Dissecting the morphological and spectroscopic properties of galaxies in the local Universe: I. Elliptical galaxies

TL;DR

This study uses a novel probabilistic classification method on SDSS data to analyze the properties and evolutionary pathways of local elliptical galaxies, revealing distinct spectral classes linked to different mass ranges and star formation histories.

Contribution

It introduces a probability-based classification approach that minimizes bias and isolates elliptical galaxy subclasses with distinct evolutionary histories.

Findings

Elliptical galaxies are distributed in three spectral classes.

Most ellipticals have old stellar populations formed early.

High-mass ellipticals show signs of more recent star formation.

Abstract

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.