Dark Matter Filaments Bind Galaxies Together

A slim bridge of dark matter – just a hint of a larger cosmic skeleton – has been found binding a pair of distant galaxies together.

According to a press release from the journal Nature, scientists have traced a thread-like structure resembling a cosmic web for decades but this is the first time observations confirming that structure has been seen. Current theory suggests that stars and galaxies trace a cosmic web across the Universe which was originally laid out by dark matter – a mysterious, invisible substance thought to account for more than 80 percent of the matter in the Universe. Dark matter can only be sensed through its gravitational tug and only glimpsed when it warps the light of distant galaxies.

Astronomers led by Jörg Dietrich, a physics research fellow in the University of Michigan College of Literature, Science and the Arts, took advantage of this effect by studying the gravitational lensing of galactic clusters Abell 222 and 223. By studying the light of tens of thousands of galaxies beyond the supercluster; located about 2.2 billion light-years from Earth, the scientists were able to plot the distortion caused by the Abell cluster. The scientists admit it is extremely difficult to observe gravitational lensing by dark matter in the filaments because they contain little mass. Their workaround was to study a particularly massive filament that stretched across 18 megaparsecs (nearly 59 million light-years) of space. The alignment of the string enhanced the lensing effect.

The team’s results were published in the July 4, 2012 issue of Nature.

“It looks like there’s a bridge that shows that there is additional mass beyond what the clusters contain,” Dietrich said in a press release. “The clusters alone cannot explain this additional mass.”

By examining X-rays emanating from plasma in the filament, observed from the XMM-Newton satellite, the team calculated that no more than nine percent of the filament’s mass could be made up of the hot gas. Computer simulations further suggested that just 10 percent of the mass was due to visible stars and galaxies. Only dark matter, says Dietrich, could make up the remaining mass.

“What’s exciting,” says Mark Bautz, an astrophysicist at the Massachusetts Institute of Technology, “is that in this unusual system we can map both dark matter and visible matter together and try to figure out how they connect and evolve along the filament.”

Refining the technique could help physicists understand the structure of the Universe and pin down the identity of dark matter (whether it’s a cold slow-moving mass or a warm, fast-moving one. Different types would clump differently along the filament, say scientists.

Image caption: Dark-matter filaments, such as the one bridging the galaxy clusters Abell 222 and Abell 223, are predicted to contain more than half of all matter in the Universe. (credit: Jörg Dietrich, University of Michigan/University Observatory Munich)

19 Replies to “Dark Matter Filaments Bind Galaxies Together”

  1. This structure has been predicted numerically. It is also consistent with the anisotropy pattern seen in the cosmic microwave background.

    1. Thanks. The question that came to my mind is whether we observe the filament directly. Even though the filament might have very weak lensing, if the angle by which light passes the filament is some angle 0< ? < ?/2, say about 30 degrees (?/6) the lensing may be increased. I want to see if there is such a geometry at play here.

      1. As I remember it, the press release noted a direct observation – the geometry was lucky or it wouldn’t have been (as) possible to see.

        But you better check that as always.

  2. These are very exciting findings which are recent in origin. I have been posting periodically when these articles come out. These dark matter filamements which were predicted are now being observed, much like exoplanets were predicted and are now being directly observed. The existence of these filaments solves in a simplistic way what were numerous puzzling conundrums about the structure of the universe and the evolution of that structure. It also will help us point our instruments exactly where they need to be in order to study and determine the nature of dark matter. I also have a pet hypothesis that most of the dark matter contained within galaxies lies in the center of them along such filaments rather than diffusely scattered throughout. When the first galaxies formed, dust would have preferentially accreted along these filaments and the SMBHs would have developed along them as well. The resulting galaxies would appear as pearls of a necklace along a dark matter string. If in fact the majority of dark matter exists wihin such filaments in the universe, then that implies that dark matter interacts gravitationally more strongly than predicted by current models.

  3. And I thought, it was magnetic fields. That’s sad.

    (Sorry, couldn’t resist. Never mind.)

    1. If you read the article backwards, you might invoke the “Electric Universe” devil!

    2. Do not invoke such things, even in jest, or the plasma universe guys may come back to haunt this website.

      1. Since they haven’t (yet), I’m fairly optimistic that they won’t. But of course, you’re right. Never wake a sleeping dragon. 😉

  4. Fascinating stuff, including the comments. Novice question: anyone care to speculate as to the structural purpose of these filaments? Assuming they exist, why would strands between galaxies be necessary? Does the universe need all its globs connected like a neural network?

    1. I believe these filaments are primodial structures that formed shortly after the big bang and before hydrogen, helium and lithium. They have served as gravitational anchors around which baryonic matter later coalesced to form the galaxys and superclusters we see today. Think of it as a cosmic web. Empty space in between these filments has been expanding exponentially under the influence of dark energy creating ever enlarging voids between these filaments. In other words it is these dark matter filaments that keep matter glued together in these web-like structures while the rest of space has been expanding wildly all around them.

    2. One other thought related to superclusters. These galaxies that form along these strings are free to move along them. They will be attracted in the direction of the largest mass that intersects the string. Along the way these galaxies sweep up any remaining dust attracted to the string in the interglactic medium. In this regard the mass of a galaxy should correlate loosely with how close the nearest galaxy is along the string. We see evidence from studies near superclusters (which are places these strings intersect) that galaxies are “falling” into the superclusters along these strings. The result is a giant smash-up of matter with fascinating consequences. Here is a reference to an abstract describing this: http://arxiv.org/abs/0804.4177

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