Using convolutional neural networks to predict galaxy metallicity from three-colour images

Abstract We train a deep residual convolutional neural network (CNN) to predict the gas-phase metallicity (Z) of galaxies derived from spectroscopic information ($Z \equiv 12 + \log (\rm O/H)$) using only three-band gri images from the Sloan Digital Sky Survey. When trained and tested on 128 × 128-pixel images, the root mean squared error (RMSE) of Zpred − Ztrue is only 0.085 dex, vastly outperforming a trained random forest algorithm on the same data set (RMSE = 0.130 dex). The amount of scatter in Zpred − Ztrue decreases with increasing image resolution in an intuitive manner. We are able to use CNN-predicted Zpred and independently measured stellar masses to recover a mass–metallicity relation with 0.10 dex scatter. Because our predicted MZR shows no more scatter than the empirical MZR, the difference between Zpred and Ztrue cannot be due to purely random error. This suggests that the CNN has learned a representation of the gas-phase metallicity, from the optical imaging, beyond what is accessible with oxygen spectral lines.