Introduction

We now take up the study of galaxy formation, which requires understanding the growth of structure in the baryonic component of the universe. This chapter uses concepts from several earlier chapters, especially Chaps. 3–5, and will be needed for Chap. 9.

The study of galaxy formation from fundamental physical considerations is made difficult by a wide variety of physical processes that we need to take into account. To begin with, galaxies by themselves show a variety of morphological and physical properties, even at z = 0. The formation process should be such that, starting from relatively structureless density enhancements at high redshifts (z ≳ 25), one is capable of producing such a variety at z = 0. Second, the observational situation as regards galaxylike structures at high redshifts is still very unsatisfactory. The samples are small in number and often we have to decide how to make the correspondence between the sources seen at high redshifts and those seen at low redshifts. This issue is further complicated by the fact that a certain kind of population could have existed at a certain interval of time and could have vanished outside this epoch. Such a conjecture of invoking new populations is often convenient, but is not very satisfactory unless we could back it with some physical reasoning.