1.Collision of two spiral galaxies :

A model of two colliding spiral galaxies can be seen in the video below. The total number of particles is equal to 60,000. Thanks to this type of simulation, astrophysicists believe that elliptical galaxies result from the merger of two spiral galaxies. Approximately two-thirds of all galaxies are spiral galaxies, with the other third divided principally between elliptical (10%) and lenticular (20%) galaxies.

The existence of two main types of galaxies, elliptical (spherical) and spiral (disk), is an age-old problem in extragalactic astronomy. According to the traditional interpretation, in which galaxies were all formed shortly after the Big Bang, the stars in spherical systems are formed very quickly from the protogalactic cloud before the dissipation of any gaseous material. The stars in disk galaxies, in contrast, appear more slowly from gas clouds that have had time to condense and flatten out into disks by rotation. It is still poorly understood why this star formation occurred at two different speeds.

Furthermore, it is possible to estimate the number of galaxy mergers since the beginning of the Universe. Based on currently merging systems displaying remnants of the collision (NGC 7252) and knowing that this transitory period lasts approximately two billion years, one can, by extrapolation, put at 10% to 20% the number of galaxies that have merged since the Big Bang. This percentage roughly corresponds to the proportion of elliptical galaxies in the Universe !

However, numerous arguments have been put forth against this theory for explaining the formation of elliptical galaxies, most notably those pointing to the great abundance of globular clusters in elliptical galaxies which cannot be explained by the merger of two spiral galaxies, where these clusters appear much less frequently. Furthermore, the merger of two lightly concentrated star systems would not be able to create a highly concentrated elliptical galaxy. Finally, dwarf elliptical galaxies would not be able to be formed by the same mechanism.

These arguments, however, can be debated. For example, that the formation of elliptical galaxies can only occur by the merger of galaxies of comparable mass: if one takes into account the capture of small companions, the formation of elliptical galaxies would occur more gradually. The merger of two gaseous, non-stellar systems would allow, by dissipation, the formation of two concentrated stellar systems.

Finally, a quite convincing argument has been put forth by Alar Toomre : if elliptical galaxies are not created by galaxy mergers, and elliptical galaxies resulted from the leftovers of all the initial collisions between galaxies, where are the remnants of these mergers, whose prototype would be NGC 7252 ? These mystery objects should represent approximately 15% of the galaxies in the Universe at the very least !