Galaxies are sociable entities; galaxies out there on their own are
quite rare. Most of them are found in galaxy clusters and groups. In
order to fully understand the evolution of galaxies, the study of the
galactic environment is thus paramount. The environment not only
includes neighbouring galaxies, but also the tenuous gas between
galaxies (the intergalactic medium, IGM, or intra-cluster medium, ICM
in cluster environments). There are many reasons why the study of
galaxy interactions and mergers is very important for our
understanding of the Universe as a whole. Perhaps one of the most
important ones is that the largely accepted cosmological model, a
dominated Cold
Dark Matter based Universe, explicitly
predicts that galaxies should form hierarchically in the merger
process. However, the theoretical study of interactions and mergers
is usually the realm of cosmological simulations and I refer the
readers to the many books and review papers devoted to the argument
[14,
294,
48,
272,
28,
80].
One of the clearest evidences of the environmental effects is the morphology-density relation [66], according to which the fraction of early-type galaxies in clusters increases with the local density of the environment. Another key observational result is the star formation-density relation [10, 299], in the sense that star formation seems to be strongly reduced in dense environments. Moreover, cluster galaxies are H I deficient compared to their field counterparts. The deficiency increases towards the cluster centre. These and other observational facts (see also [27, 105] for reviews) clearly indicate that one or more processes in cluster and group environments remove gas from galaxies or make them consume their gas more quickly. One possibility is that the dense environment promotes tidal interactions (galaxy-galaxy or galaxy-cluster). It has been shown that these interactions can remove matter from galactic halos quite efficiently [281, 184, 325, 40]. Another possible physical mechanism able to remove gas in dense environment is the combined effect of multiple high-speed encounters with the interaction of the potential of the cluster as a whole, a process that has been named "harassment" [192, 193].
By combining different processes, Boselli & Gavazzi [27] concluded that the most probable mechanism able to explain the observational differences between galaxies in clusters and in the field is ram-pressure stripping, namely the kinetic pressure that the ICM exerts on the moving galaxies. If the ram-pressure is larger than the restoring gravitational force (per unit surface) acting on a gas parcel of a galaxy moving through the ICM, this gas parcel is stripped off the galaxy [93]. There have been many simulations exploring the effect of ram-pressure stripping, with different settings and degrees of sophistication [75, 1, 257, 237, 180, 236, 128, 215, 273]. There are many indications that ram-pressure stripping is a key process, able to radically modify the evolution of DGs. Many authors even put forward the idea that ram-pressure stripping can convert gas-rich DGs into gas-poor ones. These ideas are comprehensively summarised in many excellent reviews [268, 179, 97, 267] and I refer the reader to these reviews for further details.
For the purposes of this review paper, it is more convenient to briefly summarise the results of the simulations of Marcolini and collaborators [165, 166]. These authors performed simulation of flattened, rotating DGs subject to ram-pressures typical of poor galaxy groups. Interestingly, despite the low values of the ram-pressure, some DGs can be completely stripped after 100-200 Myr. However, regions of very large surface density can be found at the front side of DGs experiencing ram-pressure stripping. This enhanced density can easily lead to a burst of star formation. If the DG experiences a galactic wind (see also Sect. 9), several parameters regulate the gas ejection process, such as the original distribution of the ISM and the geometry of the IGM-galaxy interaction. Contrary to the ISM content, the amount of the metal-rich ejecta retained by the galaxy is more sensitive to the ram-pressure action. Part of the ejecta is first trapped in a low-density, extraplanar gas produced by the IGM-ISM interaction, and then pushed back on to the galactic disc. Clearly, the interplay between galactic winds and environment is quite complex and very few studies address this issue in detail (see however [254]). This is another research field in which, in my opinion, more can be done. In particular, results of small-scale detailed simulations of individual galaxies could be used in large-scale simulations of galaxy clusters and groups, where the interaction processes between individual galaxies and the ICM cannot be appropriately resolved. This is for instance the approach followed by Creasey et al. [50], who simulate the feedback effect of SNe in a single galaxy in order to improve sub-grid models of feedback in large-scale simulations. This approach should perhaps be further extended. Also simulations like the ones of Marcolini et al. (or similar "wind tunnel" experiments) could be used to better constrain the galactic wind-ICM interactions and improve galactic cluster-scale simulations.