The formation of spheroidal stellar systems

We summarize current models of the formation of spheroidal stellar systems. Whereas globular clusters form in an efficient mode of star formation inside turbulent molecular clouds, the origin of galactic spheroids, that is bulges, dwarf ellipticals, and giant ellipticals, is directly coupled with structure formation and merging of structures in the Universe. Disks are the fundamental building blocks of galaxies and the progenitors of galactic spheroids. The origin of the various types of spheroids and their global properties can be understood as a result of disk heating by external perturbations, internal disk instabilities, or minor and major mergers. 1.1 The Realm of Spheroids Spheroids exist in the Universe with a wide range in masses and length scales. Probably the most simple, classical examples of stellar spheroids are globular star clusters with masses in the range of 104 M⊙ to 106 M⊙ and half-mass radii of order 2–10 pc (Harris 1996). These almost spherical systems appear to be stable and very long lived. Although the metallicities of different clusters in the Milky Way vary from [Fe/H] ≈ −2.5 to solar or even larger, the strikingly narrow iron abundance spreads of stars within individual clusters (Kraft 1979) and their small age spread indicate that each cluster consists of only one stellar generation that formed on a short time scale from chemically homogenized gas. Peebles & Dicke (1968) proposed that globular clusters are the first objects that formed in the Universe. More recent models assume that globulars formed at the same time as their host galaxies (Fall & Rees 1985; Vietri & Pesce 1995). As giant molecular clouds have similar masses and radii, they are considered to be the primary sites of cluster formation. Unfortunately, the formation of stars and the condensation of molecular clouds into dense, massive star clusters is still not well understood up to now (for a recent review see Lada & Lada 2003). Klessen & Burkert (2000, 2001) and Bate, Bonnell, & Bromm (2002; see also Clarke, this volume) investigated numerically the gravitational collapse of a turbulent cloud. Their models showed that the stabilizing turbulent motion of the molecular gas is dissipated on a short dynamical time scale, resulting in collapse and star formation. These models, however, neglected energetic feedback processes, which are known to play a crucial role in regulating and terminating star formation. In order to form a gravitationally bound, dense stellar cluster, high local star formation efficiencies of order ηsf ≈ 50% are required (Brown, Burkert, & Truran