Submillimetre photometry of X-ray absorbed QSOs: their formation and evolutionary status

Abstract

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. ~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 3. 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. Since 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: the forming galaxy is initially far-infrared luminous but X-ray weak similar 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.