A tool for the morphological classification of galaxies: the concentration index

TL;DR

This study evaluates the effectiveness of the concentration index from SDSS data as a morphological classifier for galaxies, finding it useful for identifying late-type galaxies but less reliable for early types, and explores related galaxy properties.

Contribution

It demonstrates the utility and limitations of the concentration index for galaxy morphology classification and analyzes its correlation with stellar mass, SFR, SSFR, and AGN activity.

Findings

A concentration index of 2.85 effectively selects late-type galaxies.

High-concentration galaxies are more massive with lower SFR and SSFR.

AGNs are more common in concentrated spiral galaxies.

Abstract

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.