New research has shown that the center of the Milky Way acts as a powerful accelerator for particles. However, there is an unknown mechanism that prevents cosmic rays reaching the cloud known as the central molecular area.
This discovery could be used to help us understand the origins and evolution of cosmic rays, particles like protons or atomic nuclei which stream through space at nearly the speed of light.
The galactic centre is a mystery zone. Although we have a good idea of what is in the center, it is so densely populated that it is difficult to study in all wavelengths, including visible and soft X-rays. This has limited what we can see.
Astronomers believe that the galactic centre is a significant source of cosmic radiations. These are nuclei and protons that have been stripped from electrons and accelerated by strong magnetic fields to relativistic speeds. The galactic center contains several objects that could be used as cosmic ray accelerators. These include supernova remnants and pulsar winds nebulae.
Based on modeling and observation data, the distribution of cosmic rays throughout the Milky Way should appear smooth and steady. The cosmic rays are produced by accelerators. They propagate in the galactic magnet field where they are likely to slow down and re-accelerate, resulting in what astronomers refer to as a cosmic radiation sea.
A source of cosmic rays with fresh energy is necessary to understand how cosmic rays can be accelerated and transported.
The cosmic rays are extremely energetic. Because their energy range produces light within the narrow wavelength range that penetrates dust, we can detect them in galactic centers.
Interstellar medium is a mixture of gas and dust in space between stars. This interaction can produce high-energy gamma ray photons with approximately 10 percent of the energy produced by their cosmic ray parents.
Xiaoyuan Huang, an astronomer at the Chinese Academy of Sciences, led a team of scientists who looked at the gamma radiation within the central molecular cloud of Milky Way with data from Fermi Large Area Telescope. They hoped to discover these sources of cosmic rays.
Gamma rays they found did not support the idea that the galactic centre is a high-energy particle accelerator, but rather, it was a region with one. They also discovered something quite surprising.
The team calculated that the density of cosmic radiations in the central molecular clouds is less than that of the cosmic-ray sea. This indicates that there is some barrier that prevents cosmic rays from reaching the central molecular clouds.
Future research will be required to determine exactly what the barrier is made of, but there are many intriguing possibilities.
Complex places are molecule clouds. Magnetic fields can be compressed if the cloud's denser areas collapse. This could create a barrier. Magnetohydrodynamic Turbulence could also be a possibility.
The solar wind modulates cosmic rays here in the Solar System. The galactic wind could play a similar function in the galactic centre. The team computed the cosmic ray density when there was a gallactic wind and returned the same result as their analysis of the data on gamma radiation.
Future research to investigate this phenomenon more in depth may be helpful to rule out some of its possible causes.
Researchers also believe that a more detailed and three-dimensional model of the galactic centre could shed more light on how cosmic rays are transported in the Milky Way. There is always more to discover.
Nature Communications published the research.