The temperatures of dust‐enshrouded active galactic nuclei

A high density of massive dark objects (MDOs), probably supermassive black holes, in the centres of nearby galaxies has been inferred from recent observations. There are various indications that much of the accretion responsible for producing these objects took place in dust‐enshrouded active galactic nuclei (AGNs). If so, then measurements of the intensity of background radiation and the source counts in the far‐infrared and submillimetre wavebands constrain the temperature of dust in these AGNs. An additional constraint comes from the hard X‐ray background, if this is produced by accretion. One possibility is that the dust shrouds surrounding the accreting AGNs are cold, about 30 K. In this event, the dusty AGNs could be some subset of the population of luminous distant sources discovered at 850 μm using the SCUBA array on the James Clerk Maxwell Telescope, as proposed by Almaini, Lawrence & Boyle. An alternative is that the dust shrouds surrounding the accreting AGNs are much hotter (>60 K). These values are closer to the dust temperatures of a number of well‐studied low‐redshift ultraluminous galaxies that are thought to derive their power from accretion. If the local MDO density is close to the maximum permitted, then cold sources cannot produce this density without the submillimetre background being overproduced if they accrete at high radiative efficiency, and thus a hot population is required. If the dust‐enshrouded accretion occurred at similar redshifts to that taking place in unobscured optical quasars, then a significant fraction of the far‐infrared background radiation measured by COBE at 140 μm, but very little of the submillimetre background at 850 μm, may have been produced by hot dust‐enshrouded AGNs which may have already been seen in recent Chandra X‐ray surveys.