Submillimetre photometry of X-ray absorbed quasi-stellar objects: 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 that 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 three. The detection rate is 42 per cent, much higher than typically reported for samples of QSOs. Detection statistics show (at the 3–4σ 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: 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); 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.