Environments of z > 5 quasars: searching for protoclusters at submillimetre wavelengths

We present submillimetre (submm) continuum images of the fields of three luminous quasars at z > 5, obtained at 850 and 450 μm using the Submillimetre Common-User Bolometer Array on the James Clerk Maxwell Telescope (JCMT). N-body simulations predict that such quasars evolve to become the central dominant galaxies of massive clusters at z= 0, but at z= 5–6 they are actively forming stars and surrounded by a rich protofilamentary structure of young galaxies. Our purpose in taking these images was to search for other luminous, star-forming galaxies in the vicinity of the signpost active galactic nuclei and thus associated with such a protocluster. Two of the quasar host galaxies are luminous submm galaxies (SMGs) in their own right, implying star formation rates ∼10³ M_⊙ yr⁻¹. Despite the coarse 850-μm beam of the JCMT, our images show evidence of extended emission on a scale of ∼100 kpc from at least one quasar – indicative of a partially resolved merger or a colossal host galaxy. In addition, at >3σ significance we detect 12 (5) SMGs at 850 μm (450 μm) in the surrounding fields. Number counts of these SMGs are comparable with those detected in the fields of z∼ 4 radio galaxies, and both samples are, at the bright end, overabundant by a factor of ≈4 relative to blank-field submm surveys. Whilst the redshift-sensitive 850 μm/450 μm and 850 μm/1.4 GHz flux density ratios indicate that some of these SMGs are likely foreground objects, the counts suggest that ∼60 per cent lie in the same large-scale structures as the quasars.