Supermassive Black Hole ‘Snowplows’ Can Stifle Star Formation in Spiral Galaxies

Strange, flickering black hole jets can shape entire galaxies

By Jacek Krywko edited by Lee Billings

For decades, astronomers have known that supermassive black holes lurk at the hearts of virtually every large galaxy, sometimes feasting on infiltrating material and emitting powerful jets. But what’s less clear is how, exactly, this activity shapes surrounding galaxies.

Researchers have discovered a crucial piece of this galactic puzzle by observing a supermassive black hole projecting a flickering jet into the galaxy VV 340A, some 450 million light-years from Earth. The jet acts like a snowplow on a cosmic scale, pushing back gas that would otherwise fuel the creation of new stars. The result was announced at this year’s winter meeting of the American Astronomical Society in Phoenix, Arizona.

“Conventionally, there are two modes of gas flow caused by supermassive black holes in galaxies,” says Justin Kader, an astrophysicist at the University of California, Irvine and first author of an associated paper published in Science. In the first so-called radiative mode, an incandescent, white-hot accretion disk of infallible matter forms around a fast-feeding supermassive black hole, heating the nearby gas. This heated gas then expands and pushes the colder gas outwards. “You can see the gas flowing out of the galaxy in these high-angle bicone structures,” Kader says.

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In the second jet-powered mode, a black hole launches fire hose-shaped jets of particles and radiation from its poles, kinetically pushing surrounding gas away from the galaxy. However, these jets typically add to a galaxy’s stellar inventory by jostling and compressing gas clouds that then collapse under gravity to produce more stars.

In VV 340A, however, Kader and his colleagues discovered jets from a supermassive black hole doing something markedly different from either of these two modes.

VV 340A is a spiral galaxy that is merging with another, VV 340B, forming a system collectively called VV 340. In the sky, the pair appears as a celestial exclamation point, with the face-up disk of VV 340B as the “dot” of the exclamation point and the edge disk of VV 340A forming the “dash”. For Kader and his colleagues, this edge-on orientation was an opportunity, allowing them to more easily probe the internal workings of the VV 340A. What they found coming out of the galaxy’s central black hole wasn’t a standard straight jet. Rather, it was an enigmatic S-shaped structure, requiring further investigation to determine its nature.

Using the infrared eyes of the James Webb Space Telescope, the team was able to pierce the thick dust shrouding the center of VV 340A to discover a massive cloud of superheated ionized plasma spanning nearly 20,000 light years, far larger than any black hole-generated plasma cloud ever seen. Subsequent optical observations at the Keck Observatory in Hawaii confirmed that this ionized plasma was not simply stationary but was being propelled outward at immense speeds. Finally, radio observations of VV 340A via two radio telescopes, the Karl G. Jansky Very Large Array and the Atacama Large Millimeter Array, showed that the plasma was perfectly aligned with the S-shaped jet coming from the black hole.

Kader and his co-authors think this S shape is the hallmark of precession, the same wobble you see in a spinning top as it slows down or in the water spewed from the spinning head of a lawn sprinkler. As the black hole spins, its jet doesn’t just point in one direction: it sweeps through space in a conical motion, pushing the star-forming gas out of the galaxy at a rate of about 20 solar masses per year. That’s enough, the researchers estimate, to shorten VV 340A’s star-forming lifetime by about 250 million years.

“Twenty solar masses a year is no big deal,” says Andrew Fabian, a British astronomer and former director of the Institute of Astronomy at the University of Cambridge, who was not involved in the study. “But a precessing jet as a driver of gas flow is something new. Indeed, it shows that it can significantly move matter in a spiral galaxy.” One of the questions still unanswered for Kader and his colleagues is what exactly causes the plane’s oscillating motion.

“These flickering jets are not common, but they have been observed before, mainly in giant elliptical galaxies,” says Kader, pointing out that there are currently thought to be two main factors explaining this behavior. One is an accretion disk instability, in which a large clump of gas falling toward the black hole pulls on the disk of material surrounding it, causing it to tip over.

The other possibility, undoubtedly more exciting, is that there is not just one black hole at the center of VV 340A but two. A binary pair of supermassive black holes orbiting each other could gravitationally whip the jet like a garden hose. “To the best of my knowledge, no binary supermassive black hole has ever been directly observed before. We do not claim to have observed thatbut it is one of two possible options,” explains Kader.

Higher-resolution radio observations, combined with studies using future observatories such as NASA’s Nancy Grace Roman Space Telescope, could help discern between these two possibilities. In the meantime, the team flagged 32 other galaxies similar to VV 340A for further examination. “What we want to see is the interaction of different gases in the process of galaxy mergers,” says study co-author Vivian U, also of the University of California, Irvine. “Being able to understand this would allow us to actually answer one of these general questions: understanding the drivers of galaxy growth.”

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