The center of the Milky Way is a powerful particle accelerator, new research reveals, but there are also some unknown mechanisms that block the invasion of cosmic rays into large clouds called the central molecular zone.
This discovery may help us better understand the origin of particles such as cosmic rays, protons and nuclei that are constantly flowing through space at near the speed of light.
The galactic center is a mysterious territory. We have a decent idea of what’s there, but it’s dusty, so we can’t study it in the wavelength range from soft X-rays to visible light. It imposes certain restrictions on what we can see and what we cannot see.
Astronomers expect the galactic center to be a major source of cosmic rays. These are protons and nuclei, which are stripped of electrons & accelerated to relativistic velocities by a strong magnetic field. In the galactic center, there are many objects that act as cosmic ray accelerators, such as supernova remnants, the pulsar nebula, the supermassive black hole in the center of the Milky Way, and Sagittarius A *.
Based on observational data and modeling, the distribution of cosmic rays along the Milky Way should be smooth and more or less steady. Cosmic rays emerge from accelerators & propagate in the galaxy’s magnetic field, where they slow down & re-accelerate to result, which can lead to what astronomy calls the ocean of cosmic rays sea.
To study how cosmic rays are accelerated and transported, source of fresh cosmic ray is essential.
Fortunately, cosmic rays are very energetic. This means that they can be detected in the galactic center. Because that energy range produces light in a limited wavelength range that dust penetrates into it.
Cosmic rays can interact with interstellar medium (gas & dust in the space between stars), which produces high-energy gamma-ray photons, which generate about 10% of energy of their comic ray parents.
A research team led by astronomer Xiaoyuan Huang of the Chinese Academy of Sciences wanted to use data from the Fermi Large Area Telescope to observe gamma rays in the central molecular cloud of the Milky Way and find these cosmic ray sources.
As expected, they discovered gamma rays that suggest that the galactic center is a high-energy particle accelerator, or at least something in the region. But they also found something really amazing.
According to the team’s calculations, the density of cosmic rays in the central molecular cloud is lower than the density of cosmic rays sea. This indicates the existence of a kind of barrier that prevents cosmic rays from passing through the central molecular cloud.
What exactly this barrier is made up of should be a topic for future research, but there are some interesting possibilities.
Molecular clouds are a difficult place. Collapse of the densest parts of the cloud can result in compression of the magnetic field. It can be an obstacle. The other can be magnetohydrodynamic turbulence.
Here in the solar system, cosmic rays are modulated by the solar wind. It’s possible that, In the Galactic Center, galaxy winds may play a similar role. The team calculated the density of cosmic rays in the presence of galaxy winds & returned similar result to the analysis of gamma ray data.
Future work to investigate this phenomenon in more detail may help to rule out some of the mechanisms that can cause it.
In addition, more detailed 3D modeling of the galactic center may help shed more light on the origin and transport of cosmic rays in the Milky Way, researchers say. In fact, there is always a lot to find.
The research has been published in Nature Communications.