Machine learning-based classification of active galaxies and estimation of supermassive black hole masses

ABSTRACT Distinguishing active galaxies from star-forming galaxies is essential for understanding galaxy evolution. Diagnostic methods like the BPT (Baldwin, Phillips, and Terlevich) diagram use optical emission-line ratios to separate galaxies. However, with growing availability of large surveys and high-resolution instruments, manually identifying galaxy types has become increasingly challenging. In this study, we investigate machine learning to classify active and star-forming galaxies using properties such as stellar mass, stellar velocity dispersion, colour, redshift, and [O III] luminosity. These new approaches enable faster active galactic nucleus/star-forming galaxy classification than the BPT diagram and provide a flexible, scalable alternative that can complement traditional diagnostics, particularly for large surveys or low-quality data. We employ four classification algorithms – Decision Tree, Random Forest, Support Vector Classifier (SVC), and k-Nearest Neighbours (KNN) – using the Galaxy Zoo 1 data set derived from the Sloan Digital Sky Survey sample. The data set contains 47 675 galaxies within the redshift range 0.02–0.05, including 17 002 pure star-forming and 2254 active galaxies, labelled using the BPT diagram. These labels train and evaluate our models through confusion matrices, learning curves, and receiver operating characteristic curves. Among the four algorithms, the SVC and Random Forest models achieve the highest accuracy of approximately 93 per cent, while KNN shows the lowest at 88 per cent. Furthermore, we estimate supermassive black hole masses using stellar velocity dispersion ($\sigma$) and the $M_{\rm BH}{-}\sigma$ relation. We apply four regression models – Random Forest Regressor, Support Vector Regressor, KNN Regressor, and Polynomial Regression. All four models produce similar results, with $R^2$ values from 0.75 to 0.77, indicating consistent performance.