A tool for the morphological classification of galaxies: the
  concentration index
  Using the galaxy data of the Sloan Digital Sky Survey Data Release 8 (SDSS
DR8), I explore whether the concentration index is a good morphological
classification tool and find that a reasonably pure late-type galaxy sample can
be constructed with the choice of r-band concentration index ci=2.85. The
opposite is not true, however, due to the fairly high contamination of an
early-type sample by late-type galaxies. In such an analysis, the influence of
selection effects is less important. To disentangle correlations of the
morphology and concentration index with stellar mass, star formation rate
(SFR), specific star formation rate (SSFR) and active galactic nucleus (AGN)
activity, I investigate correlations of the concentration index with these
properties at a fixed morphology and correlations of the morphology with these
properties at a fixed concentration index. It is found that at a fixed
morphology, high-concentration galaxies are preferentially more massive and
have a lower SFR and SSFR than low-concentration galaxies, while at a fixed
concentration index, elliptical galaxies are preferentially more massive and
have a lower SFR and SSFR than spiral galaxies. This result shows that the
stellar mass, SFR and SSFR of a galaxy are correlated with its concentration
index as well as its morphology. In addition, I note that AGNs are
preferentially found in more concentrated galaxies only in the spiral galaxy
sample.
