Unless you are an astrophysicist, you'd likely think that a black hole isn't too fussy about the kind of material it devours. Light? Check. Hydrogen? Yum! But for years researchers have believed that supermassive black holes only subsisted on a diet of hot gas. New observations of a galaxy about a billion light years away though, show that cold, clumpy cosmic rain will do just fine to fill a black hole's gaping maw.
Scientists spotted the never-before-seen phenomenon when they trained Chile's Atacama Large Millimeter/submillimeter Array (ALMA) telescope on a massive galaxy known as the Abell 2597 Brightest Cluster Galaxy, named for its spot in an atypically bright grouping of around 50 galaxies known as Abell 2597. There they determined that there were "billowy clouds of cold, clumpy gas streaming toward a black hole," according to MIT.
The clouds were traveling at speeds reaching 355 kilometers per second, or almost 800,000 miles per hour, and were likely about 150 light years away from the edge of the black hole. In cosmic terms, that's quite close – close enough to pretty much ensure the clouds will fall into the black hole. The clouds are tens of light years wide and contain as much material as a million of our own suns.
While those numbers are staggering, what really impressed the researchers was the fact that the clouds consisted of cold clumps of gas, because they believe this is the first time solid proof has been attained that such material can feed a supermassive black hole. The clouds are formed when hot gas cools, condenses and in effect creates a cosmic rainstorm. The clouds will likely not fall directly into the black hole but rather become part of its accretion disc – the swirl of material around its edges that feeds it.
This composite image of Abell 2597 Brightest Cluster Galaxy has a blue background from the Hubble Space Telescope and red foreground from ALMA, while the box shows a shadow made by one of the cold clouds
"This so-called cold, chaotic accretion has been a major theoretical prediction in recent years, but this is one of the first unambiguous pieces of observational evidence for a chaotic, cold 'rain' feeding a supermassive black hole," said Grant Tremblay, an astronomer with Yale University and lead author on a new paper detailing the findings. "It's exciting to think we might actually be observing this galaxy-spanning 'rainstorm' feeding a black hole whose mass is about 300 million times that of our Sun."
The research team didn't actually see the cold clouds directly, but rather their billion light-year-long shadows. Known as absorption features, the shadows are created when the clouds block some of the light behind them created by spiralling electrons jetting out from the black hole.
They plan to continue using ALMA to search for more cold clouds, and believe that there may be many more.
"We're only seeing this tiny sliver," says Michael McDonald, assistant professor of physics in MIT's Kavli Institute for Astrophysics and Space Research. "If there are three clouds in just our line of sight, there might be millions of clouds all around. And there's a tremendous amount of energy in just these three clouds. So if we were to look at this thing a million years later, we might see that the black hole is in outburst — much brighter, with more powerful jets, because all this high-energy material is landing on it."
McDonald is co-author on the paper about the discovery published June 8 in the journal Nature entitled "Cold, clumpy accretion onto an active supermassive black hole." The paper is available for download online.