New limits on extragalactic magnetic fields from rotation measures

TL;DR

This paper uses rotation measure data from the NRAO VLA Sky Survey to set new upper limits on the strength of extragalactic magnetic fields, constraining their intensity based on observed data and simulations.

Contribution

It provides the first robust statistical upper limits on extragalactic magnetic fields using large-scale rotation measure data and realistic simulations.

Findings

Extragalactic magnetic fields with coherence length at Jeans' length are below 1.7 nG.

Fields coherent across the observable universe are below 0.65 nG.

Limits are independent of the origin of the cosmological fields.

Abstract

We take advantage of the wealth of rotation measures data contained in the NRAO VLA Sky Survey catalogue to derive new, statistically robust, upper limits on the strength of extragalactic magnetic fields. We simulate the extragalactic magnetic field contribution to the rotation measures for a given field strength and correlation length, by assuming that the electron density follows the distribution of Lyman-$\alpha$ clouds. Based on the observation that rotation measures from distant radio sources do not exhibit any trend with redshift, while the extragalactic contribution instead grows with distance, we constrain fields with Jeans' length coherence length to be below 1.7~nG at the $2\sigma$ level, and fields coherent across the entire observable Universe below 0.65~nG. These limits do not depend on the particular origin of these cosmological fields.