Astronomers directly image a filament of the “cosmic web” for the first time

Bolshoi simulation, by Anatoly Klypin and Joel Primack. Inset: S. Cantalupo

Astronomers have long predicted that a “web” exists that is primarily composed of dark matter and could give clues about the overall structure of the Universe. Researchers were able to use light from an ancient quasar to image the hydrogen gas from a filament of the cosmic web for the first time. The study was led by Sebastiano Cantalupo of University of California at Santa Cruz and was published in Nature.

The web is predicted to be a net-like arrangement, up to 84% of which is believed to be dark matter which cannot be seen. Galaxies are not found sporadically in the Universe; they are arranged into clusters and super clusters which connect to form filaments in the web due to gravity. Overall, the cosmic web makes our Universe appear like foam or a sponge and essentially acts as the skeleton of the Universe.

Though computer simulations had consistently given the team an idea of what the web looked like, filaments have never been imaged directly. The Keck Telescope in Hawaii was used to image an ancient quasar named UM287 as it was seen over 10 billion years ago. Quasars are a variety of galactic nucleus that emits a great deal of radiation and are incredibly bright. This radiation can illuminate hydrogen gas into ultraviolet wavelengths. Because the telescope had to look so far away, the ultraviolet light coming off of the gas stretched due to the expanding Universe and made the gas visible.

The team was able to determine the original wavelength of the stretched UV light and use that formula to create a filter for the spectrometer which was ultimately used to detect gas filaments from the web. The 10 billion light year distance gave the researchers a chance to view cosmic radiation on a very large scale, allowing them to detect a gas cloud much larger than galaxy, with a gas filament of the cosmic web as the only logical conclusion. As if finding the filament was not exciting enough, it turned out to be much larger than any of them would have anticipated at two million light years across. This is the largest gas cloud ever detected.

Astronomers have imaged ancient quasars before in an attempt to spot “dark galaxies” which are dense clumps of gas completely devoid of stars. Despite previous uses of this technique, this is the first time that light was able to be seen. In this case, the team got lucky and the light from the quasar just happened to be pointed in the right direction for the gas to be detected. 

The results obtained from this study will be used to update computer simulations of the web’s composition. The dark galaxies which represent little clusters in the web could contain ten times more gas than is currently being represented in the models. These results will allow astronomers to refine the model and use the filament to understand more about how the Universe is structured.

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