Multi-messenger insights into ultra-high-energy cosmic rays from FR0 radio galaxies

Despite their low individual luminosity, Fanaroff-Riley Type 0 (FR0) radio galaxies have emerged as potentially significant contributors to the observed flux of ultra-high-energy cosmic rays (UHECRs, E≥10 EeV) due to their substantial prevalence in the local universe. Outnumbering more powerful FR radio galaxies by approximately fivefold within redshifts of z≤0.05, FR0s may contribute a considerable fraction of the total UHECR energy density. The presented comprehensive study employs CRPropa3 simulations to estimate the mass composition and energy spectra of UHECRs emitted by FR0 galaxies. These simulations, which integrate extrapolated FR0 properties and various configurations of intergalactic magnetic fields (both random and structured), are compared to recent data from the Pierre Auger Observatory using three extensive air-shower models. By fitting the simulated spectral indices, rigidity cutoffs, and elemental fractions to Auger’s observed energy spectrum and ⟨lnA⟩ composition, we probe the contribution of FR0 sources to the UHECR flux. Furthermore, we predict the secondary photon and neutrino fluxes resulting from UHECR interactions with cosmic photon backgrounds and compare these results with current upper limits and theoretical models. This multi-messenger approach provides valuable insights into the role of low-luminosity FR0 radio galaxies within the UHECR landscape.