Astrophysics: A potential barrier for cosmic rays at the galactic centre

The galactic centre of the Milky Way may contain a high-energy particle accelerator and a barrier that suppresses the passage of cosmic rays from the surrounding cosmic-ray sea into its central molecular zone. The findings, published in Nature Communications, may aid our understanding of the origins of cosmic rays.

Galactic cosmic rays are high energy particles that originate from outside the solar system and eventually reach Earth. They play an important role in aiding our understanding of high energy particles in extreme astrophysical environments, and the galactic centre of the Milky Way has been proposed as a cosmic-ray source. Previous observations have unveiled a relatively smoothly distributed sea of galactic cosmic rays throughout the galaxy. It is thought that cosmic rays could be accelerated in the Milky Way as a result of interactions with supernova remnants or stellar winds, which would enable them to propagate and spread throughout the galaxy. However, to understand the nature of very high energy cosmic rays (TeV-PeV), further exploration of the different emission components in the central molecular zone (CMZ) of the Milky Way’s galactic centre is needed.

Xiaoyuan Huang and colleagues reanalysed Fermi Large Area Telescope data of the CMZ of the Milky Way and identify a GeV-TeV cosmic ray component — the low energy component of an earlier TeV-PeV source. The authors suggest that this supports the presence of a high-energy particle accelerator at the galactic centre. The authors also found that the inferred energy density of cosmic rays in the CMZ is lower than the cosmic-ray sea component. They propose that this demonstrates the existence of a barrier, which can prevent penetration of particles from the cosmic-ray sea into the CMZ.

Huang and co-authors conclude that future 3D modelling of the galactic centre may aid our understanding of the origin of cosmic rays, as well as their transport in the galactic centre.