Another clue about the whereabouts of the missing matter in the Universe has just emerged from amid the largest local cosmic structure.
X-ray observations have revealed a massive filament of hot gas, measuring some 23 million light-years in length, in the space between four sub-clusters of galaxies in the enormous, 8,000-galaxy strong Shapley Supercluster.
“For the first time, our results closely match what we see in our leading model of the cosmos – something that’s not happened before,” says astrophysicist Konstantinos Migkas of Leiden Observatory in the Netherlands. “It seems that the simulations were right all along.”
Most matter in the Universe comprises of a ‘dark’ variety we can’t easily identify. Only around 15 percent of matter exists in the form of far more familiar protons, neutrons and electrons – what we might call ‘normal matter’.
We know more or less how much normal matter there was in the early Universe, just after the Big Bang, thanks to the Cosmic Microwave Background, the fossil radiation that propagated through space-time when the Universe became transparent.
A huge problem arises when we compare that early Universe quantity of normal matter to the amount that’s around now. All the stars, black holes, galaxies, planets, dust, gas, and everything else we can see only accounts for around half of what we’d expect to find. Matter can’t be destroyed, so where the heck did it go?
The best explanation we have is that it ended up in intergalactic space – vast amounts of material so tenuously distributed along the cosmic web that we can’t directly see it. Increasing evidence of this faint reservoir has been emerging for the last few years; and the discovery of this filament is some of the best evidence yet.
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The cosmic web is a vast network of filaments of dark matter that span intergalactic space, connecting galaxies and acting as a “superhighway” along which galaxies and matter are funneled. We can’t see these filaments easily, but Migkas and his team identified one by comparing observations from two X-ray telescopes.
The now-retired Suzaku X-ray telescope was excellent for observing faint X-radiation that is spread over a large surface area, while XMM-Newton can pick out point sources of very bright X-rays. The researchers used existing images taken by the former to detect the glow of gas within the filament, while observations from the latter allowed them to remove contaminating X-rays from sources such as black holes.
The resulting structure is a beast, stretching between two pairs of galaxy clusters named A3528S/N and A3530/32. Along its 23 million-light-year length, it contains enough material to fill 10 Milky Way galaxies, blazing at a temperature of more than 10 million degrees Celsius.
It is, the researchers say, exactly what such a filament is expected to be, based on simulations of the Universe.
“This research is a great example of collaboration between telescopes, and creates a new benchmark for how to spot the light coming from the faint filaments of the cosmic web,” says astronomer and XMM-Newton project scientist Norbert Schartel of the European Space Agency.
“More fundamentally, it reinforces our standard model of the cosmos and validates decades of simulations: it seems that the ‘missing’ matter may truly be lurking in hard-to-see threads woven across the Universe.”
The research has been published in Astronomy & Astrophysics.
