R_h surface density distribution for the reference sample (black lines) and for the best sample (in red, Σ in units of number of objects per square arcminute). The panels, from upper to lower, show the surface density over the full inspected frame for sources at R_gal ≤ R_bg for the background area at R_gal>R_bg, and for the difference between the inner and outer 23′. The vertical dashed line marks the 3.5 pc limit.