Components of the galaxy cluster Abell 2744, also known as the
Pandora Cluster: galaxies (white), hot gas (red) and dark matter (blue). Copyright: ESA/XMM-Newton (X-rays); ESO/WFI (optical); NASA/ESA & CFHT (dark matter). Hi-Res image/Low-Res image
Galaxy clusters in the cosmic web
Copyright: Image courtesy of K. Dolag, Universitäts-Sternwarte München, Ludwig-Maximilians-Universität München, German.
Galaxy Cluster Abell 2744
Components of the galaxy cluster Abell 2744, also known as the
Pandora Cluster: galaxies (white), hot gas (red) and dark matter (blue).
Galaxy clusters are the most massive cosmic structures held together by gravity, consisting of galaxies, hot gas and dark matter. They sit in the densest hubs of the filamentary 'cosmic web' that pervades the Universe.
Using ESA's XMM-Newton X-ray observatory, astronomers have detected three massive filaments flowing towards the core of Abell 2744 and connecting it with the cosmic web. The filaments also consist of galaxies, hot gas and dark matter. One of them can be seen as the elongated structure on the left side of the image, another one is visible towards the upper right, and the third one below the cluster, slightly towards the right. (These are indicated with ellipses in this image.)
There are also two structures – one on the lower left of the cluster, the other in the upper central part of the image – which are not physically linked to the cluster but are the projection of more distant structures viewed along the same line of sight.
The astronomers detected the hot gas in the cluster and filaments with X-ray observations and the galaxies with optical observations. To reconstruct the distribution of dark matter, they used the gravitational lensing effect that the mass of the cluster and filaments exerts on more distant galaxies.
Abell 2744 has a mass of almost two million billion times the mass of our Sun. Light from the cluster galaxies and gas travelled for over 3 billion years to reach Earth.
The image measures about half a degree across. The image is sprinkled with foreground stars belonging to our Galaxy, the Milky Way, which are visible as the roundish objects with diffraction spikes. Copyright: ESA/XMM-Newton (X-rays); ESO/WFI (optical); NASA/ESA & CFHT (dark matter). Hi-res image
ESA’s XMM-Newton X-ray observatory has revealed three massive filaments of hot gas flowing towards a cluster of galaxies, uncovering a portion of the cosmic skeleton that pervades the entire Universe.
Galaxies tend to congregate, forming groups and even larger agglomerates
called clusters. These clusters are the most massive cosmic structures
held together by gravity. As well as galaxies, they contain large
amounts of hot gas and even larger amounts of invisible dark matter.
On a grander scale, galaxies and galaxy clusters appear to be linked in a
gigantic filamentary network, with the most massive clusters sitting in
the densest hubs of this ‘cosmic web’.
Computer simulations indicate that the cosmic web, which consists
primarily of dark matter and some ordinary matter, behaves as the
scaffolding of the cosmos, providing the framework for stars, galaxies
and clusters to form and evolve.
In the past few decades, astronomers have detected the threadlike
structure of the cosmic web in the large-scale distribution of galaxies,
and found hints that diffuse gas is arranged in a similar way.
A new study using ESA’s XMM-Newton X-ray observatory has now uncovered a
handful of filaments made of galaxies, gas and dark matter that are
flowing towards one of the most massive galaxy clusters in the Universe,
obtaining the first, unambiguous detection of gas in the cosmic web.
“This was an unexpected and most welcome discovery,” says Dominique
Eckert of the University of Geneva, Switzerland, lead author of the
paper reporting the new results in the journal Nature this week.
The object of the study is Abell 2744, which has been nicknamed the
Pandora Cluster owing to its complex and jumbled structure. It is
composed of at least four smaller components that are merging.
“We knew that this is an incredibly massive cluster hosting active
processes at its core, and seeing its direct connection to the cosmic
web confirms our picture of how structures form in the Universe,” adds
Dr Eckert.
From 30 hours of observations by XMM-Newton in December 2014, the
astronomers detected five large structures of hot gas that seem to be
linked to the core of Abell 2744.
Comparing the X-ray data with optical observations, they identified the
galaxies that belong to the various filaments, recognising that three of
them are physically connected to the cluster, while the other two are
the projection of more distant structures viewed along the same line of
sight.
Just like the cluster, the filaments also contain plenty of dark matter.
The astronomers have reconstructed its distribution by studying the
‘gravitational lensing’ effect that the mass of the cluster and
filaments exerts on distant galaxies, modifying the path of their light
and so increasing their brightness and twisting their shapes as seen by
us.
“We initially looked at the inner core of Abell 2744 with the Hubble
Space Telescope, with the aim of using the cluster as a strong
magnifying lens to detect background galaxies that would be otherwise
too faint to observe,” explains co-author Mathilde Jauzac from the
University of Durham, UK.
“After the discovery of X-ray gas in these filaments, we decided to look
at the gravitational lensing effect also in the outskirts of the
cluster, where background galaxies are only weakly distorted and
magnified, but still enable us to study the dark matter distribution
near the cluster as well as in the nearby filaments.”
The combination of observations at different wavelengths revealed how
the various components of Abell 2744 and its surroundings coexist.
From the X-ray data, the astronomers measured the density and
temperature of the gas and compared it with the predictions from theory.
With gas temperatures of 10–20 million degrees celsius, the filaments
are much colder than the centre of the cluster, where the gas reaches
100 million degrees, but hotter than the average temperature in the
cosmic web, estimated to be several million degrees.
The gas and galaxies in the filaments amount to about a tenth of the
total mass – the rest being dark matter – which also agrees with
expectations.
While the measurements match well with the astronomers' theoretical
scenario, caution is always in order when drawing conclusions about the
Universe as a whole.
“What we observed is a very special configuration of dense filaments
close to an exceptionally massive cluster. We need a much larger sample
of less-dense filaments to investigate the nature of the cosmic web in
greater detail,” says Dr Eckert.
For more in-depth investigations, astronomers will have to wait for
ESA’s Athena X-ray telescope, planned for launch in 2028. Athena’s
extraordinary sensitivity will make it possible to survey hot gas in the
cosmic web across the sky, detecting faint and diffuse filaments and
even identifying some of the atomic elements in the gas.
“With the discovery of filaments around Abell 2744, we are witnessing
the build-up of the cosmic web in one of the busiest places in the known
Universe, a crucial step in the study of the formation of galaxies and
galaxy clusters,” says Norbert Schartel, ESA XMM-Newton Project
Scientist.
For further information, please contact:
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int
Dominique Eckert
University of Geneva, Switzerland and INAF-IASF, Milano, Italy
Email: Dominique.Eckert@unige.ch
Mathilde Jauzac
University of Durham, UKand University of KwaZulu-Natal, Durban, South Africa
Email: mathilde.jauzac@durham.ac.uk
Norbert Schartel
ESA XMM-Newton Project Scientist
ESA Directorate of Science and Robotic Exploration
Tel: +34 91 8131 184
Email: Norbert.Schartel@esa.int
Further information
“Warm-hot baryons comprise 5–10 per cent of filaments in the cosmic web,” by D. Eckert et al. is published in the 3 December 2015 issue of the journal Nature.
This work is based on X-ray data collected with ESA's XMM-Newton X-ray
observatory and on optical observations from the NASA/ESA Hubble Space
Telescope, the Canada–France–Hawaii Telescope and the European Southern
Observatory’s 2.2 m-diameter telescope.
Source: ESA
