New Hubble data suggests there is an ingredient missing from current
dark matter theories

Date:
September 10, 2020
Source:
ESA/Hubble Information Centre
Summary:
Recent observations 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.



FULL STORY ========================================================================== [This Hubble Space | 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] 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 [This Hubble Space | 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] 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 Close 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 therby pin down their masses.

"The data from Hubble and the VLT provided excellent synergy," shared
team member Piero Rosati of the Universita` 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 analysed in our paper, permitting detailed comparisons like
never before," added Stefano Borgani of the Universita` 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.


========================================================================== Story Source: Materials provided by ESA/Hubble_Information_Centre. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
*
YouTube_video:_Hubble_Sheds_Light_on_Small-Scale_Concentrations_of_Dark
Matter ========================================================================== Journal Reference:
1. Massimo Meneghetti, Guido Davoli, Pietro Bergamini, Piero Rosati,
Priyamvada Natarajan, Carlo Giocoli, Gabriel B. Caminha, R. Benton
Metcalf, Elena Rasia, Stefano Borgani, Francesco Calura, Claudio
Grillo, Amata Mercurio, Eros Vanzella. An excess of small-scale
gravitational lenses observed in galaxy clusters. Science, 2020 DOI:
10.1126/ science.aax5164 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200910150348.htm

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