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.

Cold dust emission from X-ray AGN in the SCUBA-2 Cosmology Legacy Survey: dependence on luminosity, obscuration and AGN activity

“…Our work can immediately be place in the context of these previous studies. While both Alexander et al (2005) and Wang et al (2013) probe significantly fainter in terms of X-ray luminosity, our sample fills in the luminosity gap between these studies and the individual follow-up observations of X-ray bright quasars by Stevens et al (2005). The X-ray brightest submm AGN in our sample is brighter than any of the X-ray AGN detected in the sub-mm by Alexander et al (2005) and Wang et al (2013).…”

“…The hard X-ray flux limit and z > 1 redshift limit applied prior to our stacking analysis, are denoted by the dashed lines. Both Alexander et al (2005) and Wang et al (2013) probe fainter X-ray luminosities compared to our work, while the X-ray AGN from Stevens et al (2005) that were individually followed up, probe very high X-ray luminosities. Our study is useful in bridging the gap between these different data sets.…”

“…Given the low surface density of X-ray AGN and submillimeter sources, all 19 matches have very small probabilities ( 1 per cent) of being chance associations (see . Redshift versus observed hard X-ray (2-10 keV) luminosity for the X-ray AGN that are individually detected at submillimeter wavelengths in this work and compared to individually detected submillimeter bright X-ray AGN from Alexander et al (2005) and Wang et al (2013) and submillimeter detected quasars from Stevens et al (2005). Downward triangles denote upper limits.…”

“…3, we plot the redshift versus hard (2-10 keV) X-ray luminosity for the submillimeter-detected AGN in this work as well as Alexander et al (2005) and Wang et al (2013). We also show here the submillimeter-detected X-ray absorbed quasars from Stevens et al (2005), which were selected using X-ray surveys with ROSAT, XMM-Newton and Chandra and followed up at submillimeter wavelengths using SCUBA. The Wang et al (2013) observed 0.5-8 keV luminosities have been converted to 2-10 keV luminosities assuming the intrinsic photon indices derived for each object in that paper whereas conversions from 0.5-2 keV fluxes in Stevens et al…”

“…One way to distinguish between these is to look for evidence for obscured AGN having excess star formation in their host galaxies relative to their unobscured counterparts. Several early studies of X-ray luminous AGN using the SCUBA bolometer on the James Clerk Maxwell Telescope (JCMT) showed this to be the case (Page et al 2001(Page et al , 2004Stevens et al 2005). More recently, new data at far-infrared wavelengths from the Herschel Space Observatory, has been used to study the connection between star formation in galaxies and accretion on to the central AGN (e.g.…”