Absolute magnitude M_K and Q_d index of non-stellar sources as a function of their azimuthal distance from the two-armed spiral pattern, where ΔΘ = 0° and 180° denote the relative K_s-band intensity maxima given by the phase of m = 2 FFT component. The top diagram shows M_K with colors indicating the Q_d index. The number of clusters with M_K  >  –13^m is displayed as a full drawn curve, while the hatched histogram shows the brighter clusters. The low section shows Q_d with M_K used for color coding. The full drawn histogram indicates older clusters with Q_d  <  01, whereas younger ones are shown by hatched bars.