This article originally appeared in the Yale Scientific magazine.
Almost every observed galaxy has a giant black hole at its center. Clues lead us to believe that these monsters, weighing as much a billion suns, eat copious amounts of gas from their environment and occasionally send some of it back as powerful jets, bubbles or heat. How exactly they do this, however, has been a mystery.
For over half a century, said Grant Tremblay, postdoctoral researcher at Yale, “people have simplified supermassive black hole accretion as a smooth spherical inflow of very hot plasma.” And this wasn’t necessarily a bad idea: gas that falls into a gravitational field gets heated up—the stronger the gravity, the higher the temperature. For a dense region like a cluster of galaxies, this temperature is over a hundred million Kelvin, ten times the temperature of the Sun’s core. So far, warm accretion sounds like a credible explanation.
But then there is thermal bremsstrahlung, or braking radiation. In hot plasma, electrons zoom freely around until they come close to positively charged ions; then they change trajectories and lose energy. We observe this lost energy as X-ray emission. “The cluster had to lose energy to give us this photon,” said Tremblay. “Every X-ray observation of a galaxy cluster is actually a direct measurement of galaxy cooling.” If a gas is both hot and dense, in summary, it won’t stay hot for very long. There isn’t a long-term reservoir of warm gas to feed the black hole.
In fact, Professor Megan Donahue of Michigan State University, who is a co-author on the paper, explained that we now have the opposite problem. “The gas near the centers of galaxies is very dense, and the cooling rate of a gas goes as density squared”. The more gas there is at the center, the easier it should be for electrons to find positive ions to brake against, emitting X-ray radiation and cooling down. The cluster cores should be brimming with cold gas, which in turn should form lots of stars. Instead, astronomers found them warm and star-less. This discovery motivated theorists to propose a new model: central black holes are Active Galactic Nuclei (AGN), and spit energy back into their environment in what is known as feedback.
So if hot gas doesn’t feed the AGN, what does? Donahue says the key is realizing that the gas is not all the same: “it’s like this big rain cloud that can produce raindrops that cool very rapidly.” In this model, neither uniformly warm nor uniformly cold gas feeds the black hole; rather, cold clouds precipitate out of the warm gas, and these clouds can then rain down on the black hole. Just like raindrops, on their way to the Earth through the atmosphere, don’t heat up and evaporate, so the cold clouds can maintain their structure all the way from where they formed to the cluster core. Donahue’s team designed the model by observing galaxy clusters, but they could not detect the drops —until now.
In a paper published in Nature this June, the team reported observations of the elusive drops in the Atacama Large Millimeter Array (ALMA), a collection of telescopes located in the Atacama Desert. ALMA can accurately measure both positions and speeds of any object it sees, and has such great angular resolution that it could see a dime held up in New Haven from where it stands in Chile. The team was trying to observe the cold, star-forming gas in the galaxy cluster Abell 2597, which is a tad further – one billion light years away.
The central supermassive black hole (SMBH) is accreting a lot of matter – but a black hole can only eat so fast, and so the in-falling matter settles in a large accretion disk around it. Different layers of the rotating disk generate friction as they rub against each other, which they release as heat. The cluster center should have been bright as a light bulb. “But you also have, between the observer and the light bulb, something eclipsing the light bulb and creating a shadow,” said Tremblay. These shadows were the elusive cold gas clumps, absorbing the light in certain parts of the spectrum. “This is one of the first really big pieces of unambiguous evidence for cold molecular clouds that are falling towards a supermassive black hole,” says Tremblay.
What wavelengths of the light from the black hole the clouds absorbed depended on how fast they were moving with respect to it. Tremblay saw that they were moving inwards towards the SMBH at about 300 km/s, or 67,000 miles an hour. “These things are basically on ballistic trajectories falling towards the black hole.”
Perhaps the best part is that this Nature paper was just a by-product of asking a different question—how is the cold, star-forming gas distributed in galaxies, and clusters of galaxies, and how is that shaped by AGN? We saw above that it lives in little clouds, but there’s one last fascinating detail – the clouds themselves are arranged in extended filaments, as long as the cluster itself. Megan Donahue pictures it as “not like a squid… It’s got these little tentacles.”
Tremblay thinks pasta is a better way to understand the underlying physics. After the first round of cold gas falls into the black hole, the AGN releases jets and bubbles of energy. These ejecta, he says, “drag cold gas out of the center of the galaxy, like pulling spaghetti out of hot water.” Your best bet, then, is to imagine a perpetual fountain of pasta, jetting out of the cluster center towards its outskirts, dragging little bubbles of cold gas along with it.
And so, looking for the tiniest, coldest clumps of gas everywhere in a cluster of galaxies, astronomers confirmed a model for the how the largest, hot object at its very core feeds and responds to its environment.
