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. 2019 Jan 2;5(1):eaau8227.
doi: 10.1126/sciadv.aau8227. eCollection 2019 Jan.

Filaments of galaxies as a clue to the origin of ultrahigh-energy cosmic rays

Jihyun Kim  1 , Dongsu Ryu  1 , Hyesung Kang  2 , Suk Kim  3 , Soo-Chang Rey  4
Affiliations

Filaments of galaxies as a clue to the origin of ultrahigh-energy cosmic rays

Jihyun Kim et al. Sci Adv. .

Abstract

Ultrahigh-energy cosmic rays (UHECRs) are known to come from outside of our Galaxy, but their origin still remains unknown. The Telescope Array (TA) experiment recently identified a hotspot, that is, a high concentration of anisotropic arrival directions of UHECRs with energies above 5.7 Å ~ 1019 eV. We report here the presence of filaments of galaxies, connected to the Virgo Cluster, in the sky around the hotspot and a statistically significant correlation between hotspot events and the filaments. With 5-year TA data, the maximum significance of binomial statistics for the correlation is estimated to be 6.1σ at correlation angle 3.4°. The probability that the above significance appears by chance is ~2.0 × 10-8 (5.6σ). On the basis of this finding, we suggest a model for the origin of TA hotspot UHECRs; they are produced at sources in the Virgo Cluster, and escape to and propagate along filaments, before they are scattered toward us. This picture requires the filament magnetic fields of strength ≳ 20 nG, which need to be confirmed in future observations.

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Figures

Fig. 1
Fig. 1. Hammer projection of nearby galaxies within 50 Mpc (gray dots) and TA UHECR events above 5.7 × 1019 eV (large black dots) in equatorial coordinates.
The concentration of galaxies, represented by dark gray dots, is the Virgo Cluster. The red square marks the center of the TA hotspot. The pink and sky blue lines indicate the Galactic and Supergalactic planes, respectively. The purple line indicates the field of view limit of the TA experiment.
Fig. 2
Fig. 2. Six filaments of galaxies connected to the Virgo Cluster, F1 to F6.
The color dots show the galaxies that belong to the filaments, and the color lines draw the spines of the filaments. The color codes the distance from us to the galaxies and the spines (h is the Hubble parameter). The TA events above 5.7 × 1019 eV (large black dots), nearby galaxies within 50 Mpc (gray dots), the Virgo Cluster galaxies (dark gray dots), and the center of the TA hotspot (red square) are also shown.
Fig. 3
Fig. 3. Cumulative binomial probability for the excess of TA hotspot events around filaments F1 to F3 as a function of angular separation.
Fig. 4
Fig. 4. Schematic drawing of a model for the origin of TA hotspot events.
UHECRs are postulated to be produced at a source or sources inside the Virgo Cluster. After they are confined by cluster magnetic fields and roam around for a while, UHECRs escape to the filaments connected to the cluster. Then, they propagate along the filaments. Some of them are eventually scattered by the random component of magnetic fields and may come to our Galaxy. Here, the Virgo Cluster, represented by a gray circle, and filaments F1 to F6 are plotted in the Supergalactic coordinates. Our Galaxy is located at the coordinate origin.
Fig. 5
Fig. 5. Trajectories of two UHE protons with 6 × 1019 eV around a simulated cluster.
The cluster has the x-ray weighted temperature, T = 3.5 keV, and the core magnetic field of ~1.5 μG. Colors code the magnetic field strength; the cluster is represented by the blue tone, while filaments are represented by the red tone. The box drawn with white lines has the volume of 42 × 17.5 × 17.5 (h−1 Mpc)3. The trajectories are plotted with white dots.
Fig. 6
Fig. 6. Displacement of two UHE protons, shown in Fig. 5, from their launching positions.
Case 1 is the one shown in the top panel of Fig. 5, and case 2 is in the bottom panel. Colors map the magnetic field strength that the UHECRs experience.
See this image and copyright information in PMC

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