DETECTING THE LARGEST MAGNETS - THE UNIVERSE AND THE CLUSTERS OF GALAXIES

Recently, a large sample of rotation measures of galaxies and quasars at various redshifts has been published. This data set has been employed so far for detecting a magnetic field component of about 1.5 μG in the Virgo Supercluster and for placing an upper limit of 0.1 μM for a magnetic field in the Great Attractor Supercluster. Here this data set will be used to test for the existence of an uniform component of a cosmic/ intergalactic magnetic field outside clusters of galaxies. From a total of 309 galaxies and quasars with a known redshift and a small intrinsic rotation measure value, an upper limit of about 2 rad m-2 is obtained for such a cosmic/intergalactic contribution out to a redshift z of 3.6. From this result, and a mean density of 10-5 cm-3, an upper limit of 6 × 10-12 G follows for the strength of such a cosmic/intergalactic magnetic field outside clusters of galaxies, or an upper limit of 6 × 10-11 G for a mean density of 10-6 cm-3.