Deciphering galaxy images using machine vision -- Combining variational autoencoder and principal component analysis for feature extraction

Abstract

Here, we present a machine vision approach, combining a VAE framework with PCA, to decipher galaxy images. Using mock gri-band images from the EAGLE simulation, the VAE finds that around 35 features are needed to describe the images. Adding the PCA, we identify an optimal range of 10-12 features needed to capture 99.9% of the variance in galaxy images. The exact optimal number varies with galaxy complexity: disk-dominated galaxies require 12 features, bulge-dominated galaxies need 9, and intermediate systems require 10-11 features. Correlations between extracted PCA features and structural measurements reveal that the VAE prioritizes galaxy size during reconstruction, with half-light radius strongly correlating with the highest-ranked principal components. Subsequent features capture morphology-dependent characteristics: disk-dominated galaxies emphasize size, asymmetry, and position angle; bulge-dominated systems focus on size, concentration, and axis ratio; while intermediate galaxies show enhanced attention to Sersic index, indicating greater emphasis on accurately reproducing both disk and bulge structures. The PCA process significantly reduces the entanglement of the features compared to the raw VAE latent features, decreasing the correlations with the half-light radius and the Sersic index from 14.5+-1.0 and 6.0+-1.5 features, respectively, to only 2.0+-1.0 components after PCA. Using UMAP, we construct two-dimensional visualizations that preserve neighborhood relationships from the high-dimensional feature space. This demonstrates that machine vision can effectively distinguish galaxy populations across different morphological types, including systems with atypical structures that may be overlooked by traditional classification methods, providing a data-driven complement to conventional structural measurements.