The great barrier surrounding the core of the Milky Way is encountered in 2287 by the starship NCC-1701-A under the command of Captain James T. Kirk. Supposedly, it is impenetrable, and any ship that nevertheless dares to cross it will be destroyed. Kirk and his squad from the USS Enterprise prove the legend to be false. In fact, there is a kind of barrier around the galactic center – and this barrier is not an impenetrable wall. But, as astronomers have now discovered, it is an obstacle to cosmic rays.

What exactly is the issue? Well, the Milky Way is, in a sense, swimming in a sea of cosmic rays generated by very different processes and interacting with the galactic magnetic field. This sea surges along relatively uniformly, like basically the oceans of the earth. But unlike the terrestrial waters, this ocean has a kind of bathtub drain: the central black hole Sagittarius A* at its center. On the one hand, it acts like a gigantic accelerator – the way water swirls near the outflow. The black hole is surrounded by a much larger, very dynamic cloud, the central molecular cloud (CMZ), which is heated by the black lich and feeds it material. This CMZ, in turn, now appears to promote the formation of a barrier that suppresses the passage of cosmic rays from the surrounding sea of cosmic rays into the central molecular zone. The results, published in Nature Communications, could contribute to understanding the origins of cosmic rays.

In the language of astronomers, then, it reads a bit more prosaically. Xiaoyuan Huang and colleagues have reanalyzed Fermi Large Area Telescope data for the Milky Way’s CMZ and identified a component of cosmic rays in the giga- and tera-eV range – the low-energy component of an earlier tera-peta-eV source. The authors suggest that this supports the presence of a high-energy particle accelerator at the galactic center. The authors also found that the inferred energy density of cosmic rays in the CMZ is lower than that of cosmic rays in the ocean. This very fact would indicate the existence of a barrier that at least partially prevents the penetration of particles from the cosmic ray sea into the CMZ.