The Imprint of Large-scale Structure on the Ultrahigh-energy Cosmic-Ray Sky

Ultrahigh-energy cosmic rays (UHECRs) are atomic nuclei from space with vastly higher energies than any other particles ever observed. Their origin and chemical composition remain a mystery. As we show here, the large and intermediate angular scale anisotropies observed by the Pierre Auger Observatory are a powerful tool for understanding the origin of UHECRs. Without specifying any particular production mechanism but only postulating that the source distribution follows the matter distribution of the local universe, a good accounting of the magnitude, direction, and energy dependence of the dipole anisotropy at energies above 8 × 1018 eV is obtained after taking into account the impact of energy losses during propagation (the "GZK horizon"), diffusion in the extragalactic magnetic field, and deflections in the Galactic magnetic field (GMF). This is a major step toward the long-standing hope of using UHECR anisotropies to constrain UHECR composition and magnetic fields. The observed dipole anisotropy is incompatible with a pure proton composition in this scenario. With a more accurate treatment of energy losses, it should be possible to further constrain the cosmic-ray composition and properties of the extragalactic magnetic field, self-consistently improve the GMF model, and potentially expose individual UHECR sources.