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Quasar light may have exposed dark matter

On Jan. 19, researchers at the University of California, Santa Cruz announced that they may have detected dark matter.

Processed and combined images of the field surrounding the quasar UM 287.
Sebastiano Cantalupo, et al.

Using the Keck I telescope in Mauna Kea, Hawaii, the researchers found a nebula that is 460,000 parsecs across being illuminated by a quasar. Quasars are the brightest known objects in the universe, and are the result of stars falling into supermassive black holes at the centers of galaxies.

“This is a very exceptional object: it's huge, at least twice as large as any nebula detected before, and it extends well beyond the galactic environment of the quasar,” said Sebastiano Cantalupo, first author of the paper.

Supercomputer simulations of structure formation in the universe predict that matter is distributed in a network of filaments known as the cosmic web. As the universe is composed of 4.9 percent ordinary matter and 26.8 percent dark matter (with the remainder being dark energy), almost 85 percent of the cosmic web filaments should be dark matter. Galaxies are believed to form at the nodes of this cosmic web. Gravity causes ordinary matter to follow the distribution of dark matter, so filaments of diffuse, ionised gas are expected to trace a pattern similar to that seen in dark matter simulations.

This is the first time that any direct observations concerning this theory could be made because cosmic web gas has such a low density that it emits too little light to be detected without another source of illumination. The quasar UM 287 has provided such an illumination source. By bombarding the cosmic web gas with radiation, the quasar causes the hydrogen gas to emit flourescent light. This light, known as Lyman alpha radiation, is typically in the ultraviolet part of the spectrum, but redshifting of z≈2.3 due to the expansion of the universe causes the light to appear in the visible part of the spectrum by the time it travels 10 billion light years to Earth.

“The light from the quasar is like a flashlight beam, and in this case, we were lucky that the flashlight is pointing right at the cosmic web, making some of its gas glow,” said Cantalupo.

“This quasar is illuminating diffuse gas on scales well beyond any we've seen before, giving us the first picture of extended gas between galaxies,” said J. Xavier Prochaska, coauthor of the research. “It provides a terrific insight into the overall structure of our universe.”

More limited measurements of hydrogen gas in intergalactic space have been made in the past, but those methods could only observe the cosmic web along a single line between a quasar and Earth, and could not reveal the structure of the cosmic web.

“This is the first time anyone has been able to capture an image of the cosmic web, demonstrating its filamentary structure,” said Fabrizio Arrigoni Battaia from the Max Planck Institute for Astronomy in Heidelberg, Germany.

The new observations indicate that the amount of cold gas in the cosmic web filament is more than an order of magnitude larger than predicted, suggesting that there is a missing factor in current simulations.

“If you want to know how galaxies form, you first need to understand their fuel supply, which comes from the cosmic web,” said Joseph Hennawi from the Max Planck Institute for Astronomy. “These new observations are challenging our understanding, as they suggest a large amount of gas is contained in small dense clumps, which is not currently present in our models. Resolving this tension will clearly teach us something very important.”

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