16 June 2021 News

Astronomers discover the largest rotating objects in the Universe

Artist’s impression of cosmic filaments: huge bridges of galaxies and dark matter connect clusters of galaxies to each other. Their light appears blue-shifted when they move towards us, and red-shifted when they move away. Image: AIP/Khalatyan/Fohlmeister
Artist’s impression of cosmic filaments: huge bridges of galaxies and dark matter connect clusters of galaxies to each other. Their light appears blue-shifted when they move towards us, and red-shifted when they move away. Image: AIP/Khalatyan/Fohlmeister

We know that the Moon spins around our planet, Earth spins around the Sun, and the Solar System spins around the galaxy. But it doesn’t stop there. Stretching 100, 000 light years across, the Milky Way also spins at a whopping 270 kilometers per second (168 miles per second) and takes about 200 million years to complete one rotation.

This spinning motion of our galaxy could be just a drop in the cosmic ocean though as a new study has found that long tendrils of galaxies caught up in the cosmic web are also spinning, but on the scale of hundreds of millions of light years – a rotation on such enormous scales that has never been seen before until now.

Stretched out across the Universe and separated by giant voids, the far-reaching tendrils of the cosmic web are the perfect place to grow galaxies.

These structures are so huge that the largest known filament to date, the Hercules–Corona Borealis Great Wall, is thought to be a staggering 10 billion light years long populated by several billion galaxies.

Previous research has suggested that these huge structures have a strong effect on galaxy spin, often regulating the direction of how galaxies and their dark matter halos rotate.

But whether these vast tendrils of matter hundreds of millions of light-years across are themselves spinning is difficult to measure due to their enormous size.

Inspired by work completed by theorist Dr Mark Neyrinck which suggested that filaments may spin, a team, headed by Peng Wang, an astronomer at the Leibniz Institute for Astrophysics Potsdam (AIP), found a way around the problem.

Instead of looking at the structure as a whole, the team split up the observed galaxy distribution into filament segments. Each filament was then approximated by a cylinder.

“Just as it is easiest to measure rotation in a spinning disk galaxy viewed edge on, so too is filament rotation clearly detected under similar geometric alignment,” say the authors in their recent paper published in Nature.

Then, using a sophisticated mapping method, galaxies within it were divided into two regions on either side of the filament spin, and the light from the galaxies were compared to each other.

Peng and colleagues found that galaxies on one side of the filament were redshifted in comparison to the other side. Redshifted light refers to lightwaves that have become stretched as an object moves away from something. When lightwaves become longer they move into the red end of the electromagnetic spectrum - they become redshifted.

Lightwaves from an approaching object on the other hand will appear to shorten slightly and so move to the blue end of the spectrum, or become blueshifted - a phenomena known as Doppler shifting. Edwin Hubble used this method back in the 20s to determine that galaxies were moving away from us – an idea that helped lead to the theory that the Universe was expanding.

These differences in light were then used to infer that most of the galaxies on one side of a filament were moving away from us and most on the other were coming towards us, a result that indicated the whole filament was rotating.

"Despite being thin cylinders - similar in dimension to pencils - hundreds of millions of light years long, but just a few million light years in diameter, these fantastic tendrils of matter rotate," says Noam Libeskind, initiator of the project at the AIP.

"On these scales the galaxies within them are themselves just specs of dust. They move on helixes or corkscrew like orbits, circling around the middle of the filament while travelling along it,” he added.

Some of these enormous filaments were found to be spinning at nearly 100 kilometres per second; results which signify that angular momentum can be generated on unprecedented scales.

How this is happening the team do not yet know and the work does not predict that every single filament in the universe is rotating say the authors in their research paper, but the implication is that there must be an as yet unknown physical mechanism responsible for torquing these objects, Libeskind says.

Figuring out what that mechanism is could help researchers understand how angular momentum is generated in the cosmos; a long-standing conundrum that still remains a mystery.

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