Galaxies correlating with ultra-high energy cosmic rays

The Pierre Auger Observatory reported that 20 of the 27 highest energy cosmic rays have arrival directions within 32 of a nearby galaxy in the Veron-Cetty and Veron (VCV) Catalog of Quasars and Active Galactic Nuclei (12th ed.), with a 1% probability that this would be due to chance if the cosmic ray directions were isotropic. In this paper, we examine the correlated galaxies to gain insight into the possible ultra-high energy cosmic ray (UHECR) sources. We find that 14 of the 21 correlated VCV galaxies are active galactic nuclei (AGNs) and we determine their bolometric luminosities. The remaining seven are primarily star-forming galaxies. The bolometric luminosities of the correlated AGNs are all greater than 5 × 10⁴² erg s^-1. This may explain the absence of UHECRs from the Virgo region in spite of the large number of VCV galaxies in Virgo, since most of the VCV galaxies in the Virgo region are low-luminosity AGNs. Interestingly, the bolometric luminosities of most of the AGNs are significantly lower than that required to satisfy the minimum condition for UHECR acceleration in a continuous jet. If a UHECR-AGN correlation is substantiated with further statistics, our results lend support to the recently proposed "giant AGN flare" mechanism for UHECR acceleration.