Morphological Classification of Galaxies

TL;DR

This paper reviews the history and current methods of galaxy morphological classification, highlighting traditional, crowd-sourced, and machine learning approaches, and provides a terminology cross-reference.

Contribution

It offers a comprehensive overview of galaxy classification techniques and introduces a lookup table for terminologies used in the field.

Findings

Traditional visual classification methods are complemented by crowd-sourcing approaches like Galaxy Zoo.

Machine learning techniques are increasingly used for automated galaxy morphology classification.

A cross-matching table of classification terminologies aids in standardizing galaxy morphology descriptions.

Abstract

The morphological classification of galaxies provides vital physical information about the orbital motions of stars in galaxies, and correlates in interesting ways with star formation history, and other physical properties. Galaxy morphological classification is a field with a history of more than 100 years of development, and many scientists have introduced new classification schemes, resulting in a sometimes confusing array of terminologies and overlapping classes. In this article I provide a brief historical review of galaxy classification, but focus mostly on providing a summary of how the morphological variety of galaxies seen in our expanding Universe are described. I review traditional visual classification, morphometric measurements, crowd-sourcing for large scale visual classifications (Galaxy Zoo), and of course the recent explosion of interest in making use of machine learning techniques for galaxy morphology classification. A look up table is provided for cross matching of various terminologies currently in use for galaxy morphology classification as well as brief definitions of the main morphological types.