2025MNRAS.539.2435E

We investigate ultra-fast outflows (UFOs) in active galactic nuclei (AGN) as potential sources of ultra-high-energy cosmic rays (UHECRs). We focus on cosmic-ray nuclei, an aspect not explored previously. These large-scale, mildly relativistic outflows, characterized by velocities up to half the speed of light, are a common feature of AGN. We study the cosmic-ray spectrum and maximum energy attainable in these environments with 3D CRPropa simulations and apply our method to 86 observed UFOs. Iron nuclei can be accelerated up to $\sim 10^{20}\,$ eV at the wind-termination shock in some UFOs, but the escaping flux is strongly attenuated due to photonuclear interactions with intense AGN photon fields. The maximum energy of nuclei escaping a typical UFO is limited by photodisintegration to below $\sim 10^{17}\,$ eV. However, in the most extreme $5{\small --}10~{{\ \rm per\ cent}}$ of UFOs, helium (nitrogen) [iron] nuclei can escape with energy exceeding $10^{17.4}$ ($10^{17.8}$) [$10^{18.4}$] eV. Protons and neutrons, either primaries or by-products of photodisintegration, escape UFOs with little attenuation, with half of the observed UFOs reaching energies exceeding $10^{18}\,$ eV. Thus, UFOs emerge as viable sources of the diffuse cosmic-ray flux between the end of the Galactic cosmic rays and the highest-energy extragalactic flux. For a few UFOs in our sample, nuclei escape without photodisintegration with energy up to $10^{19.8}\,$ eV. This occurs during low-emission states of the AGN, which would make UFOs intermittent sources of UHECR nuclei up to the highest observed energies. The role of UFOs as UHECR sources is testable with neutrino telescopes due to a substantial accompanying flux of PeV neutrinos.