Galaxy formation and evolution

Galaxy Merger

The formation and evolution of galaxies is still unknown in many aspects. Studies of the large scale structure of the universe revealed that galaxies live in several different environments. Some galaxies live in the field, that is they show no signs of interaction with neighbouring galaxies and the distances between the galaxy and its nearest massive neighbours are large compared to their size. Most of the field galaxies are spiral galaxies. In contrast a cluster consists of more than 100 galaxies within 2-10 Mpc and therefore is a very dense region. Galaxies in these environments have very high relative velocities and show signs of interactions. The fraction of elliptical galaxies is high compared to the field. Independent of the environment observations show a decrease of the fraction of star forming galaxies with decreasing redshift. This lead to the assumptions that elliptical galaxies are build by collisions of galaxies, which is a common event in a dense environment, while in a field environment galaxy mergers are uncommon and therefore the galaxies remain undisturbed and are supposed to mostly grow by smooth accretion.

 

 

 

General formation setup for galaxies

In the standard cosmology assumed today, the $\Lambda$ Cold Dark Matter scenario, in the early universe small perturbations collapse first and produce dark matter halos that accumulate baryons at the center. The small structures aggregate successively into larger structures. This process is called the hierarchical growth. Therefore, if we want to study galaxy formation we need to analyze and understand their assembly history. Numerical simulations have shown that there are several processes for the mass assembly: merger events between two nearly equally massive halos, called major merger, merger between a massive halo and a small sattelite halo, called minor merger, and the accretion of material that is not correlated with the assembly of a halo, for example in the course of a flyby event or by accretion of gas along filamental structures followed by in situ star formation. With increasing computational power it is now possible to perform cosmological simulations and to follow the merger and accretion history of halos from high redshifts to z=0.

 

 

Elliptical galaxies build up scenarios

For elliptical galaxies there are two main possible scenarios assumed: Major merger and minor merger (or cosmological) growth. Observations tell us that there exists ellipticals already at high redshifts, but we also see major merger currently at work (for example in the Antaennae system and the Mice system), especially in medium dense environments like galaxy groups. Simulations of major mergers between two big spiral galaxies have successfully reproduced elliptical galaxies, but they failed to explain the formation of the most massive ellipticals in the universe, the BCGs (Brightest Cluster Galaxies). Cosmological simulations recently revealed another formation scenario: Here the most massive structures grow through the continuous infall of small galaxies, so called minor mergers. In these scenario the galaxy grows fast at high redshift, where major merger are more common due to the fact that the masses of the galaxies are lower at higher redshifts (hierarchical growth) and the environment is generally denser (less evolved structures), and the accretion slowes down at lower redshifts, dominated by the infall of less massive structures. If enough of these small structures fall into a galaxy, they are able to change the morphology of the galaxy very efficiently. In addition, cosmological simulations show that about 40% of the material of a galaxy is accreted by smooth accretion. Star formation is triggered by major and minor mergers, thus in both scenarios there is (nearly) no cold gas inside the ellipticals left when the elliptical has formed, and the star formation gets to an end, the ellipticals are red and dead, as observed.

 

Joint evolution of galaxies and black holes

In the current picture of galaxy formation it is now generally accepted that every massive spheroidal galaxy host a supermassive black hole in its center. Moreover, black hole masses are found be tightly correlated to host galaxy properties, as luminosity, bulge mass and velocity dispersion. This suggests that the evolution of galaxies and the growth of black holes is not de-coupled, but goes hand and hand. E.g. feedback from actively accreting black holes is assumed to regulate black hole growth itself and also to influence the evolution of the host galaxy by quenching cooling and further star formation. Furthermore, major mergers and the corresponding infall of large amounts of cold gas, which is then available for star formation and provides also a fuel for black hole growth, are believed to be important events in establishing the observed, local relations between black hole masses and host galaxy properties. However, detailed physical processes are still poorly understood and thus, subject of current research. For example, a puzzling, open question is to explain the anti-hierarchical trend in black hole growth within a hierarchical structure formation model. Different observational studies show that - investigating the number density evolution of active galactic nuclei (AGN) - more luminous AGN peak at higher redshift than less luminous ones. This implies that more massive black holes seem to form already very early in the Universe, whereas less massive objects seem to predominantly evolve only at later times, in contrast to what is expected from our current hierarchical structure formation models. Here, semi-analytic models are appropriate tools in order to study easily the effect of different trigger mechanisms for black hole accretion, the corresponding accretion efficiencies and different prescriptions for AGN feedback in a large statistical sense, in particular the influence on the AGN number density evolution.