Issues in E Galaxy Formation: Many Problems, Few Solutions

Abstract.The quest to understand the sequence of events during the formation of large galaxies has often brought in globular cluster systems as touchstones for different models. Giant elliptical galaxies in particular confront us with the most varied tests of the models, since there are more formation channels whose end points are gE’s than for any other type of galaxy. Over the past decade or so, the debate on E galaxies has tended to crystallize around three basic approaches for the origin of globular cluster systems:Hierarchical merging from a large population of pregalactic gas clouds. There may have been two major epochs of formation (e.g. Forbes et al. 1997; Harris et al. 1998) or a long series of smaller star-forming events (e.g. Beasley et al. 2002a,b), with the majority of the “action” happening at high redshift. Late major merging of pre-existing disk galaxies (e.g. Schweizer 1987; Ashman & Zepf 1992; Zepf et al. 1999 and many other papers). In this view most of the metal-poor cluster population was formed early in the history of the progenitor disks, while many (or most) of the metal-rich clusters are formed in the merger and are thus younger. Dissipationless satellite accretion, whereby a “seed” galaxy (perhaps already a giant E-type) forms by the classic monolithic collapse and then accretes a series of smaller satellites (Côté et al. 2002). These alternatives are not mutually exclusive (though they have often been debated as if they were). It is perhaps useful to think of them instead as representing three connected points of a triangle, with the presence or absence of gas playing the crucial connecting role. Major merging resembles hierarchical merging if it takes place at extremely early times when the progenitors are still mostly gaseous; and so does satellite accretion where the incoming fragments are still quite gaseous. Furthermore, for any given gE we may be looking at the results of many possible combinations of these processes.