Dissecting the morphological and spectroscopic properties of galaxies in
  the local Universe: I. Elliptical galaxies
  We revisit the scaling relations and star-forming histories of local
elliptical galaxies using a novel selection method applied to the Sloan Digital
Sky Survey DR7. We combine two probability-based automated spectroscopic and
morphological classifications of about 600000 galaxies with z<0.25 to isolate
true elliptical galaxies. Our sample selection method does not introduce
artificial cuts in the parameters describing the galaxy but instead it
associates to every object a weight measuring the probability of being in a
given spectro-morphological class. Thus the sample minimizes the selection
biases. We show that morphologically defined ellipticals are basically
distributed in 3 spectral classes, which dominate at different stellar masses.
The bulk of the population (about 50%) is formed by a well defined class of
galaxies with old stellar populations that formed their stars at very early
epochs in a short episode of star formation. They dominate the scaling
relations of elliptical galaxies known from previous works and represent the
canonical elliptical class. At the low mass end, we find a population of
slightly larger ellipticals, with smaller velocity dispersions at fixed stellar
mass, which seem to have experienced a more recent episode of star formation
probably triggered by gas-rich minor mergers. The high mass end tends to be
dominated by a third spectral class, slightly more metal rich and with more
efficient stellar formation than the reference class. This third class
contributes to the curvature of the mass-size relation at high masses reported
in previous works. Our method is therefore able to isolate typical spectra of
elliptical galaxies following different evolutive pathways.
