What are S0 galaxies?

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Abstract
Contrary to expectation it is found that the frequency distribution of S0 galaxy luminosities is not intermediate between those of E and Sa galaxies. This may indicate that the S0 class comprises at least two subgroups (which partly overlap in luminosity): (1) Bright (M_B__T__^0i^ < -20.0) S0's that are truly intermediate between galaxies of types E and Sa, and (2) less luminous (M_B__T__^0i^ > -20.0) disklike lenticular galaxies. The majority of "ellipticals" fainter than M_B__T__^0i^ = -20.0 are probably lenticulars that are not viewed almost edge-on, which makes it difficult or impossible to recognize their disk subsystems. A third class of S0 galaxies, comprising such diverse objects as NGC 5128 and NGC 5195, may owe their present morphology to recent close encounters or mergers. It is concluded that the S0 classification type comprises a number of physically quite distinct kinds of objects that exhibit only superficial morphological similarities. The fact that most "ellipticals" fainter than M_B__T__^0i^ = -20.0 are, in fact, S0's implies that the luminosity function of E galaxies cuts off much more sharply at low luminosities than had been believed previously.

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