Simulations of structure formation in the Universe predict that galaxies are embedded in a "cosmic web" 1 , where the majority of baryons reside as rarefied and highly ionized gas 2 . This material has been studied for decades in absorption against background sources 3 , but the sparseness of these inherently one-dimensional probes preclude direct constraints on the three-dimensional morphology of the underlying web. Here we report observations of a cosmic web filament in Lyman ! emission, discovered during a survey for cosmic gas fluorescently "illuminated" by bright quasars 4,5 at z~2.3. With a projected size of approximately 460 physical kpc, the Lyman-! emission surrounding the radio-quiet quasar UM287 extends well beyond the virial radius of any plausible associated dark matter halo. The estimated cold gas mass of the nebula from the observed emission -M gas~1 0 12.0±0.5 /C 1/2 solar masses, where C is the gas clumping factor -is at least ten times larger than what is typically found by cosmological simulations 5,6 , suggesting that a population of intergalactic gas clumps with sub-kpc sizes may be missing within current numerical models. A recent pilot survey 5 using a custom-built, narrow-band (NB) filter on the Very Large Telescope (VLT) demonstrated that bright quasars can, like a flashlight, "illuminate" the densest knots in the surrounding cosmic web and boost fluorescent Lyman ! emission 4,5,7-9 to detectable levels. Following the same experiment, we imaged UM287 on UT November 12 and 13, 2012 with a custom NB filter (NB3985) tuned to Lyman ! at z = 2.28 inserted into the camera of the Low Resolution Imaging Spectrometer (LRIS) on the 10m Keck-I telescope (see Extended Data Figure 1). Figure 1 presents the processed and combined images, centered on UM287. In the NB3985 image, one identifies a very extended nebula originating near the quasar with a projected size of about 1 arcmin. Within the nebula, very few sources are identified in the broad-band images nor is any extended emission observed. This requires the NB light to be lineemission, and we identify it as Lyman-! at the redshift of UM287. Figure 2 presents the NB3985 image, continuum subtracted using standard techniques (see Methods) and smoothed with a 1 arcsec Gaussian kernel. This image is dominated by the filamentary and asymmetric Nebula that has a maximum projected extent of 55 arcsec measured from the 10 −18 erg s −1 cm −2 arcsec −2 isophotal, corresponding to about 460 physical kpc or 1.5 Mpc co-moving. Including (excluding) the emission from UM287 falling within the NB filter, the structure has a total line luminosity L Ly! = 1.43 ± 0.05 × 10 45 erg s −1 (L Ly! = 2.2 ± 0.2 × 10 44 erg s −1 ). Although Lyman-! nebulae extending up to about 250 kpc have been previously detected 10-14 , the UM287 Nebula represents so far a unique system: given its size, it extends well beyond any plausible dark matter halo associated with UM287 (see below) representing an exceptional example of emitting gas on truly Intergalactic scales. The largest Lyma...

All galaxies once passed through a hyperluminous quasar phase powered by accretion onto a supermassive black hole. But because these episodes are brief, quasars are rare objects typically separated by cosmological distances. In a survey for Lyman-α emission at redshift z ≈ 2, we discovered a physical association of four quasars embedded in a giant nebula. Located within a substantial overdensity of galaxies, this system is probably the progenitor of a massive galaxy cluster. The chance probability of finding a quadruple quasar is estimated to be ∼10(-7), implying a physical connection between Lyman-α nebulae and the locations of rare protoclusters. Our findings imply that the most massive structures in the distant universe have a tremendous supply (≃10(11) solar masses) of cool dense (volume density ≃ 1 cm(-3)) gas, which is in conflict with current cosmological simulations.

