Automatic Classification of Galaxy Morphology: a rotationally invariant supervised machine learning method based on the UML-dataset

TL;DR

This paper introduces a rotationally invariant supervised machine learning method using adaptive polar coordinate transformation for galaxy morphology classification, improving robustness and consistency in large astronomical datasets.

Contribution

The paper presents a novel adaptive polar coordinate transformation technique that enhances rotational invariance in galaxy classification, building upon previous unsupervised methods.

Findings

The method achieves consistent classifications regardless of galaxy image rotation.

Classifications align well with expected galaxy property trends.

The approach outperforms conventional data augmentation techniques.

Abstract

Classification of galaxy morphology is a challenging but meaningful task for the enormous amount of data produced by the next-generation telescope. By introducing the adaptive polar coordinate transformation, we develop a rotationally invariant supervised machine learning (SML) method that ensures consistent classifications when rotating galaxy images, which is always required to be satisfied physically but difficult to achieve algorithmically. The adaptive polar coordinate transformation, compared with the conventional method of data augmentation by including additional rotated images in the training set, is proved to be an effective and efficient method in improving the robustness of the SML methods. In the previous work, we generated a catalog of galaxies with well-classified morphologies via our developed unsupervised machine learning (UML) method. By using this UML-dataset as the training set, we apply the new method to classify galaxies into five categories (unclassifiable, irregulars, late-type disks, early-type disks, and spheroids). In general, the result of our morphological classifications following the sequence from irregulars to spheroids agrees well with the expected trends of other galaxy properties, including S\'{e}rsic indices, effective radii, nonparametric statistics, and colors. Thus, we demonstrate that the rotationally invariant SML method, together with the previously developed UML method, completes the entire task of automatic classification of galaxy morphology.