Quasars as probes of the submillimetre cosmos at z>5 -: I.: Preliminary SCUBA photometry

We present submillimetre (submm) continuum observations of a sample of some of the highest redshift quasars currently known, made with the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). 12 of the sample have redshifts z > 5 and two have z greater than or equal to 6; the median redshift of the sample is 5.3. Two of the z > 5 objects are strong (6sigma) detections, and are bright sources with S-850 mum > 10 mJy. Another firm (5sigma) detection is obtained for the z = 5.7 quasar SDSS J1044-0125; SDSS J1306+0356, at z = 6.0, is detected with a signal-to-noise ratio approximate to4. We have obtained sensitive (sigma approximate to 1.5 mJy) upper limits for much of the remainder of the sample, including the z = 6.3 quasar SDSS J1030+0524. Submm spectral indices measured for two of the sources (alpha approximate to 3.3) are consistent with thermal reradiation from dust, rather than from synchrotron emission. Sensitive upper limits at 450 mum imply that the dust is cool, requiring large dust masses (10(8-9) M.) to account for the observed fluxes, suggesting substantial prior star formation - even at z = 6, when the Universe was only 1.0 Gyr old.