That the Universe is largely composed of a cosmic web consisting of narrow filaments upon which galaxies and intergalactic gas and dust are concentrated has been known for more than a decade. While a great deal of evidence for this has accumulated, visual evidence has been difficult to find. Astronomers have now photographed what appears to be a segment of a cosmic filament stimulated into fluorescence by irradiation from a nearby quasar.
The filaments of the cosmic web are difficult to see visually. They consist primarily of dark matter and intergalactic gas and dust, none of which have a visible signature detectable across billions of light years. As a result, our knowledge of filaments primarily comes from gravitational lensing studies, radio observations, and x-ray telescopes.
Now a team, led by researchers at the University of California, Santa Cruz (UCSC), has found an unusual configuration of celestial objects that appears to make visible a part of a filament that is ten billion light years distant. The section of the filament that is visible takes the form of a huge asymmetric nebula of diffuse intergalactic gas.
Normally this gas would not emit significant amounts of light, but in this case the intergalactic gas is being irradiated by extreme UV light from a nearby quasar; the active center of a galaxy. This irradiation ionized the gas (mostly consisting of atomic hydrogen), which then emits the characteristic light of atomic hydrogen (Lyman-alpha radiation) when the ionized atoms regain their electrons. When redshift (z~2.27) is taken into account, the Lyman-alpha radiation appears to our instruments as a violet glow.
The map above is also a product of the SDSS, which used a 2.5 meter telescope to image and determine redshift (and thereby distance) for galaxies in the cosmic vicinity of the Milky Way galaxy. It includes galaxies and quasars located in a thin slice of the sky above the Earth's equator out to a distance of two billion light years. One's first impression is of a slice through a foam of luminous bodies that lay on the boundary of huge voids.
Rather solid evidence also exists for the existence of filaments with a goodly share of dark matter, as illustrated in the above figure of just such a dark matter filament. This filament stretches about sixty million light years between the galaxy clusters Abell 222 and 223. X-ray emissions from the filament suggest that nearly 10 percent of the filament's mass consists of hot gas. This filament comprises at least dark matter and intergalactic gas.
The team published a report in the January 19, 2014, issue of Nature of their discovery of a rather unusual configuration of celestial objects in the early history of the Universe (about three billion years after the Big Bang) that provides additional evidence for the existence of the cosmic web.