Abstract
It has long been established that galaxy morphology differs in clusters : elliptical or lenticular galaxies dominate, whereas spirals dominate in the field. It turns out that this observation is due to a transformation of spirals into lenticulars, acquired in the sweep of their gas by dynamic pressure (when they enter the hot gas at high speed), and not to an innate effect, where galaxies would be formed with different morphologies in the overdensities of the Universe. Ellipticals form by merging spirals, perhaps not completely in the cluster though, where relative velocities are too high, but in groups, where they are pre-processed. The groups then merge into a cluster, and the elliptical galaxies end up dominating the cluster center.
Large-scale galaxysurveys such as Sloan have revealed two sequences of galaxies: the red sequence of quiescent galaxies, no longer forming stars ( red and dead ), followed by blue galaxies, generally spirals or irregulars, which are still active. The clear separation between these two components suggests that star formation has come to an abrupt halt, known as " quenching ". Statistically, quenching is due firstly to environmental effects, and secondly to galaxy mass. In addition to gas scavenging due to dynamic pressure, environmental effects include harassment and numerous fast interactions, as well as smothering, as there is no cold gas left to refuel galaxies in clusters, as in the cosmic field and filaments. Numerical simulations have been able to quantify all these effects and have shown how cosmic filaments can also contribute to the pre-processing of galaxies, before they fall into the cluster.
