Why the Milky Way’s Black Hole is a Picky Eater

Magnetic field may be keeping our nearest supermassive black hole quiet
17 July 2019
By Karen Thomas
Milky Way’s massive black hole
Streamlines showing magnetic fields layered over a color image of the dusty ring around the Milky Way’s massive black hole. The Y-shaped structure is warm material falling toward the black hole, which is located near where the two arms of the Y-shape intersect. The streamlines reveal that the magnetic field closely follows the shape of the dusty structure. Each of the blue arms has its own field that is totally distinct from the rest of the ring, shown in pink. Credits: Dust and magnetic fields: NASA/SOFIA; Star field image: NASA/Hubble Space Telescope

Never mind the restaurant at the end of the universe, there is a diner at the center of the galaxy.

In the heart of our spiral galaxy, the Milky Way, a bright and compact astronomical radio source known as "Sagittarius A*" (Sagittarius A-Star), is considered a likely location for a supermassive black hole. Such gigantor-sized objects are believed to be at the centers of most spiral and elliptical galaxies. Black holes of this size have huge appetites, gobbling up interstellar gas, dust, stars, comets, and whatever else might be pulled in. Most black holes are highly active, emitting high-energy radiation as material is consumed. But the Milky Way's black hole is relatively quiet compared to others, and astronomers have wondered why it seems to be a picky eater.

In July 2012, NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) spacecraft caught the Milky Way's black hole having a snack. An orbiting observatory, NuSTAR is designed to capture images of violent, high-energy phenomena taking place in the universe. The telescope detected a burst of hard x-rays coming from the Milky Way's center, showing that the black hole had just devoured ... something. Although this outburst was confirmed by lower-energy observations from NASA's Chandra X-Ray Observatory and infrared data from the Keck Telescope, it still didn't answer the question of why this black hole doesn't seem to consume as much as the other massive kids on the block.

NuSTAR_ Sagittarius A-Star
NuSTAR captured these first, focused views of the supermassive black hole at the heart of the Milky Way in high-energy x-rays. Credit: NASA

Researchers suspect that invisible magnetic fields may be wrapped around some black holes. These lines of force can significantly influence the motion and evolution of matter, but since they can't be imaged directly, their role in black hole activity isn't fully understood. Using the High-resolution Airborne Wideband Camera-Plus (HAWC+), the newest instrument aboard NASA's Stratospheric Observatory for Infrared Astronomy (SOFIA), such a field was recently located at the Milky Way's black hole.

The HAWC+ detects polarized far-infrared light, invisible to the human eye, produced by grains of celestial dust that align at an angle to the magnetic fields. The new images show a structure that extends light years through space, and this may be what's keeping material from being swallowed by our black hole. These results will allow astronomers to map the shape of the field and understand the strength of this otherwise invisible force of nature.

"This is one of the first instances where we can really see how magnetic fields and interstellar matter interact with each other," says Joan Schmelz, Universities Space Research Center astrophysicist at NASA Ames Research Center. "HAWC+ is a game-changer."

Discoveries such as these made by NuSTAR, SOFIA, and HAWC+, provide multiple pieces to the puzzle of how the universe works, including the life cycles of galaxies and the black holes they host. Science is discovering not only what's on the menu for black holes, but also why some of them seem to be on a diet.

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