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A cosmic web filament revealed in Lyman-α emission around a luminous high-redshift quasar



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 gas2. This material has been studied for decades in absorption against background sources3, 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 quasars4,5 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 1012.0 ± 0.5/C1/2 solar masses, where C is the gas clumping factor—is more than ten times larger than what is typically found in cosmological simulations5,6, suggesting that a population of intergalactic gas clumps with subkiloparsec sizes may be missing in current numerical models.

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Figure 1: Processed and combined images of the field surrounding the quasar UM 287.
Figure 2: Lyman-α image of the UM 287 nebula.
Figure 3: Luminosity–size relations for previously detected, bright Lyman-α nebulae and UM 287.
Figure 4: Inferred hydrogen column densities associated with the UM 287 nebula.


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We thank the staff of the W.M. Keck Observatory for their support during the installation and testing of our custom-built narrow-band filter. S.C. thanks M. Haehnelt for comments on an earlier version of the letter and J. Primack for useful conversations. S.C. and J.X.P. acknowledge support from the National Science Foundation (NSF) grant AST-1010004. P.M. acknowledges support from the NSF through grant OIA-1124453, and from NASA through grant NNX12AF87G. The data presented here were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA. The Observatory was made possible by the financial support of the W.M. Keck Foundation. We acknowledge the cultural role that the summit of Mauna Kea has within the indigenous Hawaiian community.

Author information

Authors and Affiliations



S.C. designed the observational survey and the custom-built filter, conducted the observations, led the narrow-band imaging data reduction and analysis, performed the numerical simulations and led the theoretical interpretation, the writing of the text and the production of the figures. F.A.-B. and J.X.P. assisted with the observations, contributed to data reduction, the text and the figures. In particular, F.A.-B. reduced and calibrated the images, produced the continuum-subtracted image, the catalogues of Lyman-α emitters, and compiled data on all Lyman-α nebulae in the literature. J.X.P. reduced the spectrum of the companion quasar and contributed to the text. J.F.H. and P.M. contributed to the text and assisted with the planning and interpretation of the observations.

Corresponding author

Correspondence to Sebastiano Cantalupo.

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Extended data figures and tables

Extended Data Figure 1 Measured transmission curves of the filters used in this study.

Solid line, NB3985; dotted lines, B band (left) and V band (right). Bottom axis, observed wavelength; top axis, the rest-frame wavelength for sources at z = 2.27.

Extended Data Figure 2 Keck/LRIS spectrum of UM 287 and of the faint, radio-loud companion quasar.

Black line, spectrum of this companion quasar which is indicated by ‘b’ in Fig. 2 and is separated by about 24 arcsec from UM 287. Blue line, spectrum of UM287. Comparison of the two spectra clearly shows that this companion is a quasar at a redshift similar to that of UM 287.

Extended Data Figure 3 Pixel-to-pixel correlations for Lyman-α surface brightness for scenarios 1 and 2 in the main text.

a, Pixel-to-pixel correlation between simulated Lyman-α surface brightness (SB) divided by the clumping factor (C) and corresponding cold (T < 5 × 104 K) ionized hydrogen column densities NHII for scenario 1 (see text for details). The solid line indicates the relation NHII = 1021 × (SB)1/2 × C−1/2 (here SB is in units of 10−18 erg s−1 cm−2 arcsec−2 and C is dimensionless). b, Pixel-to-pixel correlation between simulated Lyman-α surface brightness (normalized by the quasar impact parameter squared, b2) and corresponding neutral hydrogen column density for scenario 2 (see text for details). The solid line represents the relation NHi = 1019.1 × [(SB) × (b/100)2]2 cm−2 (here b is in units of kpc).

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Cantalupo, S., Arrigoni-Battaia, F., Prochaska, J. et al. A cosmic web filament revealed in Lyman-α emission around a luminous high-redshift quasar. Nature 506, 63–66 (2014).

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