6.2 The Age of Elliptical Galaxies
Despite the discovery of the Lyman dropout population at moderate redshift, the alternative view that galaxies formed at high redshift in a short but intense period of star formation still provides the best explanation for the formation of at least some galaxies - in particular, elliptical galaxies. These constitute ~ 30% of the galaxy population seen at low redshift and ellipticals seen at z 0.5 have spectral energy distributions conststent with a passively evolving stellar population with an age of several Gyrs (see Figure 3), equivalent to a formation redshift z 5 (e.g. Bender et al. 1996, Ellis et al. 1997). So a complete picture of when galaxies form remains elusive: nearby elliptical galaxies, the most massive of any galaxy-type, contain a stellar population too old for them to be the low redshift counterparts of the Lyman dropout galaxies and yet primeval elliptical galaxies are not seen in the deep optical or near-infrared searches for monolithic PGs. A key question then is: how can the dearth of PGs in the optical and near-infrared searches (described in section 5), many of which are tuned to detect objects at z 5, be reconciled with the seemingly inevitable conclusion from evolution studies that this is exactly the epoch at which elliptical galaxies are expected to form?