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 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 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. 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.
Reference:
“Possible observational evidence for cosmic filament spin” by Peng Wang,
Noam I. Libeskind, Elmo Tempel, Xi Kang and Quan Guo, 14 June 2021, Nature
Astronomy.
Tags:
Space & Astrophysics