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

TL;DR

This study simulates gas clouds in galaxy clusters to understand star formation and age gradients caused by hot winds, revealing complex, nonmonotonic patterns due to cloud evolution and shredding.

Contribution

It provides a physically accurate simulation of star formation in accelerated gas clouds, improving upon empirical models and explaining observed stellar age gradients.

Findings

Younger stars form farther along the wind direction.

Simulations show nonmonotonic age gradients due to cloud shredding.

Velocity evolution of stars is influenced by cloud gravity.

Abstract

Satellite galaxies undergo ram pressure stripping, in which 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 of ~10$^6$ M$_\odot$ and radii of a few-100 pc) interacting with an intracluster medium wind, and include star formation. We find that our accelerating clouds do 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 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.