Simulations of the Universe on the largest scales show an unexpected resemblance to nerve cells in the human brain, with galaxy clusters playing the role of the cell body and thinner filaments of matter linking them like axons. Galaxy surveys (such as the Sloan Digital Sky Survey, or SDSS) show that galaxies do cluster like our simulations predict. But the filaments that should connect them have been harder to find. Most of the mass in the Universe is dark matter—material that neither emits nor absorbs light—and filaments are predicted to be mostly dark matter: no galaxies, little hot gas.

Einstein's general theory of relativity, however, tells us mass affects the path of light, and a group of astronomers have identified a dark matter filament by measuring this effect. Jörg P. Dietrich et al. measured the slight distortion and magnification of background galaxies by the mass in a filament between two galaxy clusters, Abell 222 and 223. By comparing the distortions with X-ray measurements, the researchers determined that the filament contains very little hot gas, lacks galaxies entirely, and is invisible in optical wavelengths.

These observations lend strong support to the theory that the Universe is built on a web of dark matter that has drawn in visible structures like galaxies and clusters.

The large-scale structure (LSS) of the Universe that's predicted by the most widely accepted cosmological model involves long filaments of dark matter. Where these filaments intersect, large dark matter halos grow, and these provide the gravity for ordinary matter to collect. The largest halos become galaxy clusters, the biggest objects in the Universe held together by their own gravity. Surveys of galaxies show that the distribution of clusters corresponds to the predictions of LSS theory, providing strong indirect evidence for the existence of dark matter.

The filaments themselves are predicted to contain little ordinary matter, though, making them hard to spot. Some observations of relatively nearby clusters have revealed hot diffuse gas in filaments, but provided no direct measurement of the dark matter content. Since other observations have revealed that dark matter comprises about 80 percent of all mass in the Universe, observing it in filaments is an essential step in verifying the theory of LSS.

The system in the current study is a supercluster consisting of galaxy cluster Abell 222 and the double cluster Abell 223 (which has two halos that overlap). The researchers used archival data from the 8.2 meter Subaru telescope in Hawaii, which includes visible and infrared observations of the supercluster. These were scanned to look for subtle changes in the light from objects behind the clusters. These can be signs of weak gravitational lensing, which would reveal the distribution of dark matter near the clusters.

Weak lensing

When light passes near a mass, gravity curves the path the light follows; the strength of the effect depends on the amount and size of the mass. The general phenomenon is known as gravitational lensing. Strong lensing, which is the most famous variant, occurs when the mass is large enough to produce multiple highly distorted images of the same object (usually a galaxy). Weak lensing, on the other hand, reveals itself primarily through a slight magnification and distortion of background galaxies. Filaments are not predicted to be massive enough for strong lensing, but in regions between nighboring clusters, they should provide an observable weak-lensing signature.



Given the lensing data, the authors used two different methods to model the distribution of mass in Abell 222/223, one that assumed the presence of the filament and the second that made no prior assumptions.

Additionally, the researchers examined X-ray data from the XMM-Newton space telescope. Since hot gas in filaments emits X-ray light, this provides an esimate of the mass of the ordinary matter. The gas between the clusters added up to about 5.8 trillion solar masses, which is too little to form a filament.

Several outcomes were consistent with the observational data. In addition to the filament hypothesis, it's possible the halos from the individual clusters were strongly distorted and actually overlap (making the whole Abell 222/223 system one huge cluster). The weak lensing signature could also have been consistent with two distinct clusters without a mass bridge connecting them. The researchers calculated the likelihood of each option based on the combined visible, infrared, and X-ray data, and found the filament model to be the clear winner.

The best explanation for the weak lensing signature is a filament comprised mostly of dark matter, with a mass about 5.8×1015 (5.8 quadrillion) times that of the Sun. This is sufficient mass to form a galaxy cluster in its own right, and while a few galaxies lie in the filament region, they are not enough to account for the entire estimated mass. This result is another strong piece of direct evidence for the existence of dark matter, lending further support to our best model of large-scale structure in the Universe.

Nature, 2012. DOI: 10.1038/nature11224 (About DOIs).