Discovery of the largest rotation in the universe

By mapping the motion of galaxies in huge filaments that connect the cosmic web, astronomers at the Leibniz Institute for Astrophysics Potsdam (AIP), in collaboration with scientists in China and Estonia, have found these long tendrils of galaxies spin on the scale of hundreds of millions of light years. A rotation on such enormous scales has never been seen before. The results published in Nature Astronomy signify that angular momentum can be generated on unprecedented scales.

Cosmic filaments are huge bridges of galaxies and dark matter that connect clusters of galaxies to each other. They funnel galaxies towards and into large clusters that sit at their ends. “By mapping the motion of galaxies in these huge cosmic superhighways using the Sloan Digital Sky survey - a survey of hundreds of thousands of galaxies, we found a remarkable property of these filaments: they too spin.” says Peng Wang, first author of the now published study and astronomer at the AIP. “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,” adds 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. Such a spin has never been seen before on such enormous scales, and the implication is that there must be an as yet unknown physical mechanism responsible for torquing these objects.”

How the angular momentum responsible for the rotation is generated in a cosmological context is one of the key unsolved problems of cosmology. In the standard model of structure formation, small overdensities present in the early universe grow via gravitational instability as matter flows from under to overdense regions. Such a potential flow is irrotational or curl-free: there is no primordial rotation in the early universe and the rotation have been already induced when the structures formed. The cosmic web in general and filaments, in particular, are intimately connected with galaxy formation and evolution. They also have a strong effect on galaxy spin, often regulating the direction of how galaxies and their dark matter halos rotate. However, it is not known whether the current understanding of structure formation predicts that filaments themselves, being uncollapsed quasi-linear objects should spin.

Motivated by the theoretical suggestion that filaments may spin the now published study examined the observed galaxy distribution to find possible filament rotation. By using a sophisticated method, the observed galaxy distribution was segmented into filaments. Each filament was approximated by a rectangle on the sky and thus the galaxies within it were divided into two regions on either side of the filament spine to measure the mean redshift difference between the two regions. The mean redshift difference is a proxy for the line-of-sight velocity difference and thus the Doppler shift from galaxies on the receding and approaching side of the filament tube and hence for the filament rotation. The study implies that depending on the viewing angle and end point mass, filaments in the universe show a clear signal consistent with rotation.

The motion of galaxies in the universe is accompanied by larger scale matter flows. The mass and velocity flows are usually transported in a hierarchical way: at first, matter collapses along the principal axis of compression forming great cosmic walls. Matter then flows in the wall plane along the direction of the intermediate axis of compression to form filaments. The final stage is the full dimensional collapse of an aspherical anisotropic density perturbation wherein matter collapses and flows along the filament axes to form clusters. “Under such a model of mass flow, one may expect that filament spin is formed at the second stage from the compression along the intermediate axis of compression. In other words, we expect motion along the filament to still be linear while motion perpendicular to the filament spine to be nonlinear,” explains Wang. ”Smaller galaxies that inhabit the regions between large galaxies are accreted onto the filament connecting the two larger galaxy clusters and then along the line connecting them. Assuming filaments can be approximated by cylindrical tendrils such a flow would naturally give rise to a rotation along the filament axis, followed by a helical movement.”

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Read the original AIP press release

Read the scientific publication

More information: Elmo Tempel, Head of the Department of Physics of Galaxies and Cosmology, Professor of Astronomy, elmo.tempel@ut.ee, 737 4534