Great Balls of Fire: Star Formation in Gas Clouds Accelerated by a Hot Wind

Satellite galaxies undergo ram pressure stripping, where their gas is directly removed by a hydrodynamical interaction with the surrounding host halo gas. In clusters, ram-pressure-stripped tails of gas have been observed to be multiphase, even forming stars within the stripped material. Some observations find a specific age gradient along the tail, with old stars closer to the galaxy disk, and a “fireball” toy model has been proposed, in which a gas cloud being accelerated away from a galaxy continuously forms stars. In this paper, we simulate individual gas clouds (with masses ∼10 ^6 M _⊙ and radii of a few–100 pc) interacting with an intracluster medium wind and include star formation. We find that our accelerating clouds generally produce a stellar age gradient, with younger stars formed farther along the wind direction and with higher velocities. However, our simulations are more physically accurate than an empirical model of monolithic cloud acceleration, leading to strongly nonmonotonic age gradients. First, the evolution of the gas cloud, both from cloud compression and collapse, as well as from the shredding of cloud material into downwind filaments, can lead to stars formed simultaneously at a range of heights and velocities. Second, the gravity from the gas and stars of the cloud can lead to the velocity evolution of newly formed stars. We conclude that the most distinct fireball stellar age gradients are formed from star-forming clouds that are rapidly accelerated and shredded by their surroundings.