The center of our galaxy is a place you don't want to be.
Conditions within the Milky Way's blindingly-bright center are identical to a colossal particle accelerator, according to new research recently published in Nature Communications. But something peculiar was also discovered: an unidentified mechanism that keeps cosmic rays from penetrating the gigantic cloud known as the central molecular zone.
This could enable a deeper grasp of how cosmic rays came to be. These consist of particles like protons, in addition to atomic nuclei that continually flow through the big black abyssal depths of space at nearly the speed of light.
Only a limited wavelength of light can penetrate the Milky Way's core
It turns out, the center of the galaxy is a mysterious place. Scientists know a few things about its contents, like a supermassive black hole signified by "Sagittarius A*", and a smattering of really old very large stars. But generally, there's so much dust that we can't look at it with the full spectrum of available wavelengths, from soft X-rays to visible light. This has imposed an empirical limit on what we can say about the Milky Way's center. But, undaunted, astronomers think it serves as a crucial source of cosmic rays: nuclei and protons stripped of all electrons and accelerated to deadly relativistic speeds via unimaginably strong magnetic fields. Some of the objects listed above might function as cosmic ray accelerators; namely, Sagittarius A*. But it could also be supernova remnants, pulsar wind nebulae, or a dark mix of all three.
Modeling and observation data suggest cosmic ray distribution across the Milky Way should be smooth, and relatively steady. When cosmic rays emerge from accelerators, they propagate through the galactic magnetic field, and undergo deceleration and re-accelerate depending on conditions of something astronomers have dubbed a cosmic ray sea. But to study how cosmic rays are accelerated and moved from place to place, you need a fresh batch of cosmic rays. And, lucky for astronomers, cosmic rays are high energy, enabling us to observe them in the galactic center, since the energy range produces light in the limited wavelength range that can penetrate the big glob of dust at the center of the galaxy.
Something mysterious is blocking cosmic rays
Cosmic rays can also interact with the stuff of the interstellar medium like dust and gas that just persists out there, and this interaction generates high-energy gamma-ray photons that have roughly 10% of the energy of the cosmic rays that spawned them. The team of researchers behind the recent study was led by Xiaoyuan Huang, an astronomer of the Chinese Academy of Sciences, and together they examined the gamma radiation in the central molecular cloud of our galaxy via data collated by the Fermi Large Area Telescope, with aims to discover the origin of fresh cosmic rays.
As expected, the gamma rays they did find hinted that the galactic center is a high-energy particle accelerator. Or, perhaps more darkly, something in there is. But the more compelling find showed that the density of cosmic rays within the central molecular cloud isn't as high as the density of the cosmic ray sea. In other words, we know there's some kind of mysterious barrier keeping cosmic rays from pushing even a little into the central molecular cloud. We need to research further to begin to grasp what this is, and there are several exciting possible theories about what it is. But with every study, we come closer to unlocking the mysteries of the most intriguing region in our galaxy.