Major Merger of two Spiral Galaxies
Merger events between two galaxies of similar masses (i.e. mass ratios from 1:1 to about 3:1) are commonly called major mergers.
These mergers are observed in the present-day universe, most often between two spiral galaxies.
As shown in the comic, this kind of merger is one known and well studied method to form an elliptical galaxy, as the masses of the two spirals add up and build one single more massive galaxy.
During the merger event, the spiral arms of the two disks get heavily distorted. Their cold gas components get shocked during the collision, and subsequently many stars are formed in a so-called starburst event.
While the two galaxies “dance” around each other, the outermost, least bound stars get stripped from the galaxies and build up shells or other “tidal features”.
The smaller the less massive galaxies, the more symmetric are these shells.
Prominent examples of such ongoing major merger events are, for example, the Antennae Galaxies (NGC4038/4039) and the Mice Galaxies (NGC4676). Our own Galaxy, the Milky Way, will most likely be involved in such a merger event with the neighbouring Andromeda Galaxy in the future.