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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 (≃1011 solar masses) of cool dense (volume density ≃ 1 cm–3) gas, which is in conflict with current cosmological simulations.

Observations of a quasar at redshift 7.54, when the Universe was just five per cent of its current age, suggest that the Universe was significantly neutral at this epoch. A massive black hole in the early Universe Despite extensive searches, only one quasar has been known at redshifts greater than 7, at 7.09. Eduardo Bañados and colleagues report observations of a quasar at a redshift of 7.54, when the Universe was just 690 million years old, with a black-hole mass 800 million times the mass of the Sun. The spectrum shows that the quasar's Lyman α emission is being substantially absorbed by an intergalactic medium containing significantly neutral hydrogen, indicating that reionization was not complete at that epoch. Quasars are the most luminous non-transient objects known and as a result they enable studies of the Universe at the earliest cosmic epochs. Despite extensive efforts, however, the quasar ULAS J1120 + 0641 at redshift z  = 7.09 has remained the only one known at z  > 7 for more than half a decade 1 . Here we report observations of the quasar ULAS J134208.10 + 092838.61 (hereafter J1342 + 0928) at redshift z  = 7.54. This quasar has a bolometric luminosity of 4 × 10 13 times the luminosity of the Sun and a black-hole mass of 8 × 10 8 solar masses. The existence of this supermassive black hole when the Universe was only 690 million years old—just five per cent of its current age—reinforces models of early black-hole growth that allow black holes with initial masses of more than about 10 4 solar masses 2 , 3 or episodic hyper-Eddington accretion 4 , 5 . We see strong evidence of absorption of the spectrum of the quasar redwards of the Lyman α emission line (the Gunn–Peterson damping wing), as would be expected if a significant amount (more than 10 per cent) of the hydrogen in the intergalactic medium surrounding J1342 + 0928 is neutral. We derive such a significant fraction of neutral hydrogen, although the exact fraction depends on the modelling. However, even in our most conservative analysis we find a fraction of more than 0.33 (0.11) at 68 per cent (95 per cent) probability, indicating that we are probing well within the reionization epoch of the Universe.

The distribution of diffuse gas in the intergalactic medium (IGM) imprints a series of hydrogen absorption lines on the spectra of distant background quasars known as the Lyman-α forest. Cosmological hydrodynamical simulations predict that IGM density fluctuations are suppressed below a characteristic scale where thermal pressure balances gravity. We measured this pressure-smoothing scale by quantifying absorption correlations in a sample of close quasar pairs. We compared our measurements to hydrodynamical simulations, where pressure smoothing is determined by the integrated thermal history of the IGM. Our findings are consistent with standard models for photoionization heating by the ultraviolet radiation backgrounds that reionized the universe.

Observations of a cosmic web filament have been made in Lyman-α emission; the filament has a projected size of approximately 460 physical kiloparsecs, and its estimated cold gas mass is more than ten times larger than what is typically found in cosmological simulations. A glimpse of structure in the cosmic web Cosmological theory and observations of the distant Universe point to the existence of a cosmic web, a network of filaments with galaxies located at nodes where the filaments intersect. Now a study of Lyman-α emissions from material surrounding the radio-quiet quasar UM2 87 may have provided a glimpse of the three-dimensional structure of the cosmic web. The redshift-2.3 quasar is illuminating the most extended cold gas reservoir so far discovered in the Universe, and the authors conclude that it traces the larger-scale filamentary structure of the cosmic web predicted by modern cosmological simulations but not previously directly detected. Simulations of structure formation in the Universe predict that galaxies are embedded in a ‘cosmic web’ 1 , where most 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-α emission, discovered during a survey for cosmic gas fluorescently illuminated by bright quasars 4 , 5 at redshift z  ≈ 2.3. With a linear projected size of approximately 460 physical kiloparsecs, the Lyman-α emission surrounding the radio-quiet quasar UM 287 extends well beyond the virial radius of any plausible associated dark-matter halo and therefore traces intergalactic gas. The estimated cold gas mass of the filament from the observed emission—about 10 12.0 ± 0.5 / C 1/2 solar masses, where C is the gas clumping factor—is more than ten times larger than what is typically found in cosmological simulations 5 , 6 , suggesting that a population of intergalactic gas clumps with subkiloparsec sizes may be missing in current numerical models.

