Compact Groups

Fossil Groups

Dwarf Galaxy Harassment

Multiple Minor Merger

Galaxy Major Merger

The galaxy cluster environment is a rather harsh environment to live in, as many processes caused by this environment have strong effects on the properties of the galaxies it harbours. One such process is the previously illustrated harassment of dwarf galaxies. Another very prominent process caused by the hot gaseous atmosphere of the galaxy cluster (which can nicely be observed through its strong X-ray emissions) is the so-called “ram-pressure stripping”: The hot gas inside the cluster environment becomes denser and denser towards the center of the cluster. A galaxy, that still has a (cold) gas component itself and moves trough this hot gas atmosphere feels an increasing pressure inflicted by the hot gas onto the galaxy’s own gas component, and this pressure ultimately “strips” the galaxy from its gas. As the gas from the galaxy is compressed while being stripped away, a star burst is triggered, before the galaxy ultimately has lost all its gas and remains “red and dead”.
This process is actually observed in both the Virgo Galaxy Cluster and the Coma Galaxy Cluster, and the galaxies that are seen to be actively stripped are called “Jellyfish Galaxies” due to the long tails of gas that stream like filaments away from the stellar body of the galaxy, thus resembling the tentacles of Jellyfish. The inflicted star bursts are also observed, and a famous class of galaxies seen inside galaxy clusters are “Post-starburst Galaxies”, that is galaxies that show clear sings of a recent starburst but are now devoid of cold gas. It can also explain the “red spirals” observed only in clusters of galaxies, as they are thought to be an intermediate state of the transformation from the gas-rich spirals to the gas-poor S0 galaxies.
Ram-pressure stripping is thus thought to be one of the major quenching mechanisms for star-forming galaxies, caused by the densest environments in the Universe, and a very good explanation for the observed Morphology-Density Relation of Galaxies.