One of the key challenges in galaxy evolution theory is to understand the star formation history of galaxies - in particular, how star formation is triggered and how it can be suddenly quenched. The environmental effects in galaxy clusters and groups are thought to play an important role, especially ram pressure stripping of the interstellar medium (ISM) as galaxies move through the surrounding intra-cluster medium (ICM). Other mechanisms include gravitational interactions between galaxies and the tidal influence of the cluster potential. In recent years numerous cluster galaxies with jellyfish-like 'tails' of stripped gas have been discovered, many containing young stars. These are dramatic examples of galaxy evolution driven by the cluster environment.

Jellyfish tails are are multiphase, consisting of gas with a wide range of densities and temperatures. They have been detected in X-rays, Hα (ionised hydrogen), HI (neutral atomic hydrogen) and recently also in CO (a tracer of cold molecular hydrogen). Explaining the presence of molecular clouds in these ram pressure stripped tails is an ongoing problem since they should be more resistant to direct stripping than any other component of the ISM. Moreover, the survival of molecular clouds in the ICM, outside the shelter of galaxy disks, is poorly understood.

Our aim is to try to understand the formation and evolution of star-forming molecular clouds in RPS tails. What are the roles of in-situ formation versus direct stripping? Can stripped low-density gas cool sufficiently in the tails to produce dense gas (and stars), or does ram pressure need to be strong enough to directly strip dense gas? How do molecular gas complexes in tails compare to those in the disks of star-forming galaxies, in terms of their mass distribution, lifetimes, and star formation efficiencies? How do star-forming molecular clouds evolve within the diverging gas flow of ram pressure stripped tails? To help address these question, we undertake high resolution observations of molecular gas using telescopes such as ALMA.

For most gas clouds, we can at least determine that they originated due to gas stripping. Others are more mysterious : certain HI clouds in Virgo are isolated, optically dark, and with high velocity widths suggestive of rotation. Could such a cloud by produced by stripping and survive long enough to become isolated, or are they galaxies in their own right that have avoided significant levels of star formation ? We are exploring this issue using both numerical simulations and observations from the Arecibo and VLA radio telescopes.