Massive young stellar objects in the Local Group spiral galaxy M 33 identified using machine learning

We present a supervised machine learning classification of stellar populations in the Local Group spiral galaxy M 33. The Probabilistic Random Forest (PRF) methodology, previously applied to populations in NGC 6822, utilizes both near and far-IR classification features. It classifies sources into nine target classes: young stellar objects (YSOs), oxygen, and carbon-rich asymptotic giant branch stars, red giant branch, and red super-giant stars, active galactic nuclei, blue stars (e.g. O-, B-, and A-type main sequence stars), Wolf-Rayet stars, and Galactic foreground stars. Across 100 classification runs the PRF classified 162 746 sources with an average estimated accuracy of similar to 86 per cent, based on confusion matrices. We identified 4985 YSOs across the disc of M 33, applying a density-based clustering analysis to identify 68 star forming regions (SFRs) primarily in the galaxy's spiral arms. SFR counterparts to known H ii regions were recovered with similar to 91 per cent of SFRs spatially coincident with giant molecular clouds identified in the literature. Using photometric measurements, as well as SFRs in NGC 6822 with an established evolutionary sequence as a benchmark, we employed a novel approach combining ratios of [H alpha]/[24 mu m] and [250 mu m]/[500 mu m] to estimate the relative evolutionary status of all M 33 SFRs. Masses were estimated for each YSO ranging from 6-27M(circle dot). Using these masses, we estimate star formation rates based on direct YSO counts of 0.63M(circle dot) yr(-1) in M 33's SFRs, 0.79 +/- 0.16M(circle dot) yr(-1) in its centre and 1.42 +/- 0.16M(circle dot) yr(-1) globally.

Automated summary

We used the Probabilistic Random Forest (PRF) methodology to classify stellar populations in the Local Group spiral galaxy M33. By utilizing both near and far-IR classification features, we accurately classified sources into nine main target classes, identified star forming regions, and estimated star formation rates and masses using photometric measurements and established evolutionary sequences.