The Formation of Early-Type Galaxies : Observations to z ∼ 1

How does the number density of early-type galaxies (E+S0) evolve with redshift? What are their star formation histories? Do their mass density profile and other structural properties evolve with redshift? Answering these questions is key to understanding how E+S0s form and evolve. I review the observational evidence on these issues, focusing on the redshift range z ∼ 0.1 − 1, and compare it to the predictions of current models of galaxy formation. 1.1 Introduction Understanding the formation and evolution of early-type galaxies (E+S0, i.e. el-lipticals and lenticulars) is not only crucial to unveil the origin of the Hubble sequence, but is also a focal point connecting several unanswered major astrophysical questions. The hypothesis that E+S0s form by mergers of disks at relatively recent times is one of the pillars of the cold dark matter (CDM) hierarchical scenario. At galactic scales, since they are the most massive galaxies, E+S0s are the key to understanding how and when dark and luminous mass are assembled in galaxies, and to test the universal form and ubiquity of dark matter halos predicted by the CDM paradigm (Navarro, Frenk & White 1997, hereafter NFW; Moore et al. 1998). On subgalactic scales, the existence of a correlation between black-hole mass and spheroid velocity dispersion suggests that the growth of black holes and the activity cycles in active galactic nuclei are somehow intimately connected with the formation of spheroids. Therefore, a unified formation scenario must ultimately be conceived (e. g. Theoretical formation scenarios are often grouped into two categories, broadly referred to as the monolithic collapse and hierarchical formation *. In the traditional picture – the monolithic collapse – E+S0s assembled their mass and formed their stars in a rapid event, of much shorter duration than their average age (Eggen, Lynden-Bell & Sandage 1962; Larson 1975; van Albada 1982). The formation process happened at high redshifts and proto early-type galaxies would be star forming and dust-enshrouded systems. These kinds of models are consistent with a variety of features (see * A complete review of the theoretical background is beyond the aims of this observational review. For more information the reader is referred to, e.