Submillimetre photometry of X-ray absorbed quasi-stellar objects: their formation and evolutionary status

We present an analysis of the submillimetre/X-ray properties of 19 X-ray absorbed, Compton- thin quasi-stellar objects (QSOs) selected to have luminosities and redshifts which represent the peak of cosmic QSO activity. i.e. � Lobjects at 1 1.5, the high star formation rates are consistent with a scenari o in which the QSOs evolve to become local luminous elliptical galaxies. Combining these results with previously published data for X-ray unabsorbed QSOs and submillimetre-selected galaxies we propose the following evolutionary sequence: the forming galaxy is initially far-infrared luminous but X-ray weak si milar to the sources discovered by the Submillimetre Common-User Bolometer Array; as the black hole and spheroid grow with time a point is reached when the central QSO becomes powerful enough to terminate the star formation and eject the bulk of the fuel supply (the Compton-thin absorbed QSO phase); this transition is followed by a period of unobscured QSO activity which subsequently declines to leave a quiescent spheroidal galaxy.