How Do Black Holes Eat?

black hole eating gas clouds
Deep in the heart of the Abell 2597 Brightest Cluster Galaxy, astronomers see a small cluster of giant gas clouds raining in on the central black hole. They were revealed by the billion light-year-long shadows they cast toward Earth. These ALMA data present the first observational evidence for predicted chaotic cold accretion onto a supermassive black hole. NRAO/AUI/NSF; Dana Berry/SkyWorks; ALMA (ESO/NAOJ/NRAO)

We all know what black holes are — superdense objects with gravity so strong that not even light can escape from them. They're popular in science fiction, but they have been known to exist for real for many years. They've been detected by their effects on nearby objects and on light (in the form of gravitational lenses). Smaller black holes can form when supermassive stars die in catastrophic explosions called Type II supernovae. Larger ones, the supermassive monsters at the hearts of galaxies, apparently form as their host galaxies interact and merge and their embedded black holes collide with each other. 

Like their smaller siblings, they support themselves by eating vast amounts of galactic gas and dust (and whatever else falls into their traps). The big ones require a LOT of material and their eating habits can affect their host galaxies in many ways. For example, they can gobble up the material needed for star formation, effectively shutting down the starbirth process in their immediate neighborhoods. 

The largest and most massive black holes can have up to millions or even billions of times the mass of the Sun, and it turns out that most galaxies (particularly spirals) have supermassive ones at their hearts. For all astronomers have learned about black holes in the relatively short time since their first discoveries of them in the 1990s, there's still a lot that remains unknown about them. One of those mysteries is being solved with innovative observations using radio telescopes: how black holes eat. 

Black Holes Chow Down

The technical term for the dining habits of black holes is "accretion". Material — usually gas — exists in a roughly spherical-shaped region around the black hole. That gas (or anything that strays too close) gets pulled into a massive disk called the accretion disk. It slow funnels the trapped material into the black hole. Think of the accretion disk as the waystation for material on the one-way trip into the singularity that holds the mass of the black hole.

Most of the time, black holes — particularly the supermassive monsters at the hearts of galaxies — subsist on a steady diet of hot gas that exists in diffuse patches in the near neighborhood. However, occasionally a wandering clump of cold gas gets caught up and the black hole quickly gobbles it down.

Checking Out The Black Hole Cafeteria

To figure out how it all works, astronomers spotted a massive black hole in a galaxy that lies about a billion light-years away. It lies at the heart of a massive cluster of galaxies. The galaxy itself is called Abell 2697,  and it is surrounded by a diffuse cloud of extremely hot gas.  At the galaxy's heart, there's a black hole chowing down on a mass of very cold gas. The galaxy itself is busily producing stars, which requires that cold gas to supply the starbirth "factories".

Astronomers wanted to know more about the cold gas and why it appeared to be "raining down" onto the black hole. So, they looked at the galaxy with a group of telescopes called the Atacama Large-Millimeter Array (ALMA, for short), to study radio emissions from the galaxy. In particular, they looked at emissions from carbon monoxide (CO) gas molecules. 

ALMA's detection of that gas helped the astronomers determine the amount of the cold CO gas, as well as where it is distributed throughout the galaxy. Carbon monoxide is a good "tracer" of the existence of the types of cold gases that eventually get used to make stars.

In fact, they mapped the temperatures of gases across the entire galaxy cluster. The more they looked into the cluster, the more gas they found, and it was cooler gas than in the outer regions and in the "intergalaxy" areas. When we say cold, we mean the range of temperatures started at the high end of millions of dees Fahrenheit to very chilly sub-zero temperatures.

Radio Data as a Speed Detector

At the very center of the target galaxy, in the immediate neighborhood of its black hole, the researchers discovered something quite unexpected: the shadows of three very cold, very clumpy gas clouds. Behind them were bright jets of material blasting away from the black hole. It's highly likely that the clouds were very close to being sucked in by the black hole.

The radio data revealed that the clouds are moving very fast: at rates of 240, 275, and 355 kilometers per second. All three are on a beeline for the black hole. They probably won't go straight into the hole directly; instead they'll probably get mixed into the accretion disk around the black hole. From there, their material will swirl around, and eventually spin into the black hole.

As astronomers study more black holes at the hearts of galaxies, including the one in the center of the Milky Way, they'll learn more about how these behemoths grow and what it is that they consume to keep their massive bulk going.