Galaxies within clusters orbit around a common center of mass in a way which differs for different types of galaxies. A recent study showed that early morphological galaxies (ellipticals and S0s) have isotropic orbits, i.e. neither too circular nor too radial, while spiral-type galaxies have more radial orbits (Mamon et al. 2019).A study carried out by an international team, published in the prestigious The Astrophysical Journal (Biviano et al. 2024) has determined the orbits of a rather rare type of galaxy, the jellyfish galaxy. This name is due to the long tails of gas and stars caused by the pressure exerted on the galaxy's gas by the hot plasma that permeates the environment of the galaxy cluster in which the galaxy orbits, a phenomenon known as "ram pressure stripping". This study, directed by Andrea Biviano, first researcher from INAF-Trieste, examined the spatial and velocity distributions of 244 galaxies subjected to ram pressure stripping in 62 galaxy clusters. These galaxies were identified by the GASP project, directed by Bianca Poggianti, director of INAF-Padua. By solving the dynamic equilibrium equations, the authors of the research found that these galaxies have even more radial orbits than spiral galaxies. This means that jellyfish galaxies follow the shortest route to the center of the cluster and therefore experience a rapid variation in the pressure exerted by the cluster hot plasma, the density of which varies wih the cluster-centric distance. Theoretical studies suggest that this rapid pressure variation is essential for the manifestation of the phenomenology of jellyfish-type morphology (Tonnesen 2019).
An example of jellyfish galaxy, JW100, taken at the VLT in Chile, with the MUSE instrument. Red indicates the light from ionized hydrogen gas, white indicates the stars. ESO/GASP collaboration: https://www.eso.org/public/italy/images/eso1725b/.
DOI: 10.3847/1538-4357/ad2c09
