Stars on the Run: Following the Fate of Stripped-Tail Star Formation in Galaxies
When galaxies plunge into the crowded and chaotic environment of a galaxy cluster, they don't make it through unscathed. Instead, they face powerful headwinds from the hot, fast-moving gas that fills the cluster — the intracluster medium (ICM). This interaction, known as ram pressure stripping (RPS), can sweep gas out of a galaxy, peeling it away from the disc and leaving behind tails of stripped material. In this paper, Nina Akerman and collaborators use high-resolution simulations to follow the journey of stars born in these stripped tails, asking where they form, how they move, and whether they stay with the galaxy or escape into the cluster.
Two Galaxies, Two Winds
To answer these questions, the team simulated two massive galaxies, each with different wind conditions. In the “W0” simulation, the wind blows directly face-on, while in “W45” the wind comes at an angle. This setup allowed them to explore how the direction of the wind affects gas stripping and star formation. They discovered that most of the stars formed in the tail were born quite early, during the first few hundred million years after stripping begins, and typically within 5 kiloparsecs (kpc) of the galaxy disc. Despite the strong winds, many of the stripped gas clouds survive long enough to collapse into stars. In fact, more stars form in the angled wind case (W45), suggesting that the gas slows down and spends more time in star-forming conditions.
Tracing Star Birth with Metallicity
To better understand the nature of the star-forming gas, the authors looked at the metallicity of the newly formed stars. Metallicity refers to the abundance of elements heavier than hydrogen and helium, which generally increases in gas that has spent more time inside a galaxy. Stars with lower metallicity likely formed from gas that mixed with the ICM, while high-metallicity stars came from relatively pure interstellar gas. Akerman’s team found that stars born farther from the galaxy had lower metallicity and higher velocities, a sign that they came from gas that had been stripped earlier and mixed more with the ICM. Meanwhile, stars closer in had higher metallicities, pointing to their origins in less-disturbed regions of the galaxy.
Stars That Fall Back
While it might seem like stripped gas would be lost forever, the simulations reveal a more nuanced picture. A significant number of stars — around 15–25% — were born from gas that fell back onto the galaxy after being initially stripped. This gas often moved into the “shadow” of the galaxy disc, where it was shielded from the wind and could cool and collapse into stars. These fallback stars generally didn’t end up in the galaxy center; instead, they spread out into an extended, thickened disc-like structure. However, even in this thicker configuration, they did not significantly contribute to a central bulge or drastically alter the galaxy’s shape.
Would We See These Stars with Hubble?
To see if this fallback population would be visible to telescopes like the Hubble Space Telescope, the team created mock UV images based on the simulated stars. These images showed that while some UV-bright features appear early in the stripping process — especially within the first 100 million years — they fade over time as star formation shuts down and the stars age. The fallback stars contribute to a slightly thicker UV-emitting disc, but only marginally. In real observations, especially if the galaxy isn’t perfectly edge-on, this effect would likely be too subtle to detect.
Complicating the Simple Picture
Even though outside-in stripping creates clear trends in metallicity and velocity, the authors emphasize that the real story is more complex. Some stars form while gas is falling back, others are born in clumps that gain extra momentum in odd directions due to galaxy rotation or pressure gradients. These effects can blur the clean age and metallicity gradients that astronomers hope to use when analyzing real galaxies. As a result, interpreting observations of star formation in stripped tails may require careful modeling of the kinds of complex dynamics seen in these simulations.
Minimal Contribution to Intracluster Light
Finally, the study explored whether tail-born stars could escape their galaxies and join the intracluster light (ICL) — the faint glow of stars that drifts between galaxies in a cluster. According to the simulations, almost all tail stars fall back onto their parent galaxies. Even when accounting for tidal forces from the cluster, only a very small number of stars escape, and their total mass is tiny compared to the overall ICL. Therefore, the team concludes that star formation in stripped tails is unlikely to be a major contributor to this diffuse light.
A Back-and-Forth Story
In summary, Akerman and collaborators show that star formation in ram pressure stripped tails is a dynamic process involving not just outflow, but also fallback. While stars do form in the gas tails, they don’t travel far — instead, most of them eventually return to their host galaxy, creating a mildly thickened disc but little else. The study adds new complexity to our understanding of how galaxies evolve in clusters and challenges some assumptions about what happens to stripped material.
Source: Akerman
