THE NATURE OF E AND S0 GALAXIES - A STUDY OF A MAGNITUDE-LIMITED SAMPLE OF GALAXIES IN THE COMA CLUSTER

Differences and similarities of E and SO galaxies have been investigat ed on basis of new CCD surface photometry in Gunn r for 171 galaxies w ithin the central square degree of the Coma Cluster; 146 of the galaxi es are classified as E or SO. The galaxies form a magnitude-limited sa mple with Gunn r less-than-or-equal-to 15.3 mag. Together with the sam ple of Virgo galaxies compiled by Caon et al. (1993), this sample is o ne of the first volume-limited samples. The isophotal shapes of the ga laxies have been studied in terms of ellipticity, c4 and c6. The sampl e is deficit of apparently round SO galaxies. The distribution of the apparent ellipticities for the SO galaxies cannot be fitted satisfacto rily with any simple distribution of the intrinsic ellipticities. This suggests that face-on SO galaxies have been classified as ellipticals . It is shown that the ellipticity-c4 diagram can be used for determin ation of relative disk luminosities, L(D)/L(tot). The distribution of L(D)/L(tot) for galaxies fainter than M(BT) = -22 mag is very broad (H -0 = 50 km s-1 Mpc-1 is used). The distribution is best fitted by a mo del where 10% of the galaxies are diskless and 90% of the galaxies are drawn from a distribution which is uniform from zero to one. This mod el also predicts distributions of apparent ellipticities, c4, and incl inations which are in agreement with the data. E galaxies brighter tha n M(BT) = -22 mag do not show any evidence of disks and may form anoth er class. The sample contains no SO galaxies brighter than M(BT) = -22 mag. Our analysis implies that the E and SO galaxies fainter than M(B T) = -22 mag form one class of galaxies with a continuous change in L( D)/L(tot). Face-on members of this class are mostly classified as E ga laxies while more edge-on members are classified as SO galaxies. Furth er, it is found that only 12% of all the galaxies in the sample are tr uly diskless systems. The diskless systems contribute almost-equal-to 30% to the total luminosity of the sample, most of this originates fro m the six brightest galaxies. It is estimated that almost-equal-to 60% of the total luminosity of the sample originates from bulges.