The discovery of quasars a few hundred megayears after the Big Bang represents a major challenge to our understanding of black holes as well as galaxy formation and evolution. Quasarsʼ luminosity is produced by extreme gas accretion onto black holes, which have already reached masses of M BH >10 9 M e by z∼6. Simultaneously, their host galaxies form hundreds of stars per year, using up gas in the process. To understand which environments are able to sustain the rapid formation of these extreme sources, we started a Very Large Telescope/Multi-Unit Spectroscopic Explorer (MUSE) effort aimed at characterizing the surroundings of a sample of 5.7<z<6.6 quasars, which we have dubbed the Reionization Epoch QUasar InvEstigation with MUSE (REQUIEM) survey. We here present results of our searches for extended Lyα halos around the first 31 targets observed as part of this program. Reaching 5σ surface brightness limits of 0.1-1.1×10 −17 erg s −1 cm −2 arcsec −2 over a 1 arcsec 2 aperture, we were able to unveil the presence of 12 Lyα nebulae, eight of which are newly discovered. The detected nebulae show a variety of emission properties and morphologies with luminosities ranging from 8×10 42 to 2×10 44 erg s −1 , FWHMs between 300 and 1700 km s −1 , sizes <30 pkpc, and redshifts consistent with those of the quasar host galaxies. As the first statistical and homogeneous investigation of the circumgalactic medium of massive galaxies at the end of the reionization epoch, the REQUIEM survey enables the study of the evolution of the cool gas surrounding quasars in the first 3 Gyr of the universe. A comparison with the extended Lyα emission observed around bright (M 1450 −25 mag) quasars at intermediate redshift indicates little variations on the properties of the cool gas from z∼6 to z∼3, followed by a decline in the average surface brightness down to z∼2.

Motivated by the recent discovery of the near-ubiquity of Lyα emission around z 3 QSOs, we performed a systematic study of QSO circumgalactic Lyα emission at z ≈ 2, utilizing the unique capability of the Keck Cosmic Web Imager (KCWI) -a new wide-field, blue sensitive integral-field spectrograph (IFU). In this paper, we present KCWI observations on a sample of 16 ultraluminous Type-I QSOs at z = 2.1 − 2.3 with ionizing luminosities of L νLL = 10 31.1−32.3 erg s −1 Hz −1 . We found that 14 out of 16 QSOs are associated with Lyα nebulae with projected linear-sizes larger than 50 physical kpc (pkpc). Among them, four nebulae have enormous Lyα emission with the Lyα surface brightness SB Lyα > 10 −17 erg s −1 cm −2 arcsec −2 on the > 100 kpc scale, extending beyond the field of view of KCWI. Our KCWI observations reveal that most z ≈ 2 QSO nebulae have a more irregular morphology compared to those at z 3. In turn, we measure that the circularlyaveraged surface brightness (SB) at z ≈ 2 is 0.4 dex fainter than the redshift-corrected, median SB at z 3. The Lyα SB profile (SB Lyα ) of QSOs at z ≈ 2 can be described by a power law of SB Lyα,z≈2.3 = 3.7 × 10 −17 × (r/40) −1.8 erg s −1 cm −2 arcsec −2 , with the slope similar to that at z 3. The observed lower redshift-corrected, circularly-averaged SB may be mainly due to the lower covering factor of cool gas clouds in massive halos at z ≈ 2.

We present a study of the metal-enriched cool halo gas traced by Mg ii absorption around 228 galaxies at z ∼ 0.8 − 1.5 within 28 quasar fields from the MUSE Analysis of Gas around Galaxies (MAGG) survey. We observe no significant evolution in the Mg ii equivalent width versus impact parameter relation and in the Mg ii covering fraction compared to surveys at z ≲ 0.5. The stellar mass, along with distance from galaxy centre, appears to be the dominant factor influencing the Mg ii absorption around galaxies. With a sample that is 90% complete down to a star formation rate of ≈0.1  $\rm M_\odot yr^{-1}$PLXINSERT-, and up to impact parameters ≈250 − 350  kpc from quasars, we find that the majority ($67^{+12}_{-15}$% or 14/21) of the Mg ii absorption systems are associated with more than one galaxy. The complex distribution of metals in these richer environments adds substantial scatter to previously-reported correlations. Multiple galaxy associations show on average five times stronger absorption and three times higher covering fraction within twice the virial radius than isolated galaxies. The dependence of Mg ii absorption on galaxy properties disfavours the scenario in which a widespread intra-group medium dominates the observed absorption. This leaves instead gravitational interactions among group members or hydrodynamic interactions of the galaxy haloes with the intra-group medium as favoured mechanisms to explain the observed enhancement in the Mg ii absorption strength and cross section in rich environments.