The nature of the SDSS galaxies in various classes based on morphology, colour and spectral features -- I. Optical properties

TL;DR

This study classifies SDSS galaxies into 16 classes based on morphology, colour, and spectral features, analyzing their physical parameters and implications for galaxy evolution.

Contribution

It provides a detailed classification scheme and physical analysis of SDSS galaxies, revealing star formation and AGN activity patterns across classes.

Findings

Over half of red early-type galaxies show star formation or AGN activity.

Blue early-type galaxies have structural features similar to red early-types but with bluer centers.

Most blue late-type galaxies are actively forming stars.

Abstract

We present a comprehensive study of the nature of the SDSS galaxies divided into various classes based on their morphology, colour, and spectral features. The SDSS galaxies are classified into early-type and late-type; red and blue; passive, HII, Seyfert, and LINER, returning a total of 16 fine classes of galaxies. We examine the luminosity dependence of seven physical parameters of galaxies in each class. We find that more than half of red early-type galaxies (REGs) have star formation or AGN activity, and that these active REGs have smaller axis ratio and bluer outside compared to the passive REGs. Blue early-type galaxies (BEGs) show structural features similar to those of REGs, but their centres are bluer than REGs. HII BEGs are found to have bluer centres than passive BEGs, but HII REGs have bluer outside than passive REGs. Bulge-dominated late-type galaxies have red colours. Passive red late-types are similar to REGs in several aspects. Most blue late-type galaxies (BLGs) have forming stars, but a small fraction of BLGs do not show evidence for current star formation activity. Differences of other physical parameters among different classes are inspected, and their implication on galaxy evolution is discussed.