, 1991c), improving on the earlier system of de Vaucouleur.
A further analysis of a 'complete sample' of galaxies in the Virgo supercluster of galaxies was made by Vallée (1993b), using the well-calibrated minimum-energy magnetic field obtained from a consistent set of 1.4 GHz radio observations, and using the Virgo-centered supercluster coordinate system. A trend for a larger neutral hydrogen mass in spiral galaxies located farther from the plane of the Virgo Supercluster was found, but there was no change of the magnetic field value as one goes away from the supercluster center.