Galaxy’s “Bunny Ears” Reveal Cosmic Recycling Secrets

Astronomers are observing a distant galaxy’s dramatic transformation. This “jellyfish galaxy,” named NGC 4858, is experiencing intense forces that are shaping its appearance. The galaxy is revealing insights into how galaxies evolve within crowded cosmic environments.

A Supersonic Wind Tunnel

Galaxy clusters, containing thousands of galaxies, are filled with hot plasma, significantly hotter than the Sun’s surface. As spiral galaxies move through this intracluster medium, they experience a headwind. NGC 4858 is undergoing some of the most forceful ram-pressure stripping ever witnessed.

According to Harrison Souchereau, a Yale University astronomer and lead author of a new study, the galaxy is essentially in a wind tunnel, experiencing gas being stripped away by the wind. This “jellyfish galaxy” classification highlights the trailing streams of gas and young stars extending from one side.

Optical images show distinct blue trails of young stars. These features confirmed that ram-pressure winds eliminated the galaxy’s interstellar gas, compressing it into new star-forming knots. However, optical light does not reveal the cold molecular clouds, providing the raw materials for new stars.

Bunny Ears and Galactic Fountains

The ALMA images reveal the galaxy’s leading edge, compressed by the wind. On its trailing side, a plume of gas curves towards the disk. Above the disk, two arms create a “bunny ears” shape. Souchereau suggests this results from the environmental wind and the galaxy’s rotation.

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

—Harrison Souchereau, Yale University Astronomer

Models demonstrate that headwinds can unevenly drag on rotating galaxies, bending arms and accumulating gas on one side. Stars can still form where the gas density peaks, even when other areas lack gas. Recent research shows star formation in these environments. One recent study found that star formation efficiency is reduced in ram pressure stripped galaxies. ( Astrophysical Journal 2023).

Recycling and Galaxy Evolution

The study highlights evidence for “fallback,” a process where gas pushed out of the disk does not always escape. If it doesn’t reach the cluster’s escape velocity, gravity reverses its course. This creates a galactic fountain, causing the gas to fall back, often concentrating in the distorted spiral arms on one side of the inner tail.

The study suggests that a galaxy’s transformation is potentially messier and more gradual than previously thought. Gas recycling may prolong a galaxy’s star-forming life, steering the stripped gas with angular momentum and magnetic fields. Upcoming observations with optical and Webb telescopes will analyze the chemical makeup of gas entering and exiting the galaxy.

By comparing NGC 4858 to other jellyfish galaxies, scientists aim to build a timeline of how ram-pressure interactions unfold. The galaxy’s unique bunny-ear silhouette illustrates how ram-pressure winds bend spiral arms, how gas cycles in a galactic fountain, and how galaxies reignite star formation.