What Are S0 Galaxies?

TL;DR

This paper analyzes the luminosity and classification of S0 galaxies, suggesting many may be misclassified ellipticals and that S0 galaxies likely evolved by losing significant luminous material.

Contribution

It challenges traditional classifications by showing S0 galaxies are generally fainter than ellipticals and spirals, proposing many ellipticals may actually be misclassified S0s.

Findings

S0 galaxies are about a magnitude fainter than ellipticals and spirals.

Many E5-E7 galaxies may be misclassified S0 galaxies.

E7 galaxies might all be misclassified S0_1 galaxies.

Abstract

The data collected in the Shapley-Ames catalog of bright galaxies show that lenticular (S0) galaxies are typically about a magnitude fainter than both elliptical (E) and early spiral (Sa) galaxies. Hubble (1936) was therefore wrong to regard S0 galaxies as being intermediate between morphological types E and Sa. The observation that E5-E7 galaxies are significantly fainter than objects of sub-types E0-E5 suggests that many of the flattest 'ellipticals' may actually be misclassified lenticular galaxies. In particular it is tentatively suggested all E7 galaxies might actually be misclassified S0_1 (7) galaxies. The present results are consistent with the view that galaxies belonging to the S0 class evolved in environments in which they typically lost more than half of their original luminous material.