Dissecting the morphological and spectroscopic properties of galaxies in the local Universe

Aims. 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. Methods. We combine two probability-based automated spectroscopic and morphological classifications of ~600 000 galaxies with z < 0.25 to isolate true elliptical galaxies. Our sample selection method does not introduce artificial cuts into the parameters describing the galaxy but instead associates every object with a weight measuring the probability of being in a given spectro-morphological class. Thus, the sample minimizes the selection biases. Results. We show that morphologically defined ellipticals are basically distributed into three spectral classes, which dominate at different stellar masses. The bulk of the population (~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 ellipticals with relatively large shapes and 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 that is slightly more metal rich and hosts more efficient stellar formation than the reference class. This third class contributes to the curvature in the mass-size relation at high masses reported in previous works. Our method is therefore able to isolate the typical spectra of elliptical galaxies following different evolutionary pathways