Galaxies Correlating with Ultra-high Energy Cosmic Rays

TL;DR

This study investigates the correlation between ultra-high energy cosmic rays and nearby galaxies, finding that many are active galactic nuclei with high luminosities, supporting the giant AGN flare hypothesis for UHECR acceleration.

Contribution

It provides new insights into the types and luminosities of galaxies correlated with UHECRs, suggesting a potential acceleration mechanism involving giant AGN flares.

Findings

Majority of correlated galaxies are AGNs with high bolometric luminosity.

Correlated galaxies are mostly outside the Virgo region, explaining the lack of UHECRs there.

Results support the giant AGN flare mechanism for UHECR acceleration.

Abstract

The Pierre Auger Observatory reports that 20 of the 27 highest energy cosmic rays have arrival directions within 3.2 deg of a nearby galaxy in the Veron-Cetty & Veron Catalog of Quasars and Active Galactic Nuclei (12th Ed.), with ~5 of the correlations expected by chance. In this paper we examine the correlated galaxies to gain insight into the possible UHECR sources. We find that 14 of the 21 correlated VCV galaxies are AGNs and we determine their bolometric luminosities. The remaining 7 are primarily star-forming galaxies. The bolometric luminosities of the correlated AGNs are all greater than 5 x 10^{42} erg/s, which 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 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.