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Astronomers Used Quasars as Flashlights to Illuminate the Dark Matter Between Galaxies

No, literally. Quasars make great cosmic flashlights in a pinch.
January 20, 2014, 7:45pm

A macro image of dark matter simulation, with an inset of a high-resolution section of the cosmic web. Background image via Anatoly Klypin and Joel Primack, inset via S. Cantalupo.

Dark matter is notoriously difficult to study. Because it neither emits or absorbs light, traditional observation methods are moot, despite the  relative abundance (it's believed to account for about a quarter of the universe's matter) of the mysterious stuff.


That's why it's handy to have massive cosmic searchlights working in your favor. A team of scientists led by the UCSC's Sebastiano Cantalupo discovered that a quasar named UM 287 could provide such a service. The result is the first direct observation of a cosmic web filament—the signature of dark matter, as well as the universe's favorite building block.

Cosmologists think that as the early universe cooled, a network of dark matter evolved into an intergalactic web, stringing together receding clusters of regular matter. Galaxies were more likely to develop at the crossroads of these dark matter threads, while isolated strands stretched between galaxies: sort of like an off-roader's map to the universe. While simulations of the web's geometry have been based on observational data, direct imaging of its filaments in three dimensions has not been achieved until now.

Cantalupo's team searched for quasars in their study because they are among the most luminous phenomena in the universe. These distant beacons are supermassive galactic nuclei, and they barf out ridiculous amounts of electromagnetic radiation. UM 287 is 10 billion light years away, but it's so bright that Cantalupo could observe it with the large Keck Telescope in Hawaii, using a custom filter.

The Keck Observatory in Mauna Kea, Hawaii. Photo via T. Wynne/JPL.

I know what you're thinking: if dark matter doesn't respond to light than what does it matter if a nearby quasar shoots a photonic blast its way? It's true that the quasar doesn't illuminate dark matter itself, but it does shed light on a trail of hydrogen gas that adorns the filament from its host galaxy all the way out into intergalactic space.


The gas trail is about two million light years across, and it betrays the location and geometry of the underlying filament. While cosmologists have long known that these gas threads existed, they have been too dim to observe without the reflected glow of a well-placed quasar.

The team published their results in the esteemed science journal Nature yesterday, and for the most part, their study confirms the existing cosmological model. The only major exception was the mass of the filament, which the team estimated to be the equivalent of a thousand billion suns.

“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,” Cantalupo said in a statement on UCSC's website.

“We think there may be more gas contained in small dense clumps within the cosmic web than is seen in our models. These observations are challenging our understanding of intergalactic gas and giving us a new laboratory to test and refine our models.”

In short: watch out, dark matter. We're on to your tricks.