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 investigated on basis of new CCD surface photometry in Gunn r for 171 galaxies within the central square degree of the Coma Cluster; 146 of the galaxies are classified as E or SO. The galaxies form a magnitude-limited sample with Gunn r less-than-or-equal-to 15.3 mag. Together with the sample of Virgo galaxies compiled by Caon et al. (1993), this sample is one of the first volume-limited samples. The isophotal shapes of the galaxies have been studied in terms of ellipticity, c4 and c6. The sample is deficit of apparently round SO galaxies. The distribution of the apparent ellipticities for the SO galaxies cannot be fitted satisfactorily 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 determination 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 model 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 model also predicts distributions of apparent ellipticities, c4, and inclinations which are in agreement with the data. E galaxies brighter than M(BT) = -22 mag do not show any evidence of disks and may form another 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(BT) = -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 galaxies while more edge-on members are classified as SO galaxies. Further, it is found that only 12% of all the galaxies in the sample are truly diskless systems. The diskless systems contribute almost-equal-to 30% to the total luminosity of the sample, most of this originates from the six brightest galaxies. It is estimated that almost-equal-to 60% of the total luminosity of the sample originates from bulges.