The SCUBA Bright Quasar Survey (SBQS): 850-μm observations of the z >~ 4 sample

We present initial results of a new, systematic search for massive star formation in the host galaxies of the most luminous and probably most massive z>~4 radio-quiet quasars (M_B<=-27.5 νL_ν(1450Å)>10¹³L_solar). The survey, undertaken at 850μm using SCUBA at the James Clerk Maxwell Telescope (JCMT), has a target sensitivity limit of 3σ~10mJy, set to identify sources suitable for detailed follow-up, e.g. continuum mapping and molecular line diagnostics. A total of 38 z>~4 radio-quiet quasars have been observed at 850μm, of which eight were detected (>3σ) with S_850μm>~10mJy (submillimetre-loud). The new detections almost triple the number of optically selected, submillimetre-loud z>~4 radio-quiet quasars known to date. We include a detailed description of how our quasar sample is defined in terms of radio and optical properties. As a by-product of our selection procedure, we have identified 17 radio-loud quasars with z>~4. There is no strong evidence for trends in either detectability or 850-μm flux with absolute magnitude, M_B. We find that the weighted mean flux of the undetected sources is 2.0+/-0.6mJy, consistent with an earlier estimate of ~3mJy based on more sensitive observations of a sample z>~4 radio-quiet quasars. This corresponds to an inferred star formation rate of ~1000M_solaryr^-1, similar to Arp220. The typical star formation time-scale for the submillimetre-bright sources is ~1Gyr, 10 times longer than the typical accretion-driven e-folding time-scale of ~ 5×10⁷yr. Our 850-μm detection of the z=4.4 quasar PSS J1048+4407, when analysed in conjunction with 1.2-mm single-dish and interferometric observations, suggests that this source is resolved on angular scales of 1-2arcsec (6-12kpc). In addition, we present a new optical spectrum of this source, identifying it as a broad absorption line (BAL) quasar. The new redshift is outside that covered in a recent CO line search, highlighting the need for accurate redshifts for the observation and interpretation of high-redshift line studies.