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A filament of dark matter between two clusters of galaxies


It is a firm prediction of the concordance cold-dark-matter cosmological model that galaxy clusters occur at the intersection of large-scale structure filaments1. The thread-like structure of this ‘cosmic web’ has been traced by galaxy redshift surveys for decades2,3. More recently, the warm–hot intergalactic medium (a sparse plasma with temperatures of 105 kelvin to 107 kelvin) residing in low-redshift filaments has been observed in emission4 and absorption5,6. However, a reliable direct detection of the underlying dark-matter skeleton, which should contain more than half of all matter7, has remained elusive, because earlier candidates for such detections8,9,10 were either falsified11,12 or suffered from low signal-to-noise ratios8,10 and unphysical misalignments of dark and luminous matter9,10. Here we report the detection of a dark-matter filament connecting the two main components of the Abell 222/223 supercluster system from its weak gravitational lensing signal, both in a non-parametric mass reconstruction and in parametric model fits. This filament is coincident with an overdensity of galaxies10,13 and diffuse, soft-X-ray emission4, and contributes a mass comparable to that of an additional galaxy cluster to the total mass of the supercluster. By combining this result with X-ray observations4, we can place an upper limit of 0.09 on the hot gas fraction (the mass of X-ray-emitting gas divided by the total mass) in the filament.

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Figure 1: Mass reconstruction of Abell 222/223.
Figure 2: Posterior probability distributions for cluster virial radii and filament strength.
Figure 3: Surface mass density of the best fit parametric model.


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J.P.D. was supported by NSF grant AST 0807304. A.S. acknowledges support from the National Aeronautics and Space Administration through Einstein Postdoctoral Fellowship Award Number PF9-00070.

Author information




J.P.D. led the project, reduced the optical data, performed the weak lensing analysis and wrote the manuscript. N.W. contributed to the writing of the manuscript. N.W., A.F. and A.S. performed the X-ray analysis and estimated the gas mass. L.M. and T.K. wrote the shear estimation code. The timing argument was made by D.C. All authors discussed all results and commented on the manuscript.

Corresponding author

Correspondence to Jörg P. Dietrich.

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Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

This file contains Supplementary Text and additional references, Supplementary Figure 1 which shows the E- and B-mode reconstruction of the A 222/223 supercluster field and Supplementary Figure 2 which shows the posterior probability distributions for the 8 free parameters when we leave the ellipticity of A 222 and A 223-S free. (PDF 479 kb)

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Dietrich, J., Werner, N., Clowe, D. et al. A filament of dark matter between two clusters of galaxies. Nature 487, 202–204 (2012).

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