Astronomers using the Atacama Large Millimeter Array (ALMA) have identified an awesome new form of cosmic weather: black hole rain. The observations, described Wednesday in Nature, offer the first direct evidence of cold, dense clouds condensing around a supermassive black hole and providing it with an additional food source as part of the general process known as black hole accretion. The occurrence has been long-theorized by astronomers, but until now has gone unseen.
Astrophysics and astronomy are overloaded with metaphors, but, in this case, black hole rain is almost a literal description. The precipitation we experience here on Earth happens as moisture-laden air cools and condenses. It turns out that something similar happens among clusters of galaxies characteristically found commingling with clouds of hot, ionized plasma—some regions of gas cool and fall inward as they condense.
The target of the ALMA group was a bright cluster of about 50 galaxies known as Abell 2597. At the center of the cluster is one massive spiral galaxy called the Abell 2597 Brightest Cluster Galaxy, which features a black hole at its center that's some 300 million times the mass of our Sun. The black holes found at the centers of large galaxies are already known to feed on the aforementioned haze of hot ionized plasma via the process of accretion, in which a black hole steadily accumulates its gaseous neighborhood into a large flat disc before eventually devouring it.
"Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales," the paper explains. "The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas."
Recent theory and simulations have offered the possibility of something more, however. In some cases, black hole accretion may be dominated by randomly distributed clumps of cold molecular gas rather than the more typical meal of hot plasma. And now this theory has some real-world evidence behind it.
What the astronomers found is three large clumps of cold gas falling toward the Abell 2597 black hole at some 300 kilometers per second. Each one of these clouds, which appear in observations as dark "shadows" against the bright black hole background, could be as massive as a million Suns and span up to 10 light-years from end to end. These three may even be only part of the whole picture and could just be the most visible parts of a galaxy-spanning cosmic rainstorm currently deluging Abell 2597 Brightest Cluster Galaxy. Right now, they're about 300 light-years from falling into the black hole itself, which, in terms of our rain metaphor, would be like milliseconds before the raindrop meets the puddle.
This is only the first, limited observation of the "rainstorm" phenomenon, but theory suggests it's likely common enough. The next step for the astronomers is to find more of them in other galaxies, a task that the ALMA observatory remains well-equipped for.