2.3.1. Age Controversy

Although ellipticals have been considered to be old for the last 40 years, the term ``early-type'' to describe these galaxies (based on the original misapprehensions of Hubble) remains entrenched. Perhaps this will turn out to be a blessing in disguise, since the age distribution of elliptical galaxies, long thought a settled issue, has recently become enlivened by controversy. Ellipticals (and S0 galaxies) are predominantly found in dense environments, and consequently most local work has focussed on the properties of ellipticals in rich clusters. But hierarchical models for galaxy formation introduced the possibility that elliptical galaxies in the field are continuously formed from merging systems. In a controversial paper, Kauffmann et al. [50] claim observational evidence, from a colour-based analysis of data from the Canada-France Redshift Survey (CFRS), for a factor of three decrease in the abundance of ellipticals by z = 1. The claimed decrease is in good agreement with the predictions of semi-analytical approximations to hierarchical formation models for galaxy evolution. Kauffmann et al. also point out that cluster samples are ill-suited to testing the hierarchical picture as they represent accelerated regions so far as structure growth is concerned. It is quite possible that most of the rich cluster ellipticals are truly old as their homogeneously distributed colours imply. The ultimate test of Kauffmann et al.'s conjecture lies in the analysis of ellipticals in field samples. The currently-available samples are too small for robust results [76, 4], although Zepf [87] claims the absence of red objects at very faint limits in the HDF is consistent with hierarchical predictions. Glazebrook et al. [44] have also claimed evidence for younger field ellipticals in data from the HST Medium Deep Survey, on the basis of I - K colours.