Classifying galaxies according to their Hi content

Abstract

Abstract We use machine learning to classify galaxies according to their Hi content, based on both their optical photometry and environmental properties. The data used for our analyses are the outputs in the range z = 0 − 1 from Mufasa cosmological hydrodynamic simulation. In our previous paper, where we predicted the galaxy Hi content using the same input features, Hi rich galaxies were only selected for the training. In order for the predictions on real observation data to be more accurate, the classifiers built in this study will first establish if a galaxy is Hi rich ($\rm {log(M_{H{\small I}}/M_{*})} > -2$) before estimating its neutral hydrogen content using the regressors developed in the first paper. We resort to various machine learning algorithms and assess their performance with some metrics such as accuracy, f1, AUC PR, precision, specificity and log loss. The performance of the classifiers, as opposed to that of the regressors in previous paper, gets better with increasing redshift and reaches their peak performance around z = 1 then starts to decline at even higher z. Random Forest method, the most robust among the classifiers when considering only the mock data for both training and test in this study, reaches an accuracy above $98.6 \%$ at z = 0 and above $99.0 \%$ at z = 1, which translates to a AUC PR above $99.93\%$ at low redshift and above $99.98\%$ at higher one. We test our algorithms, trained with simulation data, on classification of the galaxies in RESOLVE, ALFALFA and GASS surveys. Interestingly, SVM algorithm, the best classifier for the tests, achieves a precision, the relevant metric for the tests, above $87.60\%$ and a specificity above $71.4\%$ with all the tests, indicating that the classifier is capable of learning from the simulated data to classify Hi rich/Hi poor galaxies from the real observation data. With the advent of large Hi 21 cm surveys such as the SKA, this set of classifiers, together with the regressors developed in the first paper, will be part of a pipeline, a very useful tool, which is aimed at predicting Hi content of galaxies.