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Scientists have found the energy limit for particles accelerated by the black hole at the center of the Milky Way.
The center of our galaxy is a bustling place. There's a supermassive black hole, gas clouds swirling around it, and it all shines in gamma rays, the most energetic form of light. Telescopes in the Namibian desert collected this light for 16 years, amassing 365 hours of observations. It turned out that the glow isn't steady but has a kink: at a certain energy, it suddenly weakens.
Gamma rays are born when cosmic rays, fast charged particles, slam into gas. The gamma glow reveals how energetic those particles are.
The scientists built a three-dimensional model: where the gas is, where the particles are, and how they collide. It turned out that particles at the galactic center don't accelerate indefinitely. Their energy is capped at roughly 15–50 TeV. It's as if the galaxy's heart could only beat with a certain strength, no stronger. The acceleration source sits very close to the black hole, within 20 parsecs. That's less than the distance from the Sun to the nearest star.
Previously, interference from other sources obscured the energy limit. Now that it's been found, we can better understand how the black hole accelerates particles, and why its "heart" beats the way it does.
The central molecular zone contains about 10% of the Galaxy's total molecular gas while occupying only 1% of its volume, making it the densest gas reservoir in the Milky Way.