The ($\log R_e$, $\mu_e$) Plane of Hot Stellar Systems

Two families of hot stellar systems, named {\it `ordinary'\/} and {\it 'bright'\/}, are identified in the ($\log R_e, \mu_e$) plane built with a luminosity--limited sample of ellipticals and bulges of S0s and spirals of the Virgo and Fornax clusters. This finding, based on {\it ad hoc\/} new observations, is confirmed by a much larger set of literature data for $\sim1500$ galaxies. The {\it `ordinary'\/} family is biparametric: $L_T\propto I_e\,R_e^2$; its members are fainter that $M_B\simeq-19.3$ and smaller than $\re\simeq3$ kpc (whatever $M_B$ is). The {\it 'bright'\/} family is uniparametric ($\mu_e$ depends on $R_e$ alone) and hosts brightest cluster members and QSO parent galaxies. We show that the segregation in the ($\log R_e, \mu_e$) plane has an important counterpart in the behavior of various physical parameters, which is markedly different for galaxies smaller ({\it `ordinary'\/} family) and larger ({\it 'bright'\/} family) than $R_e=3$ kpc.