Compact Groups

Fossil Groups

Dwarf Galaxy Harassment

Multiple Minor Merger

Galaxy Major Merger

In the previous entry, major merger events were explained. However, merger cannot just happen between galaxies of similar mass, but also between a massive galaxy (like the Milky Way) and a much smaller galaxy (like the Magellanic Clouds). These mergers between two galaxies of very different masses are commonly called “minor merger”, as a single such merger usually does not strongly alter the appearance of the main galaxy (while, of course, the small one gets consumed by the big one). However, if such mergers happen multiple times, the equivalent of a major merger event can be added to the main galaxy in mass, and the main galaxy will be transformed. Simulations have shown that such multiple minor merger events will even puff up the resulting elliptical galaxy in comparison to a single merger of the same mass as the sum of the small galaxies. This could explain the large extents found for some of the observed elliptical galaxies.
This is the second, commonly excepted formation mechanism for elliptical galaxies, and one of the challenges in modern extragalactic astronomy is to find out if there is a way to determine the main formation channel of an elliptical from its measurable properties. However, such minor mergers also occur to disk galaxies, and they can cause many more features (which will be shown in comics at a later point for sure).