Extragalactic magnetic field constraints from ultrahigh-energy cosmic rays from local galaxies

We interpret the correlation between local star-forming galaxy positions and ultrahigh-energy cosmic-ray (UHECR) directions, recently detected by the Pierre Auger Observatory (PAO), in terms of physical parameters: the local density of sources and the magnetic fields governing the UHECR propagation. We include a Galactic magnetic field model on top of a random extragalactic magnetic field description to determine the level of UHECR deflections expected from an ensemble of source positions. Besides deflections in magnetic fields, we also take into account energy losses with background photon fields as well as spectrum and composition measurements by the PAO. We find consistency between the PAO anisotropy measurement and the local star-forming galaxy density for large extragalactic magnetic field strengths with B gt 0.2 rm nG (for a coherence length of 1 rm Mpc) at the 5σ confidence level. Larger source densities lead to more isotropic background and consequently allow for weaker extragalactic magnetic fields. However, the acceleration of UHECR by such abundant sources is more challenging to motivate. Too large source densities and extragalactic magnetic field strengths, on the other hand, are also disfavoured as that decreases the expected level of anisotropy. This leads to upper limits of B lt 22 rm nG and rho 0 lt 8.4 times 10 {-2} rm Mpc {-3} at the 90 per cent confidence level.