Astronomers discover enormous 'barrier' separating the center of the Milky Way from the cosmic ray sea

An artist's impression of the Milky Way's center, using data from the Fermi Gamma-Ray Space Telescope. (Image credit: NASA Goddard)

According to recent research, the Milky Way's core may be considerably stranger than scientists assumed.

A team of researchers from the Chinese Academy of Sciences in Nanjing investigated a map of radioactive gamma-rays — the universe's highest-energy form of light, which can arise when extremely high-speed particles known as cosmic rays collide with ordinary matter — blasting in and around the galaxy's centre for the study.

According to the map, something in the galactic core seems to be accelerating particles to mind-boggling speeds — extremely close to the speed of light and producing an abundance of cosmic rays and gamma-rays just beyond the galactic centre. However, something in the Milky Way's centre blocks a massive amount of cosmic rays from other areas of the cosmos from entering, the scientists reported in the journal Nature Communications on Nov. 9.

The researchers characterize the impact as an invisible "barrier" that wraps around the galactic core, keeping the density of cosmic rays there much lower than the rest of our galaxy's baseline level. In other words, cosmic rays may leave the galactic core but find it difficult to enter.

It's still unclear how or why this cosmic barrier exists.

Monster in the middle

In the constellation Sagittarius, around 26,000 light-years from Earth, lies the galaxy's core. It's a crowded and dusty environment, with more than a million times the number of stars per light-year as the whole solar system, all encircled by a supermassive black hole with a mass of around 4 million times that of the sun.

Scientists have long hypothesized that this black hole, known as Sagittarius A*, or another object at the galactic core, is speeding protons and electrons to near-light speed, resulting in cosmic rays that travel across our galaxy and beyond. These rays travel across our galaxy's magnetic fields, forming an ocean of high-energy particles with a density that is pretty uniform throughout the Milky Way. The cosmic ray sea is the name given to this constant soup of particles.

In their latest study, the researchers matched the density of cosmic rays in this sea to the thickness of cosmic rays in the galactic core. Cosmic rays can't be seen directly, but they can be found in gamma-ray space maps, which indicate where cosmic rays have impacted other forms of matter.

The scientists established that something in the galactic core is definitely working as a massive particle accelerator, spewing cosmic rays out into the galaxy, using the Fermi Large Area Telescope data. Sagittarius A*, the remnants of ancient supernovas, or even strong stellar winds from the many stars crammed into the galactic centre, could be the culprits because black holes could theoretically shoot certain particles into space while gobbling up everything else around them.

However, the image also showed the enigmatic "barrier," a visible location at the perimeter of the galactic centre where the quantity of cosmic rays reduces substantially. According to the researchers, the cause of this phenomenon is difficult to determine, although it might entail a tangle of magnetic fields at our galaxy's dense centre.

The scientists proposed in their study that massive clouds of dust and gas surrounding the galactic core may collapse on themselves, compressing the magnetic fields and producing a cosmic-ray-proof barrier. Perhaps stellar winds from the galactic centre's countless stars are pushing back against the cosmic ray sea, similar to how the solar wind does.

To understand out precisely what is going on in the strange depths of our galaxy, further investigation is needed.

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