Density plot (2D histogram) of Gaia sources in the PM plane (μ_l, μ_b) in Galactic coordinates, with parallax π >  5 and relative error on parallax π/Δ_π >  10, Gaussian-smoothed, and with filters (1) and (3) applied (see Sect. 2 for details). The circle at (μ_l, μ_b) = (−40, −28) indicates the 1 − σ size of the smoothing Gaussian. The small cross at (μ_l, μ_b) = (−40, −25) represents the median PM error. Black dots indicate the HIPPARCOS Sco OB2 likely members from de Zeeuw et al. (1999). The color scale on the top axis indicates source density is in units of sources/(mas/yr)². The red polygon enclosing all HIPPARCOS members defines the extraction region of PM-selected Gaia Sco OB2 candidate members.