Supermassive black holes chow down on their surrounding stars, and in doing so create the right conditions for new stars to form—a kind of feedback loop that regulates galaxies around them, according to scientists who studied one of the biggest galaxy clusters known to humans.
Astrophysicists including Brian McNamara (of the University of Waterloo), Michael McDonald (of MIT) and Helen Russell (of the University of Cambridge) published research from the National Radio Astronomy Observatory describing this in the Astrophysical Journal today.
Using the Atacama Large Millimeter Array (ALMA) in Chile, the group traced cold molecular gas around a supermassive black hole in the Phoenix Cluster, which is about 5.7 billion light years away. In an incredibly hot environment, the presence of this cold gas—"enough of it to create 10 billion suns," Russell said—shows us how black holes regulate the expansion of galaxies while powering their own growth.
When hydrogen (the most abundant element in the universe) is cold, as in about -273℃, it collapses, McNamara told me on the phone. That becomes the fuel for new star bursts.
"So the picture is one of a galaxy with a big ball of stars, with a black hole at the centre that's about a billion times the mass of the Sun," he said. Even though the black hole is relatively small compared to the rest of the galaxy, like a grape beside the Earth, it packs more potential energy than all the stars combined. "We're trying to understand how stars are forming in these galaxies, and what's making these black holes active."
To do this, the group used ALMA to look at various molecules in the galaxy, as well as visual telescopes to look at the stars, and X-ray telescopes to study the hot gas surrounding them. They observed that the jets of particles being launched from supermassive black holes were creating explosions around the galaxy, like fireballs.
"If you were to toss a grenade into a room, when it blows up—for a very brief period of time—all the oxygen in the room will get blown away," McNamara explained. "That's sort of what happens in these galaxy clusters: it creates these giant bubbles."
The bubbles last for millions of years. The group's X-ray data found a trail of gas behind bubbles in the Phoenix Cluster—and in that molecular gas, they saw stars forming. "The implication is that the black holes that are generating these very powerful outbursts not only heat the hot gas, but paradoxically, also cause some cooling."
This cooling is needed to create stars (because cold hydrogen equals stars), which then feed the black hole, which then creates more stars.
It's kind of like rain, McNamara said—the perfect temperature and air density have to come together with a few other factors, and then we get a loop of water falling from the very clouds it evaporated to form, and so on.
"If the black hole heats this hot gas up enough, it won't cool out. What we see is the black hole is heating this stuff up, and it's heating it up very rapidly, but paradoxically when we looked to see where most of the rapid cooling was occurring, it was right behind these heating bubbles," he said. "We realized these bubbles were not only heating the hot gas, but they were promoting the cooling of the hot gas in their wake."
Russell, who started this research as a postdoc in McNamara's lab, said it's this feedback loop in an incredibly hot environment that creates the cooling effect needed to form stars. She added that this newly discovered mechanism sheds light on how black holes are fueled (AKA in the most badass fashion: they fuel themselves), and the role they play in galaxies.
Given that in the centre of every large galaxy lies a supermassive black hole, Russell said, this loop could explain how black holes regulate the speed of star formation in their respective galaxies. Scientists know there's a relationship between the size of a galaxy and its black hole, McNamara added—but until now, the reason why has been a mystery.
"We think it's the black hole that actually acts like a thermostat to regulate the growth of the galaxy. These observations show us very clearly how that might be occurring," he said. "It means for the last eight billion years, black holes have stunted the growth of galaxies. And in the process of stunting the growth of galaxies, they grow bigger themselves."
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