2.3. Ellipticals

The sequence E0-E6, based on apparent ellipticity, was originally conceived of as an angular momentum sequence in young galaxies. However, it did not take long for colour-based work to suggest that ellipticals are old [7], and formed quickly in a short burst of star-formation. Furthermore, it is now known that ellipticals are pressure-supported by anisotropic velocity distributions, with rotation playing a subsidiary role (except in less luminous systems [24]). Since ellipticals are known to be triaxial, the E0-E6 sequence is seriously contaminated by orientation effects.

The observational evidence supporting the view that ellipticals are old and roughly coeval is based primarily on the small scatter in the colour-magnitude diagram of low-redshift cluster ellipticals [72, 13]. This test has also been extended to higher redshift cluster samples with morphological classifications from the Hubble Space Telescope (HST) [38, 80], where the scatter remains small, suggesting that the epoch of elliptical formation in rich clusters was well in the past even at high redshifts.