Detection of Large-Scale Cosmic Magnetic Fields
  Observational evidence for the existence of cosmic magnetic fields in
intergalactic space coherent over Mpc scales is presented. Using an
unprecedentedly large sample of Faraday rotation measures of radio sources from
the NRAO VLA Sky Survey data and the photometric redshift galaxy catalog from
the sixth Data Release of the Sloan Digital Sky Survey (SDSS), we measure the
cross-correlations between the rotation measures and the galaxy density field
distributed along the source sightlines. It is shown that the rotation measures
for sightlines passing through high density regions at separation r >= 1 Mpc/h
from the locations of background radio sources are significantly enhanced. We
discuss possible generators of this enhancement and interpret it to be
intergalactic magnetic fields coherent over 1 Mpc/h with mean field strength
B~30 nG.
