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How the tentacles of jellyfish galaxies feed supermassive black holes

Some galaxies wander the cosmos, tentacles extending like hungry jellyfish — and now scientists have found active supermassive black holes at the center of six of them. Those unbelievably huge black holes are gobbling up lots of gas, and what keeps them well fed is the same mechanism that forms the jellyfish galaxies in the first place, according to new research.

The discovery is surprising because most supermassive black holes around the Universe are dormant, sleeping giants that don’t devour much gas. So the findings, published yesterday in Nature, can help astronomers understand why certain black holes suck in lots of matter and some don’t. And because black holes and galaxies are interconnected, it can help us better understand how galaxies — and ultimately the Universe — evolve.

Jellyfish galaxies get their name from their long “tentacles” of gas. These tentacles are formed when the galaxy enters a cluster of hundreds of galaxies. Clusters swim in hot, dense gas, and when a single galaxy falls into them, it experiences this gas as a sort of “wind,” according to Swinburne University of Technology. This wind basically strips away gas contained within the single galaxy, blowing it into tentacles, in a process called ram-pressure stripping.

Jellyfish galaxy JO194, described in the Nature study.
Illustration by Callum Bellhouse and the GASP collaboration

Jellyfish galaxies are losing gas because of this, which creates a problem: when gas is removed, new stars can’t form in the galaxies. So today’s discovery was something of an accident. The astronomers were hoping to learn more about how the gas is stripped away. They weren’t looking for active supermassive black holes specifically, says study co-author Bianca Poggianti at the INAF – Astronomical Observatory of Padova.

Poggianti’s team found that six of seven jellyfish galaxies had an active black hole in the middle, an unusually high number. Only 3 percent of regular galaxies inside clusters have active black holes, Poggianti says. “The unusual thing is the frequency of the phenomenon among the jellyfish galaxies,” Poggianti tells The Verge.

The reason for the black holes may be the gas. Not only is it being blown out to form the “tentacles,” part of it is also being funneled toward the center of the galaxy, into the black holes, she says. The black holes are gobbling up so much matter that they brighten up, forming an active galactic nucleus, or AGN. AGNs are some of the most powerful, luminous objects in the Universe, and because they can last billions of years, we can study the ones that formed shortly after the Big Bang. That allows astronomers to learn how the Universe has evolved since then.

The findings are “intriguing,” says Harald Ebeling, an astronomer at the University of Hawaii’s Institute for Astronomy, who did not take part in the study. But many more jellyfish galaxies should be studied to make sure that there’s a universal connection between ram-pressure stripping and AGNs, he tells The Verge. Ebeling and his team have been studying about two dozen jellyfish galaxies, and only a minority of them have active supermassive black holes at their center, he says. “Prompted by her study, I think I’ll take a closer look at our data and see whether if we dig a little deeper, we can find more AGN there,” Ebeling says.

Poggianti says she and her team plan to observe 114 galaxies in total, most of them jellyfish. And they’re also going to use radio telescopes around the world. At the heart of her project is to study how galaxies evolve — because that tells us how the Universe evolves.

Jellyfish galaxies are just “a tiny piece of the puzzle,” Poggianti says, they a revealing one at that.

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