written by Julia Weniger (E-mail)
Galaxy interactions are very common in the universe, because the distance between galaxies is relatively small in comparison to their size. Typically, the distance is only ten times their size, whereas the distance between stars in a galaxy is ten million times the size of a star. When galaxies come very close to each other, tidal forces change their morphology, e.g. a bridge connecting the interacting partners might evolve or a long tidal tail might form. Two spiral galaxies of about equal size could merge to also form an elliptical galaxy. This scenario is an example of a major merger. Another well studied galaxy interaction involves the passing of a small galaxy through a larger one at high speed, creating a so called ring galaxy. Such galaxy interactions are not only important in today's universe, but were already important in the past. In particular, the standard cosmological paradigm, called ΛCDM, claims that structures, such as galaxies, are formed hierarchically, i.e. small structures merge to form larger ones.
However, galaxies are very complex objects and not only gravity and its tidal forces are important for describing their evolution. For example, it is also known from observations and shown by simulations that the star formation rate is influenced by galaxy interactions. Since stars are formed from gas, namely the ISM, the physics of gas must be considered in order to understand what happens during galaxy interactions. There are also many other physical processes involved, e.g. supernova explosions, planetary nebulae, stellar winds, phase transitions within the ISM, magnetic fields and black hole accretion. All of these physical processes might have a great effect on the evolution of galaxies and may even have had a greater effect in the past when galaxies contained more gas. The importance of various physical processes during galaxy interactions in today's universe as well as in the early universe is studied at the Department of Astrophysics of the University of Vienna by means of numerical simulations.
Figure 1:
This figure shows a major merger shortly after their first passage. From top to bottom surface density map of stars, a surface density map of clouds, a density map of the diffuse ISM, and a temperature map of the diffuse ISM is illustrated. One can clearly distinguish a bridge connecting the two spiral galaxies and two elongated tidal tails originating from the opposite sides of the bridge.