Using Gaia to identify and remove foreground sources for M33. (A) Field-of-view of Gaia sources (black dots) for M33, with the green ellipse marking the galaxy’s boundary. The blue dots highlight the sources in M33 with known spectral classification. (B) Foreground region for M33. (C) Distribution of the proper motion over its error for Dec, for all Gaia sources in the foreground region (fitted with a spline function). (D) Distribution of the proper motion over its error for Dec (solid line), for all sources along the line-of-sight of M33, including both foreground and galactic (M33) sources. The dashed line refers to the scaled spline (accounting for the number of foreground sources expected inside M33) and the dotted line to a Gaussian function. The vertical dashed lines correspond to the 3σ threshold of the Gaussian. Any source with values outside this region is flagged as a potential foreground source. (E) Gaia CMD of all sources identified as galactic (red points) and foreground (gray). The majority of the foreground sources lie on the yellow branch of the CMD, which is exactly the position at which the largest fraction of the contamination is expected.