We introduce a data-reduction package written in Interactive Data Language (IDL) for the Magellan Echellete Spectrograph (MAGE). MAGE is a medium-resolution ( R ∼ 4100 R ∼ 4100 ), cross-dispersed, optical spectrograph, with coverage from ∼ 3000 – 10000     Å . The MAGE Spectral Extractor (MASE) incorporates the entire image reduction and calibration process, including bias subtraction, flat fielding, wavelength calibration, sky subtraction, object extraction, and flux calibration of point sources. We include examples of the user interface and reduced spectra. We show that the wavelength calibration is sufficient to achieve∼5 km s-1 ∼ 5     km   s - 1 rms accuracy and relative flux calibrations better than 10%. A lightweight version of the full reduction pipeline has been included for real-time source extraction and signal-to-noise estimation at the telescope.

The physical and chemical properties of the circumgalactic medium at z  ≳ 6 have been studied successfully through the absorption in the spectra of background quasi-stellar objects 1 – 3 . One of the most crucial questions is to investigate the nature and location of the source galaxies that give rise to these early metal absorbers 4 – 6 . Theoretical models suggest that momentum-driven outflows from typical star-forming galaxies can eject metals into the circumgalactic medium and the intergalactic medium at z  = 5–6 (refs. 7 – 9 ). Deep, dedicated surveys have searched for Lyα emission associated with strong C  iv absorbers at z  ≈ 6, but only a few Lyα-emitter candidates have been detected. Interpreting these detections is moreover ambiguous because Lyα is a resonant line 10 – 12 , raising the need for complementary techniques for detecting absorbers’ host galaxies. Here we report a [C  ii ] 158 μm emitter detected using the Atacama Large Millimeter Array that is associated with a strong low-ionization absorber, O  i , at z  = 5.978. The projected impact parameter between O  i and [C  ii ] emitter is 20.0 kpc. The measured [C  ii ] luminosity is 7.0 × 10 7 solar luminosities. Further analysis indicates that strong O  i absorbers may reside in the circumgalactic medium of massive halos one to two orders of magnitude more massive than expected values 8 , 14 . [C  ii ] 158 μm emission associated with a strong low-ionization absorber at z  = 5.978 indicates a dark matter halo mass of around 4 × 10 11 solar masses, one to two orders of magnitude more massive than typical values predicted from cosmological simulations.

Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the Universe. The cold-dark-matter model of the formation of large-scale structures (that is, clusters of galaxies and even larger assemblies) predicts 1 , 2 , 3 , 4 , 5 , 6 the existence of quasars gravitationally lensed by concentrations of dark matter 7 so massive that the quasar images would be split by over 7 arcsec. Numerous searches 8 , 9 , 10 , 11 for large-separation lensed quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars known 12 , including the first lensed quasar discovered 13 , have smaller separations that can be explained in terms of galaxy-scale concentrations of baryonic matter. Although gravitationally lensed galaxies 14 with large separations are known, quasars are more useful cosmological probes because of the simplicity of the resulting lens systems. Here we report the discovery of a lensed quasar, SDSS J1004 + 4112, which has a maximum separation between the components of 14.62 arcsec. Such a large separation means that the lensing object must be dominated by dark matter. Our results are fully consistent with theoretical expectations 3 , 4 , 5 based on the cold-dark-matter model.

Simulations of structure formation in the Universe predict that galaxies are embedded in a 'cosmic web'1, where most 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-α emission, discovered during a survey for cosmic gas fluorescently illuminated by bright quasars (4,5) at redshift z ≅ 2.3. With a linear projected size of approximately 460 physical kiloparsecs, the Lyman-α emission surrounding the radio-quiet quasar UM 287 extends well beyond the virial radius of any plausible associated dark-matter halo and therefore traces intergalactic gas. The estimated cold gas mass of the filament from the observed emission--about [10.sup.12.0 ± 0.5]/[C.sup.1/2] solar masses, where cis the gas clumping factor--is more than ten times larger than what is typically found in cosmological simulations (5,6), suggesting that a population of intergalactic gas clumps with subkiloparsec sizes may be missing in current numerical models.

We conduct a survey for Lyman break galaxies (LBGs) and Lyman alpha emitters (LAEs) in the environs of six and 17 z ∼ 4 quasars respectively, probing scales of R ≲9 h −1 Mpc. We detect an enhancement of galaxies (both LBGs and LAEs) in quasar fields, a positive and strong quasar-galaxy cross-correlation function, consistent with a power-law shape, and a strong galaxy auto-correlation function in quasar fields. The three mentioned results are all indicators that quasars trace massive dark matter halos in the early universe.

We present AGNfitter: a Markov Chain Monte Carlo algorithm developed to fit the spectral energy distributions (SEDs) of active galactic nuclei (AGN) with different physical models of AGN components. This code is well suited to determine in a robust way multiple parameters and their uncertainties, which quantify the physical processes responsible for the panchromatic nature of active galaxies and quasars. We describe the technicalities of the code and test its capabilities in the context of X-ray selected obscured AGN using multiwavelength data from the XMM-COSMOS survey.