When astronomers began to develop the first dynamical models for galaxy formation in a CDM dominated universe, it immediately became clear that most baryonic material is predicted to cool and form stars. This is because in these ‘hierarchical’ structure formation models, small dense halos form at high redshift and cooling within them is predicted to be very efficient. This disagrees badly with observations, which show that only a relatively small fraction of all baryons are in cold gas or stars . Apparently, some physical process must either prevent the gas from cooling, or reheat it after it has become cold.
Even the very first models suggested that the solution to this problem might lie in feedback from supernovae, a class of exploding stars that can produce enormous amounts of energy . The radiation and the blast waves from these supernovae may heat (or reheat) surrounding gas, blowing it out of the galaxy in what is called a galactic wind...
Another important feedback source for galaxy formation is provided by active galactic nuclei (AGN), the active accretion phase of supermassive black holes (SMBH) lurking at the centers of almost all massive galaxies. This process releases vast amounts of energy – this is why AGN are bright and can be seen out to large distances, which can be tapped by surrounding gas. Although only a relatively small fraction of present-day galaxies contain an AGN, observations indicate that virtually all massive spheroids contain a nuclear SMBH . Therefore, it is believed that virtually all galaxies with a significant spheroidal component have gone through one or more AGN phases during their life.
Although it has become clear over the years that feedback processes play an important role in galaxy formation, we are still far from understanding which processes dominate, and when and how exactly they operate. Furthermore, to make accurate predictions for their effects, one also needs to know how often they occur. For supernovae this requires a prior understanding of the star-formation rates and the IMF. For AGN it requires understanding how, when and where supermassive black holes form, and how they accrete mass.
Fig. 1. A flow chart of the evolution of an individual galaxy. The galaxy is represented by the dashed box which contains hot gas, cold gas, stars and a supermassive black hole (SMBH). Gas cooling converts hot gas into cold gas, star formation converts cold gas into stars, and dying stars inject energy, metals and gas into the gas components. In addition, the SMBH can accrete gas (both hot and cold) as well as stars, producing AGN activity which can release vast amounts of energy which affect primarily the gaseous components of the galaxy. Note that in general the box will not be closed: gas can be added to the system through accretion from the intergalactic medium and can escape the galaxy through outflows driven by feedback from the stars and/or the SMBH. Finally, a galaxy may merge or interact with another galaxy, causing a significant boost or suppression of all these processes.
It should be clear from the above discussion that galaxy formation is a subject of great complexity, involving many strongly intertwined processes. This is illustrated in Fig. 1, which shows the relations between the four main baryonic components of a galaxy: hot gas, cold gas, stars, and a supermassive black hole. Cooling, star formation, AGN accretion, and feedback processes can all shift baryons from one of these components to another, thereby altering the efficiency of all the processes. For example, increased cooling of hot gas will produce more cold gas. This in turn will increases the star-formation rate, hence the supernova rate. The additional energy injection from supernovae can reheat cold gas, thereby suppressing further star formation (negative feedback). On the other hand, supernova blast waves may also compress the surrounding cold gas, so as to boost the star-formation rate (positive feedback). Understanding these various feedback loops is one of the most important and intractable issues in contemporary models for the formation and evolution of galaxies.
