Beyond the Hubble Sequence -- Exploring Galaxy Morphology with Unsupervised Machine Learning

TL;DR

This paper introduces an unsupervised machine learning approach combining feature extraction and hierarchical clustering to analyze galaxy morphology, revealing well-separated clusters that correlate with physical properties and challenge traditional visual classifications.

Contribution

The study presents a novel methodology integrating feature learning with clustering, considering multiple thresholds and galaxy orientation, leading to more physically meaningful galaxy classifications.

Findings

27 distinct galaxy clusters identified with shape and structure features

Clusters correlate strongly with physical properties like color-magnitude and mass-size relations

Achieved 87% accuracy in binary classification of galaxy types, matching real distributions

Abstract

We explore unsupervised machine learning for galaxy morphology analyses using a combination of feature extraction with a vector-quantised variational autoencoder (VQ-VAE) and hierarchical clustering (HC). We propose a new methodology that includes: (1) consideration of the clustering performance simultaneously when learning features from images; (2) allowing for various distance thresholds within the HC algorithm; (3) using the galaxy orientation to determine the number of clusters. This setup provides 27 clusters created with this unsupervised learning which we show are well separated based on galaxy shape and structure (e.g., S\'ersic index, concentration, asymmetry, Gini coefficient). These resulting clusters also correlate well with physical properties such as the colour-magnitude diagram, and span the range of scaling-relations such as mass vs. size amongst the different machine-defined clusters. When we merge these multiple clusters into two large preliminary clusters to provide a binary classification, an accuracy of $\sim87\%$ is reached using an imbalanced dataset, matching real galaxy distributions, which includes 22.7\% early-type galaxies and 77.3\% late-type galaxies. Comparing the given clusters with classic Hubble types (ellipticals, lenticulars, early spirals, late spirals, and irregulars), we show that there is an intrinsic vagueness in visual classification systems, in particular galaxies with transitional features such as lenticulars and early spirals. Based on this, the main result in this work is not how well our unsupervised method matches visual classifications and physical properties, but that the method provides an independent classification that may be more physically meaningful than any visually based ones.