Dwarf Galaxy Harassment
Galaxy clusters are a very special environment, in which many interesting physical processes are happening. One such process is called “harassment”.
The total mass of galaxy clusters is so huge, that the internal velocities of small objects accreted into such an environment is very large as well. The orbiting velocity of galaxies inside a cluster can reach up to 1000km/s or even more. If two galaxies inside a cluster meet a such a speed, they will not merge (like shown in the major merger scenario), but pass right through each other, as the energy in such encounter is much larger than the binding energy. If there is gas left in the colliding galaxies, it will be shock-heated and most likely form stars, however, as most of the galaxies inside a galaxy cluster are gas poor this is not the dominant effect. The stars in such galaxies do not really “feel” the encounter, as they are small compared to the extent of the galaxy and as such stellar encounters only happen very rarely. Thus, each galaxy will loose some stars to the other galaxy in the process, but otherwise they hardly feel the encounter.
The latter is true for the larger galaxy, at least. The smaller galaxy involved in such encounter, on the other hand, is changed by it. It is “puffed up”. This means that the smaller galaxy, temporarily sitting inside the potential well made by a much more massive structure, looses some of its structural binding, and the resulting structure is smeared out over a much larger radius than before, its central density being decreased significantly.
This process if one of the most prominent processes discussed as the formation scenario for the newly discovered class of galaxies, called ultra-diffuse galaxies (UDGs).