Top left: three-colour composite image with VMC Y (blue) and K_s bands (green), and Spitzer IRAC 8.0 μm (red). The rectangle shows the region covered by our analysis. Small crosses mark the centres of grid elements for the 40″ grid that are classified as PMS-only (yellow) or UMS+PMS (cyan). Several prominent regions are highlighted and labelled (spatial sizes according to Bica et al. 2008) and discussed further in Sect. 6. Top right: inverted grey-scale map of Hα emission (Smith et al. 2005). Bottom: PMS density contours for elements classified as PMS-only or UMS+PMS derived from the residual maps of the grid elements. The outermost contour represents ΔnPMS = 2.4 stars arcmin⁻² (see Sect. 6.2 for details); every subsequent contour represents an increase in density by 3 × Δn_PMS.