Jellyfish Galaxies and ram-pressure stripping

Galaxy evolution is complex and involves the interplay of various physical mechanisms, including gravity, gas dynamics, radiation, star formation, feedback from stars and active galactic nuclei (AGN) as well as the influence of dark matter. This is a multiscale mechanism ranging from individual star formation within galaxies up to the clustering of galaxies in cosmic filaments and clusters. When a galaxy enters a cluster, it begins to lose gas because of the ram pressure of the ambient gas (Gunn & Gott 1972) and tidal forces (tidal stripping). In galaxy clusters, the effect of dynamic pressure dominates (e.g. Simpson et al. 2018). Ram-pressure stripping is documented not only by hydrodynamic simulations (e.g. Steinhauser et al. 2016), but also by the observation of “jellyfish galaxies” (Ebeling et al. 2014, Boselli et al. 2023). The more extensive hot gas is removed first, then it is the turn of the cold atomic gas (Bekki 2009). Molecular gas is very difficult to extract (and resolve in simulations; Hausammann et al. 2019). Often these perturbing mechanisms start to be active in groups of galaxies which fall into a cluster, as suggested by numerical simulations (Haggar et al. 2024, Lokas et al. 2023). In this case, it is the intergalactic gas from the group which will be removed first. Interest in this topic comes from the importance of the environment on the evolution of cluster galaxies and their quenching. Indeed, the impact of ram pressure modifies the star formation histories and chemical abundances of cluster galaxies. Ram pressure could increase the metal content of galaxies if it stops the infall of external gas and, at the same time, it compresses the gas in a way to accelerate star formation inside galaxies (Koutsouridou & Cattaneo 2019). Alternatively, tidal effects can also impact the stellar content in case of encounters and the role of the AGN of the central galaxy on satellites is still uncertain. The aim of this workshop is to discuss quantitative comparison of simulations/models with observations. What is the current state of the art on the observational side and on the simulation one? Are the simulations capable of accounting for the ram pressure tails observed in jellyfish galaxies? On the observational side, the main questions are the mass of the gas in the ram pressure tails, the length of the tails, the speed of the satellite galaxy and the density of the ambient medium. On the simulation side, several galaxies/groups of galaxies with characteristics similar to the observed sample have been identified. The recent citizen science project named Fishing for Jellyfish project based on IllustrisTNG simulations makes a real breakthrough in the domain (Zinger et al. 2024, Göller et al. 2023, Rohr et al. 2023) with 5307 visually identified jellyfish galaxies. This sample is key to study how to disentangle the different effects in action.