The Milky Way's Supermassive Black Hole Has an Accretion Disk That's 25x Larger Than the Solar System

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As the most massive objects in existence, black holes usually have accretion disks, rings of gas and other materials that reach blazing hot temperatures, sometimes even emitting powerful, luminous x-rays.

Accretion disks have been spotted around other black holes before, but never around our galaxy's own supermassive black hole, Sagittarius A*. The immense astronomical object rests at the center of the Milky Way roughly 26,000 light years from Earth and tips the scales at an estimated 4,000,000 solar masses.

On Wednesday, a team of astronomers led by Caltech astrophysicist Elena M. Murchikova announced that it had finally detected and imaged Sagittarius A's accretion disk using the Atacama Large Millimeter Array (ALMA) in Chile. Their finding is published in Nature.

The disk is primarily composed of hydrogen gas – equal to roughly one-tenth the mass of Jupiter. It's heated to around 18,000 degrees Fahrenheit, though it gets far hotter closer to the black hole. The disk extends out about a hundredth of a light year, about 1,000 times the distance from the Sun to the Earth. Assuming a roughly circular shape, that would give the accretion disk a diameter about 25 times longer than our solar system's!

ALMA image of the disk of cool hydrogen gas flowing around the supermassive black hole at the center of our galaxy.

Murchikova and her collaborators imaged the accretion disk and even traced its motion by mapping the faint radio signal it produces. On one side of the disk, the wavelength was redshifted, meaning it was moving away from the Earth. On the other side, the wavelength was blueshifted, meaning it was moving towards the Earth. This suggests that the disk is rotating.

Sagittarius A*, the monster black hole in the middle of the Milky Way, comes more into focus with each passing year.

"We are also probing accretion onto the black hole," Murchikova said in a statement. "Even so, we still have no good understanding of how its accretion works. We hope these new ALMA observations will help the black hole give up some of its secrets."

Source: Elena M. Murchikova, E. Sterl Phinney, Anna Pancoast & Roger D. Blandford. "A cool accretion disk around the Galactic Centre black hole." Nature 570, 83–86 (2019)

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