FAMILIES OF GALAXIES IN THE MU(E)-R(E) PLANE

Using a complete sample of Virgo cluster galaxies and a larger set of heterogeneous literature data, we show that ellipticals, early-type dwarfs, and bulges of S0s and of spirals form two distinct families in the plane of the effective parameters, R(e) and mu(e). The 'ordinary' family is composed of all galaxies fainter than M(B) = congruent-to - 19.3. Their effective parameters range over a wide interval for the same total luminosity: R(e) varies by at least approximately 0.7 dex and mu(e) by 3.5 mag. Moreover, the interval spanned by R(e) is the same at any luminosity (down to M(B) congruent-to - 12.0), with a sharp upper boundary at R(e) congruent-to 3 kpc. The 'bright' family is composed of the brightest cluster members and the host galaxies of QSOs. Their effective radii extend up to congruent-to 300 kpc, and are directly related to the effective surface brightness: mu(e) congruent-to 3 log R(e) + constant. There are indications that the objects belonging to this family have probably undergone merging phenomena. The existence of the above dichotomy has a bearing on the use of the Fundamental Plane to probe extragalactic distances, and increases interest in the (log R(e), mu(e)) plane as a diagnostic diagram for galaxy formation and evolution.