Upper panel: dust mass (Red solid line) as function of galactic age. The cumulative contribution to the dust mass by SNIa, SNII, AGB and QSO (fiducial case)) as well as the integral of the the dust growth and destruction are shown. The observed values are given by the shaded area. The arrow shows the time at which we predict the conditions for the galactic wind to occur in most of the galaxy. The model refers to a galaxy of 10¹²   M_⊙ (model M12d). Lower panel: as above, in this case only the total dust and the QSO contribution are plotted. The QSO fiducial dust production is compared to a case in which we assume the metallicity of the dust forming regions 0.8 dex higher than the average (Case A), and to two cases in which the relation between BH mass and stellar mass is “super-Magorrian” (Cases B and C, see text). Finally, we present a model (Case D) in which the QSO is fine-tuned to be the only source of dust.