by Leibniz-Institut für Astrophysik Potsdam (AIP)
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 that 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 toward
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 spin," says Peng Wang,
first author of the now published study and astronomer at the AIP.
Noam
Libeskind, initiator of the project at the AIP, says, "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. On these scales,
the galaxies within them are themselves just specks of dust. They move
on helixes, or corkscrew-like orbits, circling around the middle of the filament while
traveling 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. As such, any rotation must
be generated as structures form. 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 suggestion from the theorist Dr. Mark Neyrinck that filaments
may spin, we examined the observed galaxy distribution, looking for
filament rotation," says Noam Libeskind. "It's fantastic to see this
confirmation that intergalactic filaments rotate in the real universe,
as well as in computer simulation."
By
using a sophisticated mapping method, the observed galaxy distribution
was segmented into filaments. Each filament was approximated by a
cylinder. Galaxies within it were divided into two regions on either
side of the filament spine (in projection) and the mean redshift
difference between the two regions was carefully measured. The mean
redshift difference is a proxy for the velocity difference (the Doppler
shift) between galaxies on the receding and approaching side of the
filament tube. It can thus measure the filament's 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.
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