Cross-Correlation between UHECR Arrival Distribution and Large-Scale Structure

TL;DR

This study finds a positive correlation between ultra-high-energy cosmic ray arrival directions and nearby large-scale structures, suggesting extragalactic origins and providing constraints on intergalactic magnetic fields.

Contribution

It demonstrates a significant angular correlation between UHECRs and IRAS galaxies, especially in the southern sky, and discusses implications for cosmic ray source models and magnetic fields.

Findings

Positive correlation of PAO UHECRs with IRAS galaxies within 15°

No clear correlation observed in AGASA data due to smaller exposure

Correlation within 8° when combining all-sky data sets

Abstract

We investigate correlation between the arrival directions of ultra-high-energy cosmic rays (UHECRs) and the large-scale structure (LSS) of the Universe by using statistical quantities which can find the angular scale of the correlation. The Infrared Astronomical Satellite Point Source Redshift Survey (IRAS PSCz) catalog of galaxies is adopted for LSS. We find a positive correlation of the highest energy events detected by the Pierre Auger Observatory (PAO) with the IRAS galaxies inside $z=0.018$ within the angular scale of $\sim 15^{\circ}$. This positive correlation observed in the southern sky implies that a significant fraction of the highest energy events comes from nearby extragalactic objects. We also analyze the data of the Akeno Giant Air Shower Array (AGASA) which observed the northern hemisphere, but the obvious signals of positive correlation with the galaxy distribution are not found. Since the exposure of the AGASA is smaller than the PAO, the cross-correlation in the northern sky should be tested using a larger number of events detected in the future. We also discuss the correlation using the all-sky combined data sets of both the PAO and AGASA, and find a significant correlation within $\sim 8^{\circ}$. These angular scales can constrain several models of intergalactic magnetic field. These cross-correlation signals can be well reproduced by a source model in which the distribution of UHECR sources is related to the IRAS galaxies.