Unveiling the nature of fireballs in ram pressure stripped jellyfish galaxies

Abstract

Galaxies residing in clusters environment undergo dramatic transformations due to their interaction with the IntraCluster Medium (ICM). Among the most dominant environmental processes is Ram Pressure Stripping (RPS), which drives the formation of jellyfish galaxies characterized by long, spectacular tails of ionized, stripped gas. Observations have consistently revealed pockets of active star formation within these tails, commonly referred to as fireballs or star-forming clumps. The standard theoretical model predicts that these newly formed structures should exhibit a clear stellar age gradient, with the youngest stars located furthest from the galactic disk. In this work, we investigate the nature, internal dynamics, and long-term evolution of fireballs using a high-resolution hydrodynamical simulation. The simulation was done with the ENZO code in a wind-tunnel setup, modeling a gas-rich galaxy (analogous to observed objects like JO201) interacting with a high-speed variable wind representative of a massive cluster environment. Our methodology involved a Friends-of-Friends (FoF) algorithm combined with a novel frame-to-frame probabilistic tracking technique, which allowed us to identify, and follow the evolutionary history of 272 individual fireballs over a duration of ∼ 600 M yr. Our results demonstrate that the evolution of these structures is substantially more complex than the current models suggest. We find that a significant fraction of fireballs exhibits ”mixed” or ”inverted” age gradients, where older stellar populations lead the younger star-forming knots or where there is not a clear morphology for it. We provide evidence from the simulation that suggests that this age gradient inversion is a direct physical consequence of the deceleration of the dense gas clump due to the Ram Pressure drag force, while the recently formed stars—decoupled from the gas—continue their ballistic trajectory only affected by gravity. Furthermore, our statistical analysis indicates that high-velocity fireballs undergo short, intense starburst episodes in the outer regions of the tail. Crucially, the long-term orbital tracking reveals that a significant amount of the star-forming material does not escape the galaxy, but rather undergoes fallback, following damped oscillatory trajectories back towards the galactic disk. This study contribute to(...).