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 that represent the peak of cosmic QSO activity, i. e. similar to L* objects at 1 < z < 3. Of these, we present new data for 11 objects not previously observed at submillimetre wavelengths and additional data for a further three. The detection rate is 42 per cent, much higher than typically reported for samples of QSOs. Detection statistics show ( at the 3 - 4 sigma level) that this sample of absorbed QSOs has a higher submillimetre output than a matched sample of unabsorbed QSOs. We argue that the far-infrared luminosity is produced by massive star formation. In this case, the correlation found between far-infrared luminosity and redshift can be interpreted as cosmological evolution of the star formation rate in the QSO host galaxies. Because the submillimetre luminous phase is confined to z > 1.5, the high star formation rates are consistent with a scenario 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: ( i) the forming galaxy is initially far-infrared luminous but X- ray weak similar to the sources discovered by the Submillimetre Common-User Bolometer Array ( SCUBA); ( ii) 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); (iii) this transition is followed by a period of unobscured QSO activity, which subsequently declines to leave a quiescent spheroidal galaxy.