2.4. S0 Galaxies
It interesting to note that on the basis of the early versions of the tuning fork, S0 galaxies were essentially predicted to exist by Hubble even before they were observationally established (``The transitional stage, S0, is more or less hypothetical'' - Hubble 1936). The natural way in which subsequent observations entrenched S0 systems as key components in the galaxy population has been rightly hailed as a major success of the Hubble system. But are S0 galaxies, as predicted by Hubble, a truly transitional physical ``bridge'' between spirals and ellipticals? The evidence is equivocal, and the question remains among the most interesting in physical morphology.
Figure 2. Histograms showing the luminosity distributions of elliptical, S0, Sa, and Sb galaxies in the Revised Shapley-Ames Catalog (Sandage 1981). The S0 distribution is repeated from panel to panel. Note how S0 systems appears to be systematically fainter than both elliptical and Sa galaxies, suggesting that S0 galaxies are not a morphological bridge between these. Figure taken from van den Bergh (1998).
As pointed out by van den Bergh (1998), the luminosity distribution of both ellipticals and early-type spirals peaks ~ 1.5 mag brighter than that of S0 galaxies, suggesting S0 galaxies are not a smooth transition class between spirals and ellipticals (Figure 2). Other evidence seems to be building that suggests that S0 systems are relics of spirals. For example, the ``Morphs'' collaboration  have used HST imaging to show that much the gradual bluing in the cluster population with redshift (the Butcher-Oemler effect) is due to fairly-normal looking late-type spirals that are seen in the cores of high redshift systems, but which are absent in the cores of local clusters. The elliptical fraction as a function of redshift seems constant, and the clear implication is that spirals are being transformed into S0 systems. Coming at this issue from the other direction, other studies [12, 66] have shown that some ellipticals contain boxy inner disks, and other studies have on the whole lent support to the notion that early-type galaxies can be physically differentiated on the basis of having disk components of varying strengths [54, 40, 70], possibly as a function of luminosity. An interesting synthesis of these ideas has been proposed by Kormendy & Bender (1997), who suggest that the present ranking of the early-types along the tuning fork (based on apparent eccentricity) should be replaced by a sequence in which ellipticals are ranked according to boxiness of isophotes . S0 systems would then indeed form a transitional bridge between spirals and ellipticals with boxy isophotes. This proposed system is a radical alteration of the Hubble sequence, but one which preserves the spirit of the S0 class as a transitional bridge as envisioned by Hubble.