A Tale of Tails: Stripping and Star Birth in Jellyfish Galaxies

In a new study, Catherine Gibson and collaborators explore how galaxies get reshaped when they fall into the extreme environments of galaxy clusters. Their focus is on MACS J0138.0-2155, a massive cluster about 4 billion light-years away, where several galaxies are being transformed into so-called jellyfish galaxies. These galaxies earn their nickname from long, tentacle-like gas tails that form when the pressure from the hot cluster environment strips gas away from their disks.

Why Jellyfish Galaxies Matter

Galaxies in dense clusters often stop forming stars and become “red and dead.” One culprit behind this transformation is ram-pressure stripping (RPS), which works like wind blowing gas off a moving galaxy. This stripped gas can form spectacular tails, where new stars sometimes light up. Understanding these processes helps astronomers learn how galaxies evolve over cosmic time.

Observations and Data

The team examined four galaxies in the cluster, labeled J1–J4. Three of them (J1, J2, and J3) show clear jellyfish features, while J4 may sit in the foreground. To study them, the researchers used the MUSE spectrograph on the Very Large Telescope, which lets them measure the motions of stars and gas across each galaxy. They also relied on images from the Hubble Space Telescope to see the galaxies’ shapes. By studying light from hydrogen (especially the Hα line), they measured star formation rates and identified where new stars were forming.

Tails, Heads, and Star Formation

Both J1 and J2 show a clear separation between their “heads” (the galaxy disks) and “tails” (the stripped gas). Their heads display strong Balmer absorption lines, a signal of post-starburst galaxies that recently shut down star formation. But in their tails, star formation continues, with bright knots of new stars shining within the stripped gas. J3 looks less disrupted, showing both absorption and emission features, with its strongest star formation reaching nearly 0.5 solar masses per year per square kiloparsec. J4, by contrast, has weaker activity and may not fully qualify as a jellyfish galaxy.

Kinematics and Stripping Stages

The researchers found that all three jellyfish galaxies show velocity gradients in their tails, evidence that gas is being stripped as the galaxies move through the cluster. J1 and J2 appear to be in later stages of stripping, already quenched in their centers but still forming stars in their tails. J3 seems to be in an earlier stage, still forming stars across much of its body. The gas and stars in these galaxies move differently, another hallmark of RPS.

Piecing It All Together

Gibson and her team conclude that ram-pressure stripping is driving both quenching and tail star formation in these galaxies. J1 and J2 are clear examples of post-starburst jellyfish, while J3 is caught earlier in its transformation. J4 shows some star formation but may not be part of the same process. Overall, these results reinforce that galaxy clusters are not just collections of galaxies, they are environments that actively reshape them.

Looking Ahead

The authors suggest that future X-ray observations could reveal even more about how the hot cluster medium interacts with infalling galaxies. By combining multiwavelength data, astronomers can better understand the balance between star formation and stripping, and how galaxies go from vibrant star factories to “red and dead” cluster members.

Source: Gibson