The supermassive black holes lurking at the centers of galaxies have been known to chow down on anything unlucky enough to pass too close, but the opportunity to see that in action rarely occurs at the Milky Way's quiet core. Now, astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) observatory in Chile have imaged a very active black hole at the center of spiral galaxy M77, which is apparently feasting on the universe's largest donut.
The heart of M77 is what's known as an active galactic nucleus (AGN), meaning that gas and matter is constantly being sucked into the central black hole and giving off intense light. These active regions in the universe could help unlock the mysteries of how galaxies and the supermassive black holes at their cores develop in tandem.
Apart from the occasional spectacle, our own galactic center isn't very active, so astronomers need to look further away to find those clues. The team, made up of researchers from the National Observatory of Japan, SOKENDAI and Kagoshima University, used ALMA to image M77's galactic nucleus.
The team spotted a compact gaseous structure, resembling a gigantic donut, surrounding the black hole. The cloud stretches some 20 light-years out from the center, and is spinning around the black hole. The existence of these spinning torus structures has been hypothesized for decades, but according to the researchers, this marks the first time one has been directly observed.
"To interpret various observational features of AGNs, astronomers have assumed rotating donut-like structures of dusty gas around active supermassive black holes," says Masatoshi Imanishi, lead author on a paper describing the find. "This is called the 'unified model' of AGN. However, the dusty gaseous donut is very tiny in appearance. With the high resolution of ALMA, now we can directly see the structure."
The resolution of ALMA's images can't hog all the credit for the discovery. The researchers say it was also important to focus on specific molecular emission lines, and microwave emissions were detected from hydrogen cyanide (HCN) molecules and formyl ions (HCO+). Since these molecules only emit microwaves in dense gas, it tells the team a lot about the donut's density.
But there's more to the story. The researchers say that the torus isn't spinning perfectly in line with the black hole's gravity – instead, there's a high degree of randomness to its motion. That implies a violent past, which may have involved a collision with a smaller galaxy.
The research was published in Astrophysical Journal Letters.
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