Hierarchical galaxy formation

With the initial conditions specified, it became possible to simulate galaxy formation. Three distinct approaches have emerged numerical, semi-analytical and hybrid. The fully numerical approach canot yet cope with the complexities of star formation, but has been instrumental in guiding us towards an understanding of the dark matter distribution. The semi-analytical approach has had most success, because it can cope with a wide dynamic range via the extended Press-Schechter formalism, to which is added a prescription for star formation 

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Based on currently available elemental abundance data and age determinations, the thick disk could have formed either through a violent, heating merger or through accretion of (substantial) satellites in a hierarchical galaxy formation scenario. The fast monolithic-like collapse is getting more and more problematic as data are gathered. It would be especially crucial to establish if there is an age-metallicity relation in the thick disk or not as in that case the thick disk could not have formed in that way (since the models indicate that the formation time-scale for the stars in the thick disk would be very short, see [7]). 

The main difference between the monolithic collapse scenario and the hierarchical merging relies in the time of galaxy formation, occurring quite early in the former scenario and continuously in the latter scenario. As we will see, there are arguments either in favour of the monolithic or the hierarchical scenario. 

Figure 7. Predicted [a/Fe] vs. velocity dispersion for elliptical galaxies. Data are compared with predictions obtained by adopting the star formation history assumed in hierarchical clustering models for galaxy formation.The models and the figure are from Thomas et al. (2002).

The Sagittarius dwarf Spheroidal galaxy (Sgr dSph) is currently disrupting under the strain of the Milky Way (MW) tidal field. The study of the Sgr chemical composition allows us to study at the same time the star formation history of a dwarf galaxy and the relevance of the hierarchical merging process for the formation of large galaxies such as the MW. 

After my talk at the Caucasus Winter School Zeldovich offered me collaboration in the study of the universe. He was developing a theory of formation of galaxies (the pancake theory, Zeldovich 1970) an alternative whirl theory was suggested by Ozernoy (1971), and a third theory of hierarchical clustering by Peebles (1971). Zeldovich asked for our help in solving the question Can we find some observational evidence which can be used to discriminate between these theories ... 

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