QUANTITATIVE MORPHOLOGY OF E-SO GALAXIES .4. ELLIPTICALS AND LENTICULARS AS A SINGLE POPULATION

The geometrical properties of E and SO galaxies have been intercompare d using the data collected in Paper III (Michard & Marchal 1994) for 1 08 RSA objects in a complete, luminosity and distance limited, sample. As the apparent flattening (largely an effect of projection along the line of sight), is a determining factor in the segregation between E and SO objects, the working hypothesis has been made that an important bias is introduced in the recognition of the two classes. It is perha ps as well to assume that galaxies of both Hubble types belong, but fo r a small(?) minority, to a common population of objects with similar structures. This hypothesis receives strong support from the frequency -diagrams of the ellipticity epsilon(max) measured near its maximum or at the isophote of surface brightness V=21.5. The diagram for S0's al one cannot be generated by the random projection of any objects: it is clearly biased by the shift to the E type of many S0's of moderate in clination and relatively modest disk. This limits the significance of the same diagrams for E galaxies. The noted bias is much reduced if SO 's and disky E's are considered together. Because of the strong outwar ds decrease of the ellipticities in disky E's and in the SO's with non -thin envelopes (thick disks and spheroidal haloes), the frequency dia grams of the ellipticities measured at the classical B=25, or at V=25, do not show the bias noted above for SO's. The lack of round E's requ ires the spheroidal components to be faintly triaxial, as recently emp hasized by other authors. Our hypothesis is also supported by the over lap of E and SO galaxies in ad hoc classification schemes of elliptici ty profiles and envelope geometry, and in such correlation diagrams as : the ellipticity in the envelope, i.e. near mu(V)=25, against the int ermediate maximum ellipticity the extremum of the Carter's coefficient e4 (or a4 or c4 in other similar works) against the maximum elliptici ty the disk extent, as far as it can be estimated without detailed mod eling, against the maximum ellipticity. The ''standard'' structure of E - SO objects includes a spheroidal, nearly oblate component, that ca nnot be of constant ellipticity: this parameter often increases outwar ds from its bulge value, then decreases again in the envelope. Disks o f various brightness and extent, between dominating and vanishing, are the second essential component. Obviously, their relative importance is , together with the dust content, a physical criterion involved in the morphological separation between E and SO objects, besides the inc lination effect. In the present sample, the disk cannot be detected in nearly half of E classified objects, and is probably vanishing in abo ut 40% of this class, or circa 17% of the total of studied early-type objects. This minority contains mostly boxy E's with nonstandard ellip ticity profiles, and, as is well known, no significant rotational supp ort. From the convergent results of quantitative morphology and kinema tics, the early-type galaxies should be segregated into the majority c lass of disky fast oblate rotators and a minority class of non-rotatin g ellipsoids.