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This supermassive black hole feeds on frigid gas

Abell 2957 is a mammoth cluster of galaxies, a billion light years away. At the center of the cluster, smack in the middle of a stellar nursery, is one of the brightest galaxies in the universe. It’s imaginatively called the Brightest Cluster Galaxy, or the BCG for short. The BCG is a giant elliptical galaxy, tens of thousand light years across, with a supermassive black hole at its core.

Lately we’ve been listening to that region of space with the Atacama Large Millimeter/submillimeter Array of radio telescopes. ALMA started up in 2013 — it’s so new it practically still has the plastic on it. But a team of researchers working with ALMA just hit pay dirt. Streaking toward the Brightest Cluster black hole are three enormous, lumpy clouds of cold gas, and they’re all raining into the black hole at the same time.

That black hole is “sitting at the bottom of a gravitational funnel,” said Michael McDonald, coauthor of the paper and assistant professor of physics at MIT’s Kavli Institute for Astrophysics and Space Research, “and all the gas from a thousand galaxies is available to it. These are the galaxies that are the most massive, with the most massive black holes in the universe, and the most potential for star formation.”

Pointing ALMA at the BCG’s central 97,000 light-years revealed that the galaxy contained not just the warm, ionized gas we expected, but also a central bubble of colder molecular gas. The outside of the bubble is probably being heated by the extreme radiation present in the region, given that it’s a star nursery to some of the universe’s biggest and brightest. But its interior stays comparatively cool and dim, shielded from radiative heating by sheer mass. That’s what shelters these dense little clouds from the greater turbulence in the area, preventing them from being consumed or dispersed by the birth and death of nearby stars, nebulae and even other black holes. Instead, they drift ever inward toward the black hole at the center of the Brightest Cluster galaxy, “sustaining star formation amid a kiloparsec-scale molecular nebula that inhabits its core.”

“This very, very hot gas can quickly cool, condense, and precipitate in much the same way that warm, humid air in Earth’s atmosphere can spawn rain clouds and precipitation,” said Grant Tremblay, lead author of the study. “The newly condensed clouds then rain in on the galaxy, fueling star formation and feeding its supermassive black hole.”

Although they were obscured by clouds of gas and plasma and washed out by the glow of their hot, bright surroundings, the team found these three galactic-scale rain clouds just before they began to brush against the black hole’s event horizon, because they were starkly backlit against the black hole’s accretion disk jets. (How’s that for a sense of scale?) “We got very lucky,” said McDonald. “Seeing three shadows at once is like discovering not just one exoplanet, but three in the first try. Nature was very kind in this case.”

Don’t let that fool you into thinking cold gas is inert, though. Cold gas is what condenses to make stars in stellar nurseries like this. Now we know that it apparently also feeds black holes. McDonald expects that these three cold gas clouds probably won’t diffuse gently into the black hole, but will instead collect in its accretion disc. This sharply contradicts our spherical-cow assumptions about how black holes take up matter, which are — as the report extremely politely puts it — “largely unconstrained by observations.”

“The simple model of black hole accretion consists of a black hole surrounded by a sphere of hot gas, and that gas accretes smoothly onto the black hole, and everything’s simple, mathematically,” says McDonald. “But this is the most compelling evidence that this process is not smooth, simple, and clean, but actually quite chaotic and clumpy.”

“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,” said Tremblay.We might even get to watch this unfold in real time: “If the clouds really are that close to the black hole, we should see them physically move on human timescales — that is, six months to a few years. […] We might actually see a movie of cold black hole accretion! That would be really, really cool.”

The paywalled paper will appear in Nature on June 9, 2016, but the ALMA observatory made it available as a PDF. Go read it!

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