The (z′−Y)_AB colours of all objects in our catalogue (dots), as a function of Y-band magnitude. The vertical dotted line indicates a 10σ detection (Y_AB= 28.0). Our primary z′-drop selection region where (z′−Y)_AB > 1.3 and Y_AB < 28.5 is marked by the dashed lines – candidates lie in the upper left, and those z′-drops which are undetected at short wavelengths (absent from the ACS v band) are marked as crosses, or arrows denoting 2σ lower limits on their colours if they are undetected in z′ band.