Jellyfish galaxy grew ‘bunny ears’ while battling galactic wind

Galaxy clusters pack thousands of galaxies into a volume only a few million light-years wide. They also fill the space between them with a thin plasma hotter than the Sun’s surface.

When a spiral galaxy plunges through this intracluster medium, the hot plasma acts like a supersonic headwind, pushing on the galaxy’s cooler gas. NGC 4858 is currently enduring some of the strongest ram-pressure stripping ever observed.

“This galaxy, NGC 4858, is traveling very quickly through the Coma Cluster,” said Harrison Souchereau, an astronomer at Yale University and lead author of a new study exploring this galaxy.

“It is effectively in a wind tunnel, and its gas is in the process of being stripped away by the wind,” he said.

A survey of jellyfish galaxies

NGC 4858 is classified as a “jellyfish galaxy.” This type of galaxy has long, trailing streams of gas and young stars extending from one side – making it resemble a jellyfish with flowing tentacles.

Earlier optical pictures showed distinct blue tentacles of young stars trailing behind NGC 4858. These features confirmed that ram-pressure winds removed the galaxy’s interstellar gas and compressed it into new star-forming knots.

But optical light cannot reveal the cold molecular clouds that provide the raw material for future stars.

To map that component, Souchereau and Yale professor Jeffrey Kenney used ALMA Observatory’s high resolution and sensitivity at radio wavelengths.

Their observations – part of a broader survey of jellyfish galaxies – resolved structures just a few hundred light-years across. These were small enough to compare directly with computer simulations of ram-pressure stripping.

Bunny ears and inner tails

The ALMA images show that the galaxy’s leading edge, which faces into the wind, appears sharply compressed. On the trailing side, a short, hook-shaped plume of CO-bright gas curves back toward the disk.

Above the disk plane, two arched spiral arms stretch outward and backward, giving the galaxy a cartoonish set of “bunny ears.”

“This is likely due to a combination of the environmental wind pushing on the gas and the rotation of the galaxy,” Souchereau explained.

Models show headwinds drag unevenly on spinning galaxies, bending arms and piling gas onto one side of the disk. Stars can still form where the gas density peaks, even as other parts of the disk go without.

The galactic fountain effect

The study highlights one of its most striking findings: clear evidence for “fallback,” a long-theorized but rarely documented process.

When ram pressure pushes gas out of the disk, the material does not always escape into intergalactic space. If it fails to reach the cluster’s escape velocity, gravity eventually reverses its course.

“Most people think of ram pressure stripping as removing the gas from galaxies, which is the main effect and a big deal, since gas is the raw material for star formation,” Kenney said.

“But sometimes gas can get pushed out but not stripped, since it never reaches the escape speed. So it falls back, creating a kind of galactic fountain. When this happens, the gas falling back often concentrates in distorted spiral arms on one side of the inner tail.”

In NGC 4858, the returning gas streams converge inside the bunny-ear arms and along the inner tail, triggering bursts of star formation in regions that would otherwise be quiescent.

The galaxy shows both destruction and renewal, as some forces gut certain regions while others briefly revive with recycled gas.

Recycling may change a galaxy’s fate

Astronomers consider ram-pressure stripping one of the fastest ways to convert blue, star-forming spirals into red, gas-poor ellipticals inside clusters.

The new ALMA data indicate that the transformation can be messier and more gradual than simple one-way stripping models predict.

Gas recycling may prolong a galaxy’s star-forming life, albeit in distorted patches rather than orderly spiral arms. The uneven fallback suggests that angular momentum and magnetic fields steer stripped gas – key factors for next-generation simulations to model.

Upcoming optical and Webb telescope observations will analyze the chemical makeup of gas flowing into and out of the galaxy. These studies will test whether recycled material can eventually build a fresh, stable disk or simply fuel brief starbursts.

Testing galaxy survival limits

Astronomers study NGC 4858 as a compelling case of extreme environmental effects on galaxies because it lies nearby and sits in one of the densest known clusters.

By comparing it to other jellyfish in the Coma sample – some at different angles of motion and stages of stripping – astronomers hope to build a timeline of how ram-pressure interactions unfold.

For now, the galaxy’s whimsical bunny-ear silhouette offers three key lessons. Ram-pressure winds compress and bend spiral arms; gas can cycle out and back in a galactic fountain; and even under relentless external assault, galaxies can briefly reignite pockets of star formation.

Observing these processes in real time provides a critical benchmark for theories of galaxy evolution in the universe’s most crowded neighborhoods.

A preprint of the study can be found on arXiv.

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