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

Abstract

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 (MB ≤ -27.5; vLv(1450 Å) > 1013 L⊙). The survey, undertaken at 850 μm using SCUBA at the James Clerk Maxwell Telescope (JCMT), has a target sensitivity limit of 3σ ∼ 10 mJy, 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 850 μm, of which eight were detected (>3σ) with S850μm ≳ 10 mJy (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, MB. We find that the weighted mean flux of the undetected sources is 2.0 ± 0.6 mJy, consistent with an earlier estimate of ∼3 mJy based on more sensitive observations of a sample z ≳ 4 radio-quiet quasars. This corresponds to an inferred star formation rate of ∼ 1000 M⊙ yr-1, similar to Arp220. The typical star formation time-scale for the submillimetre-bright sources is ∼1 Gyr, 10 times longer than the typical accretion-driven e-folding time-scale of ∼5 × 107 yr. 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 1-2 arcsec (6-12 kpc). 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.