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 deep Keck narrow-band Lyα images of the luminous z > 3 radio galaxies 4C 41.17, 4C 60.07, and B2 0902+34. The images show giant, 100−200 kpc scale emission line nebulae, centered on these galaxies, which exhibit a wealth of morphological structure, including extended low surface brightness emission in the outer regions, radially directed filaments, coneshaped structures and (indirect) evidence for extended Lyα absorption. We discuss these features within a general scenario where the nebular gas cools gravitationally in large Cold Dark Matter (CDM) halos, forming stars and multiple stellar systems. Merging of these "building" blocks triggers large scale starbursts, forming the stellar bulges of massive radio galaxy hosts, and feeds super-massive black holes which produce the powerful radio jets and lobes. The radio sources, starburst superwinds and AGN radiation then disrupt the accretion process limiting galaxy and black hole growth, and imprint the observed filamentary and cone-shaped structures of the Lyα nebulae.1 Based on observations made at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation.

In this paper, we combine the stellar spectral synthesis code STARBURST 99, the nebular modelling code MAPPINGS IIIq, a 1-D dynamical evolution model of H II regions around massive clusters of young stars and a simplified model of synchrotron emissivity to produce purely theoretical self-consistent synthetic spectral energy distributions (SEDs) for (solar metallicity) starbursts lasting some 10 8 years. These SEDs extend from the Lyman Limit to beyond 21 cm. We find that two ISM parameters control the form of the SED; the pressure in the diffuse phase of the ISM (or, equivalently, its density), and the molecular cloud dissipation timescale. In particular, the shape of the FIR (dust re-emission) bump is strongly dependent on the mean pressure in the star-forming or starburst galaxy. This can explain the range of FIR colors seen in starburst galaxies. In the case -2of objects of composite excitation, such diagrams potentially provide a means of estimating the fraction of the FIR emission that is contributed by an active nucleus. We present detailed SED fits to Arp 220 and NGC 6240, and we give the predicted colors for starburst galaxies derived from our models for the IRAS and the Spitzer Space Observatory MIPS and IRAC instruments. Our models reproduce the spread in observed colors of starburst galaxies. From both the SED fits and the color:color diagrams, we infer the presence of a population of compact and ultra-compact H II regions around single OB stars or small OB clusters. Finally, we present absolute calibrations to convert observed fluxes into star formation rates in the UV (GALEX), at optical wavelengths (Hα), and in the IR (IRAS or the Spitzer Space Observatory). We show that 25µm fluxes are particularly valuable as star formation indicators since they largely eliminate one of the parameters controlling the IR SED.