The most massive galaxies in the present-day Universe are found to lie in the centres of rich clusters. They have old, coeval stellar populations suggesting that the bulk of their stars must have formed at early epochs in spectacular starbursts 1 -luminous phenomena at submillimetre wavelengths 2 . The most popular model of galaxy formation predicts that these galaxies form in protoclusters at high-density peaks in the early Universe 3 . Such peaks are signposted by massive high-redshift radio galaxies 4 . Here we report deep submillimetre mapping of seven high-redshift radio galaxies and their environments. These data confirm not only the presence of spatially extended massive star-formation activity in the radio galaxies themselves, but also in companion objects previously undetected at any wavelength. The prevalence, orientation, and inferred masses of these submillimetre companion galaxies suggest that we are witnessing the synchronous formation of the most luminous elliptical galaxies found today at the centres of rich galaxy clusters.-1 -Whilst existing submillimetre studies of high-redshift radio galaxies (hereafter HzRGs) have shown that their star-formation rates are large enough to build a massive galaxy in < 1 Gyr 2,5,6,7 they have provided no information on the spatial extent of this emission or on the star-formation activity in their environments. We have therefore mapped a sample of seven objects with redshifts ranging from 2.2 to 4.3 at a wavelength of 850 µm with the Submillimetre Common-User Bolometer Array (SCUBA) 8 on the James Clerk Maxwell Telescope (JCMT). The targets were chosen from those sources found to be submillimetre bright in the previous SCUBA surveys of HzRGs 6,7 . Our new maps illustrate the distribution of dust-reradiated emission in and around the HzRGs on scales from 5 ′′ to 160′′ , or 30 kpc to 1 Mpc. We illustrate the seven submillimetre maps from this survey in Figure 1; the orientation of the radio jets of each HzRG is represented by tick marks on these maps.One of the most striking aspects of the submillimetre maps is that the dust emission from the central radio galaxy is resolved in at least five of the seven sources -even with the coarse beam of the JCMT. In Figure 2 and Table 1 we present simple two-dimensional Gaussian fits to the data which, while not giving a true reflection of the physical situation, at least provide a quantitative measure of the spatial extent of the dust emission. This emission is sometimes in the form of several partially-resolved or merged clumps (typified by 8C 1909+722), sometimes in an apparently smoother distribution (e.g. 4C 60.07), and is more extended than the radio emission in most cases. The extent of the dust emission ranges from 50 to 250 kpc, a physically interesting size because (1) the corresponding half-light radii (30-150 kpc) are equivalent to those of brightest cluster galaxies in the local Universe 9 , and (2) gas-dynamical simulations of major galaxy mergers predict that star formation should peak when the galaxies are sepa...

We report the detection of 1.3mm continuum and near-infrared K-band (2.2-micron) emission from the submillimeter galaxy SMM J00266+1708. Although this galaxy is among the brightest sub-mm sources detected in the blank-sky surveys (L~10^{13} L[sun]), SMM J00266+1708 had no reliable optical/near-infrared counter-part. We used sensitive interferometric 1.3mm observations with the Owens Valley Millimeter Array to accurately determine the position of the sub-mm galaxy. Follow-up near-infrared imaging with the Keck I telescope uncovered a new faint red galaxy at K=22.5 mag which is spatially coincident with the 1.3mm emission. This is currently the faintest confirmed counter-part of a sub-mm galaxy. Although the redshift of SMM J00266+1708 is still unknown, its high sub-mm/radio spectral index suggests that the system is at high redshift (z>2). Approximately 50% or more of the sub-mm galaxies are faint/red galaxies similar to that of SMM J00266+1708. These ultraluminous obscured galaxies account for a significant fraction of the total amount of star-formation at high redshift despite being missed by optical/ultraviolet surveys.Comment: 7 pages, includes 3 figures, submitted to AJ (minor revisions after referee's report

Context. The submillimetre-bright galaxy population is believed to comprise, aside from local galaxies and radio-loud sources, intrinsically active star-forming galaxies, the brightest of which are lensed gravitationally. The latter enable studies at a level of detail beyond what is usually possible by the observation facility. Aims. This work focuses on one of these lensed systems, HATLAS J142935.3−002836 (H1429−0028), selected in the Herschel-ATLAS field. Gathering a rich, multi-wavelength dataset, we aim to confirm the lensing hypothesis and model the background source's morphology and dynamics, as well as to provide a full physical characterisation. Methods. Multi-wavelength high-resolution data is utilised to assess the nature of the system. A lensing-analysis algorithm that simultaneously fits different wavebands is adopted to characterise the lens. The background galaxy dynamical information is studied by reconstructing the 3D source plane of the ALMA CO (J:4 → 3) transition. Near-IR imaging from HST and Keck-AO allows to constrain rest-frame optical photometry independently for the foreground and background systems. Physical parameters (such as stellar and dust masses) are estimated via modelling of the spectral energy distribution taking source blending, foreground obscuration, and differential magnification into account. Results. The system comprises a foreground edge-on disk galaxy (at z sp = 0.218) with an almost complete Einstein ring around it. The background source (at z sp = 1.027) is magnified by a factor of μ ∼ 8−10 depending on wavelength. It is comprised of two components and a tens-of-kpc-long tidal tail resembling the Antennae merger. As a whole, the background source is a massive stellar system (1.32 +0.63−0.41 × 10 11 M ) forming stars at a rate of 394 ± 90 M yr −1 , and it has a significant gas reservoir M ISM = 4.6 ± 1.7 × 10 10 M . Its depletion time due to star formation alone is thus expected to be τ SF = M ISM /SFR = 117 ± 51 Myr. The dynamical mass of one of the components is estimated to be 5.8 ± 1.7 × 10 10 M , and, together with the photometric total mass estimate, it implies that H1429−0028 is a major merger system (1:2.8 +1.8 −1.5).