SCUBA2 High Redshift Bright Quasar Survey: Far-infrared Properties and Weak-line Features

We present a submillimetre continuum survey (SCUBA2 High rEdshift bRight quasaR surveY, hereafter SHERRY) of 54 high redshift quasars at $5.6<z<6.9$ with quasar bolometric luminosities in a range of (0.2$-$$ 5)times10^{14},L_{odot}$, using the Submillimetre Common-User Bolometer Array-2 (SCUBA2) on the James Clerk Maxwell Telescope. About 30% (16/54) of the sources are detected with a typical 850$mu$m rms sensitivity of 1.2 $rm mJy,beam^{-1}$ ($Srm _{ u,850,mu m} = 4$-5 mJy, at $>3.5sigma$). The new SHERRY detections indicate far-infrared (FIR) luminosities of $rm 3.5times10^{12}$ to $rm 1.4times10^{13}$ $L_{odot}$, implying extreme star formation rates of 90 to 1060 $M_{odot}$ yr$^{-1}$ in the quasar host galaxies. Compared with $z =$ 2$-$5 samples, the FIR luminous quasars ($L_{rm FIR} > 10^{13},L_{odot}$) are more rare at $z sim 6$. The optical/near-infrared (NIR) spectra of these objects show 11% (6/54) of the sources have weak Ly$alpha$, emission line features, which may relate to different sub-phases of the central active galactic nuclei (AGNs). Our SCUBA2 survey confirms the trend reported in the literature that quasars with submillimeter detections tend to have weaker ultraviolet (UV) emission lines compared to quasars with nondetections. The connection between weak UV quasar line emission and bright dust continuum emission powered by massive star formation may suggest an early phase of AGN-galaxy evolution, in which the broad line region is starting to develop slowly or is shielded from the central ionization source, and has unusual properties such as weak line features or bright FIR emission.