Jellyfish Galaxy Sheds Spiral Arms in “Bunny Ear” Tails

Some 320 million light-years away, a galaxy streams through its cluster, leaving behind rivulets of gas where new stars begin to form. In the dense sea of galaxies that makes up the cluster, hot gas in the environment blasts through the traveling galaxy like wind, blowing off its gas while leaving its stars unperturbed.

This type of galaxy is known as a “jellyfish” due to the billowing appearance of the gaseous tails. Presenting at a press conference at the 246th American Astronomical Society meeting and publishing in the Astrophysical Journal, a team of astronomers described this galaxy-stripping act in detail.

Harrison Souchereau (Yale University) led observations using the Chilean Atacama Large Millimeter/submillimeter Array (ALMA), of 23 jellyfish galaxies in nearby clusters via the ALMA-JELLY survey. Not only can ALMA produce stunning images of the stripped galaxies, but it can also measure the velocity of the cold gas in their tails.

“From this, we know not just where the gas is located on the sky, but how this cold gas is moving,” Souchereau says. “This allows us to understand the underlying physics of this galaxy on a much more fundamental level.” Observing NGC 4858, a galaxy in the Coma Cluster, the team found not just one but two tails, dubbed “bunny ears,” that originated in an edge-on collision with the cluster’s headwind. By understanding the impact of the the galaxy’s interaction with its local environment, the team hopes to shed light on how these behemoths of stars, gas, and dust evolve and continue forming stars.

Swimming the Galactic Sea

When galaxies pass through a galaxy cluster, they often miss the other galaxies entirely. But the hot gas suffusing the cluster has an outsize effect known as ram pressure stripping. Even on a windless day, when you stick your hand out of a moving car window, you still feel a wind on your hand that’s created by your own motion — that’s ram pressure, Souchereau explains. “You’re experiencing the same physics that galaxies in really dense regions such as clusters are also experiencing,” he says.

NGC 4858 is traveling at a zippy 56,000 miles per hour through the Coma cluster. Souchereau’s team measured brilliant blue clumps of starlight in its large gas tails, indicating recent star formation. Since ram pressure is not strong enough to strip stars as well as gas, these stars had to be born in the tails themselves, “formed from the gas that has been stripped away from the galaxy and is now in the tail,” Souchereau says.

Using the ALMA data, the team was able to render the galaxy’s two components — its disk of gas and its bunny ear tails — and notice crucial asymmetries. The tails’ cold gas, for example, lie mostly on one side of the galaxy. “What we think is going on here is that this is likely an effect of the wind first striking the galaxy in an edge-on way,” Souchereau explains.  

“This means the wind is only seeing a small fraction of the total gas disk of the galaxy,” Souchereau says. Picture the galaxy like a frisbee flung into the wind — the wind is only striking the edge of the frisbee, not its top or bottom. As a result, one side of the galaxy rotates into the wind, while the other side rotates against it.

To understand the impact of the ram pressure on the galaxy without the confounding effect of rotation, the team subtracted the galaxy’s rotation from their observations. That left the motions in the gas due to ram pressure alone.

Oddly, the team found that the base of the tail (that is, the base of the bunny ears) gas was moving in the opposite direction from what they would expect.

Souchereau thinks that gas might actually be a galactic wind that’s now falling back into the disk. Ram pressure first pushes gas out along the side of the disk that’s rotating with the wind, where it can be much more easily stripped. But the gas still continues to rotate, too, and once it reaches the other side of the disk, its movement goes against the ram pressure wind. “This gas will eventually collect and that is where we would expect fallback to occur,” Souchereau explains.

The team simulated galaxies in a sort of wind tunnel and saw a similar mechanism emerge — especially for galaxies hitting the wind edge-on.

This picture can emerge for galaxies without any kind of spiral structure, but NGC 4858 has strong spiral arms. The team thinks that’s what produces the bunny-ear tails. “The two bunny-ear-like structures that we’re seeing in this galaxy are probably two entire spiral arm components that were stripped away altogether [at different times]”, Souchereau says, “and are at different evolutionary stages.”

The falling back of cold gas onto a galaxy like this “has never been observed before in such a clear and unambiguous way,” Souchereau says. Their discovery sheds light on galaxy evolution in these dense clusters, where a galaxy’s swim through a sea of other galaxies produces these evolving tails of gas.

“In the broader context, understanding these extreme events will give us insight into how the ram pressure works for galaxies where you can’t as easily see these effects,” Souchereau says. This discovery is just one result that came out of the ALMA-JELLY survey, which aims to reveal how different galaxies interact with the broader cosmic environment.

Studying NGC 4858’s interactions with its environment provides a window into some of the most extreme conditions in the universe. These jellyfish galaxies highlight the complex dynamics at play throughout a galaxy’s lifetime. They can also help us learn more about star formation in these volatile environments. “By studying the most extreme events, we can understand how this works for all types of galaxies in the universe,” Souchereau says.