1999MNRAS.309L...1M

We present sensitive 850-µm SCUBA photometry of a statistically complete sample of six of the most luminous (M_B←27.5; νL_ν(1450Å)≳10¹³L_☉), z>4 radio-quiet quasars, reaching noise levels (1σ∼1.5mJy) comparable with the deep blank sky surveys. These observations probe the rest-frame far-infrared region (∼150µm), at luminosity levels for thermal sources comparable with those that IRAS permitted for low redshift quasars. One quasar (BR2237-0607; z=4.55) is detected at 850µm with a flux of 5.0±1.1mJy (4.5σ), whilst a second (BR0019-1522; z=4.52) has a detection at the 2σ level. When combined with our previous millimetre measurements of z>4 quasars, we find that there is a large range (5-10) in far-infrared (FIR) luminosity (L_FIR) at fixed UV luminosity, and that the typical quasar has a L_FIR and mass of cool (50K) dust similar to that of the archetyepal low redshift (z=0.018) ultraluminous IRAS galaxy (ULIRG) Arp220 [L_FIR∼5x10¹²L_☉; M_d(cool)∼10⁸M_☉]. If one assumes a fiducial FIR luminosity of 5x10¹²L_☉ for for all quasars with M_B← 23, we find that ≳15 per cent of the sources in the SCUBA deep surveys could be classical broad-lined radio-quiet active galactic nuclei (AGN). Thus if one considers the observed ratio of Seyfert II to Seyfert I galaxies at low redshift and any contribution from totally optically obscured AGN, a significant fraction of the SCUBA source population will harbour AGN and hence the inferred star formation rates from submm fluxes may be overestimated if the active nuclei are bolometrically dominant or the initial mass function (IMF) is top heavy.