We present the results of the kinematic study of the extended gas in a sample of 10 high‐redshift radio galaxies (z∼ 2.5) based on high signal‐to‐noise ratio Keck II and Very Large Telescope (VLT) long‐slit spectroscopy. In addition to the typical high surface brightness kinematically perturbed regions (FWHM and velocity shifts >1000 km s−1), we find in all objects giant low surface brightness haloes that show quieter kinematics with typical emission‐line FWHM and velocity shifts of ∼several hundred km s−1. The giant haloes often extend for more than 100 kpc and sometimes beyond the radio structures. They emit lines other than Lyα (C iv, He ii and N v in some cases), typically found in the spectra of high‐redshift active galaxies. Continuum is also often detected. The haloes are enriched with heavy elements at tens of kpc from the active nucleus. Typical Lyα luminosities and surface brightness (within the slit) are in the range 1043−44 erg s−1 and several × 10−17 to −16 erg cm−2 s−1 arcsec−2, respectively. Estimated densities are in the range ∼17–150 cm−3. The quasar continuum is the dominant source of ionization of the quiescent haloes along the radio axis. The implied total quasar ionizing luminosities are in the range ∼several × 1045–1047 erg s−1, in the same range as radio‐loud quasars at comparable redshift. The detection of giant quiescent haloes in all objects suggests that they could be a common ingredient of high‐redshift radio galaxies. The radio galaxies seem to be embedded within the haloes. The nature and the origin of the haloes, together with the cosmological implications, are also discussed.

The largest galaxies in the Universe reside in galaxy clusters. Using sensitive observations of carbon-monoxide, we show that the Spiderweb Galaxy -a massive galaxy in a distant proto-cluster -is forming from a large reservoir of molecular gas. Most of this molecular gas lies between the proto-cluster galaxies and has low velocity dispersion, indicating that it is part of an enriched inter-galactic medium. This may constitute the reservoir of gas that fuels the widespread star formation seen in earlier ultraviolet observations of the Spiderweb Galaxy. Our results support the notion that giant galaxies in clusters formed from extended regions of recycled gas at high redshift.The formation of the largest galaxies in the Universe is thought to be a two-stage process. For the last 10 Gyr, these giant galaxies have grown mostly by cannibalizing smaller galaxies (1,2). However, computer simulations predict that in an earlier phase, lasting a few Gyr, their stars condensed directly out of large reservoirs of accreted gas (3,4).We present observational evidence for an extended gas reservoir fueling star formation in the massive Spiderweb Galaxy, MRC 1138-262, which is located in a proto-cluster at a redshift of z = 2. 161 (5-9). The Spiderweb Galaxy is not a single galaxy, but an aggregation of protocluster galaxies. They are embedded in a giant halo of atomic (neutral and ionized) hydrogen gas, which radiates Lyα emission across a region of ~200 kpc (6). The central proto-cluster galaxy has a super-massive black hole at its core, which emits jets of relativistic particles visible in radio observations (5). Observations suggest that the proto-cluster galaxies will eventually merge and evolve into a single, giant elliptical galaxy in the center of the cluster (10). We therefore refer to the Spiderweb Galaxy as the entire region encompassed by the Lyα halo, and to the gas between the proto-cluster galaxies as the inter-galactic medium (IGM).Earlier observations of line emission by carbon-monoxide revealed the presence of large amounts of molecular gas in the Spiderweb Galaxy (11). Molecular gas is the raw fuel for the formation of stars, so observations of molecular gas give us insight into the processes driving the evolution of the distant Spiderweb Galaxy. We have obtained new, sensitive observations of 12 CO (J=1→0) with the Australia Telescope Compact Array (ATCA, 90 hour exposure time) and the Karl G. Jansky Very Large Array (VLA, 8 hour exposure time) (12). The ATCA observations were optimized for detecting low-surface-brightness emission from broadly distributed CO, with a 4.8ʺ″×3.5ʺ″ resolution. The VLA observations complement the ATCA data with a higher 0.7ʺ″×0.6ʺ″ resolution, sensitive to small-scale features but not to large-scale ones. Sampling these different spatial scales allows us to obtain a complete picture of the CO distribution, from the gas in the individual proto-cluster galaxies to that across the IGM. Figure 1 shows that the CO emission in the ATCA data covers a region of ~70 kpc around the central...