Constraints on large-scale magnetic fields in the intergalactic medium using cross-correlation methods

Large-scale coherent magnetic fields in the intergalactic medium (IGM) are presumed to play a key role in the formation and evolution of the cosmic web, and in large-scale feedback mechanisms. However, they are theorized to be extremely weak, in the nano-Gauss regime. To search for a statistical signature of these weak magnetic fields, we perform a cross-correlation between the Faraday rotation measures (RMs) of 1742 radio galaxies at z > 0.5 and large-scale structure at 0.1 < z < 0.5, as traced by 18 million optical and infrared foreground galaxies. No significant correlation signal was detected within the uncertainty limits. We are able to determine model-dependent 3 sigma upper limits on the parallel component of the mean magnetic field strength of filaments in the IGM of similar to 30 nG for coherence scales between 1 and 2.5Mpc, corresponding to a mean upper bound RM enhancement of similar to 3.8 rad m(-2) due to filaments along all probed sightlines. These upper bounds are consistent with upper bounds found previously using other techniques. Our method can be used to further constrain intergalactic magnetic fields with upcoming future radio polarization surveys.