New Hubble data suggests there is an ingredient missing from current dark matter theories
From:
ESA/Hubble Information Center
September
10, 2020 -- This Hubble Space Telescope image shows the massive galaxy cluster
MACSJ 1206. Embedded within the cluster are the distorted images of distant
background galaxies, seen as arcs and smeared features. These distortions are
caused by the dark matter in the cluster, whose gravity bends and magnifies the
light from faraway galaxies, an effect called gravitational lensing. This
phenomenon allows astronomers to study remote galaxies that would otherwise be
too faint to see. Astronomers measured the amount of gravitational lensing
caused by this cluster to produce a detailed map of the distribution of dark
matter in it. Dark matter is the invisible glue that keeps stars bound together
inside a galaxy and makes up the bulk of the matter in the Universe. The Hubble
image is a combination of visible- and infrared-light observations taken in
2011 by the Advanced Camera for Surveys and Wide Field Camera 3.
Credit:
NASA, ESA, G. Caminha (University of Groningen), M. Meneghetti (Observatory of
Astrophysics and Space Science of Bologna), P. Natarajan (Yale University), and
the CLASH team
Observations
by the NASA/ESA Hubble Space Telescope and the European Southern Observatory's
Very Large Telescope (VLT) in Chile have found that something may be missing
from the theories of how dark matter behaves. This missing ingredient may
explain why researchers have uncovered an unexpected discrepancy between
observations of the dark matter concentrations in a sample of massive galaxy
clusters and theoretical computer simulations of how dark matter should be
distributed in clusters. The new findings indicate that some small-scale
concentrations of dark matter produce lensing effects that are 10 times
stronger than expected.
Dark matter is the invisible glue that keeps
stars, dust, and gas together in a galaxy. This mysterious substance makes up
the bulk of a galaxy's mass and forms the foundation of our Universe's
large-scale structure. Because dark matter does not emit, absorb, or reflect
light, its presence is only known through its gravitational pull on visible
matter in space. Astronomers and physicists are still trying to pin down what
it is.
Galaxy clusters, the most massive and recently
assembled structures in the Universe, are also the largest repositories of dark
matter. Clusters are composed of individual member galaxies that are held
together largely by the gravity of dark matter.
"Galaxy clusters are ideal laboratories
in which to study whether the numerical simulations of the Universe that are
currently available reproduce well what we can infer from gravitational
lensing," said Massimo Meneghetti of the INAF-Observatory of Astrophysics
and Space Science of Bologna in Italy, the study's lead author.
"We have done a lot of testing of the
data in this study, and we are sure that this mismatch indicates that some
physical ingredient is missing either from the simulations or from our
understanding of the nature of dark matter," added Meneghetti.
"There's a feature of the real Universe
that we are simply not capturing in our current theoretical models," added
Priyamvada Natarajan of Yale University in Connecticut, USA, one of the senior
theorists on the team. "This could signal a gap in our current
understanding of the nature of dark matter and its properties, as these
exquisite data have permitted us to probe the detailed distribution of dark
matter on the smallest scales."
The distribution of dark matter in clusters is
mapped by measuring the bending of light -- the gravitational lensing effect --
that they produce. The gravity of dark matter concentrated in clusters
magnifies and warps light from distant background objects. This effect produces
distortions in the shapes of background galaxies which appear in images of the
clusters. Gravitational lensing can often also produce multiple images of the
same distant galaxy.
The higher the concentration of dark matter in
a cluster, the more dramatic its light-bending effect. The presence of smaller-scale
clumps of dark matter associated with individual cluster galaxies enhances the
level of distortions. In some sense, the galaxy cluster acts as a large-scale
lens that has many smaller lenses embedded within it.
Hubble's crisp images were taken by the
telescope's Wide Field Camera 3 and Advanced Camera for Surveys. Coupled with
spectra from the European Southern Observatory's Very Large Telescope (VLT),
the team produced an accurate, high-fidelity, dark-matter map. By measuring the
lensing distortions astronomers could trace out the amount and distribution of
dark matter. The three key galaxy clusters, MACS J1206.2-0847, MACS
J0416.1-2403, and Abell S1063, were part of two Hubble surveys: The Frontier
Fields and the Cluster Lensing And Supernova survey with Hubble (CLASH)
programs.
To the team's surprise, in addition to the
dramatic arcs and elongated features of distant galaxies produced by each
cluster's gravitational lensing, the Hubble images also revealed an unexpected
number of smaller-scale arcs and distorted images nested near each cluster's
core, where the most massive galaxies reside. The researchers believe the
nested lenses are produced by the gravity of dense concentrations of matter
inside the individual cluster galaxies. Follow-up spectroscopic observations
measured the velocity of the stars orbiting inside several of the cluster
galaxies to thereby pin down their masses.
"The data from Hubble and the VLT
provided excellent synergy," shared team member Piero Rosati of the
Università degli Studi di Ferrara in Italy, who led the spectroscopic campaign.
"We were able to associate the galaxies with each cluster and estimate
their distances."
"The speed of the stars gave us an
estimate of each individual galaxy's mass, including the amount of dark
matter," added team member Pietro Bergamini of the INAF-Observatory of
Astrophysics and Space Science in Bologna, Italy.
By combining Hubble imaging and VLT
spectroscopy, the astronomers were able to identify dozens of multiply imaged,
lensed, background galaxies. This allowed them to assemble a well-calibrated,
high-resolution map of the mass distribution of dark matter in each cluster.
The team compared the dark-matter maps with
samples of simulated galaxy clusters with similar masses, located at roughly
the same distances. The clusters in the computer model did not show any of the
same level of dark-matter concentration on the smallest scales -- the scales
associated with individual cluster galaxies.
"The results of these analyses further
demonstrate how observations and numerical simulations go hand in hand,"
said team member Elena Rasia of the INAF-Astronomical Observatory of Trieste,
Italy.
"With high-resolution simulations, we can
match the quality of observations analyzed in our paper, permitting detailed
comparisons like never before," added Stefano Borgani of the Università
degli Studi di Trieste, Italy.
Astronomers, including those of this team,
look forward to continuing to probe dark matter and its mysteries in order to
finally pin down its nature.
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https://www.sciencedaily.com/releases/2020/09/200910150348.htm
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