Systematic Survey of the Correlation between Northern HECR Events and SDSS Galaxies

TL;DR

This study systematically analyzes the spatial correlation between ultra-high-energy cosmic rays and SDSS galaxies, revealing potential signals of correlation at certain redshifts and galaxy types, which may shed light on HECR origins.

Contribution

It is the first comprehensive survey examining how HECR arrival directions correlate with galaxy properties and redshift, identifying specific galaxy types linked to HECR sources.

Findings

Potential correlation at small angular scales (<10°) with a chance probability less than 5%.

Strongest correlation found with early-type galaxies in 0.012 ≤ z < 0.018 at 90% confidence level.

Correlation signals suggest certain galaxy populations may be sources of HECRs.

Abstract

We investigated the spatial correlation between the arrival directions of the highest energy cosmic rays (HECRs) detected by the Akeno Giant Air Shower Array (AGASA) with energies above $4 \times 10^{19}$ eV and the positions of galaxies observed by the Sloan Digital Sky Survey (SDSS) within $z = 0.024$. We systematically tested the dependence of the correlation on the redshift ranges and properties of the galaxies, i.e., absolute luminosity, color, and morphology, to understand where HECR sources are and what objects are HECR sources. In the systematic survey, we found potential signals of the positive correlation at small angular scale ($<10^{\circ}$) with the (non-penalized) chance probability less than 5% in intermediate redshift ranges. Then, we estimated penalized probabilities to compensate the trial effects of angular scan, and found that the strongest correlation is produced by early-type galaxies in $0.012 \leq z < 0.018$ at 90% C.L. The possible origin of HECRs which correlating galaxies imply is also discussed.