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National Aeronautics and Space Administration
“Seeing” Dark Matter
Douglas Clowe
Taken from:
Hubble 2007: Science Year in Review
Produced by NASA Goddard Space Flight Center and Space
Telescope Science Institute.
The full contents of this book include more Hubble science
articles, an overview of the telescope, and more. The complete volume and its component sections are available for
download online at:
www.hubblesite.org/hubble_discoveries/science_year_in_review
Hubble 2007: Science Year in Review
“Seeing” Dark Matter
Douglas Clowe
For more than 70 years, astronomers have known that most of the matter in the universe is invisible. Fritz Zwicky discovered
this astonishing fact in 1933, while studying the random motions of galaxies in the Coma cluster of galaxies. He could prove
the cluster is indeed a real structure—that the galaxies are truly bound together—but he could not account for the force
needed to do the job. The mutual gravitational attraction of all the stars in the galaxies was not enough. Indeed, if that were
all, the Coma cluster would have dispersed and disappeared long ago. Zwicky proposed two possible solutions: either 99%
of the mass of the cluster is some unknown, unseen material, which he called “dark matter,” or gravity does not work the
same for galaxy clusters as it does for smaller systems, like binary stars, the Solar System, or apples falling from trees. Either
solution could deliver the additional force needed to hold a cluster together. Since Zwicky’s work, dark matter has remained a
mystery. Is it real? What is it? Or is the other solution correct? Could it be that we do not fully understand how gravity—one
of the basic principles of the universe—works over cosmic distances?
Recent observations of colliding clusters have confirmed dark matter’s enigmatic properties and, for the first time, have
shown that dark matter must exist—regardless of whether gravity behaves differently in galaxy clusters. To appreciate these
new results, we need to be aware of two related discoveries about the cosmic inventory of ordinary matter.
In the 1970s, x-ray astronomers discovered a new component of ordinary matter: hot gas permeating the space between
the galaxies. This gas is mostly hydrogen and helium ions. Because its temperature is so high—typically 100 million
degrees—intergalactic gas emits light in the x-ray region of the spectrum. The total mass of this new component is typically
10 times the total mass of all the stars in all the galaxies in a cluster. Therefore, ordinary matter can actually account for about
10% of the gravity needed to hold clusters together, considerably more than just the 1% Zwicky thought. After this discovery,
dark matter needs to supply the remaining 90%.
Demonstration of the reality of dark matter in the aftermath of a collision between two clusters of galaxies 100 million years ago. The hot
intergalactic gas (red) now lags behind, due to mutual friction, while the dark matter (blue) and galaxies move apart, hardly affected. The
system of clusters, named 1E 0657–56, is located in the southern constellation Carina.
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Also by the 1970s, astrophysicists learned to estimate the total amount of ordinary matter in the universe. For this, they used
the theory of nucleosynthesis—the original production of atomic nuclei—in the Big Bang. According to this theory, the
Big Bang initially created all ordinary matter in the form of hydrogen. For a brief period—only a few minutes long—while
the universe was still small, the density of matter was high enough for nuclear fusion to convert some hydrogen nuclei into
helium nuclei. The theory dictates that the more matter created in the Big Bang, the greater would be the resulting ratio of
helium to hydrogen. Astrophysicists can estimate this ratio by observing ancient astronomical sources—sources so distant,
and thus so young at the time we see them, that the ratio is effectively still unmodified. Then, they used the theory to estimate
the total amount of ordinary matter produced. They found that—within an uncertainty of a factor of two or so—the totality
of observed stars and hot intergalactic gas of galaxy clusters in the universe accounts for the total original production of
ordinary matter. This discovery ruled out the possibility that the apparent need for dark matter could be satisfied by some
form of ordinary matter—that is, ruled out the possibility that dark matter is simply ordinary matter that we cannot see.
The chief impediment to learning more about dark matter has been the collocation of all the sources of gravity: stars, intergalactic gas, and dark matter. Although it is impossible, we would really like to bring a galaxy cluster into the laboratory, extract
and weigh the various components of matter, and analyze the dark matter. For now, cluster collisions—the largest collisions
in nature—offer the next best opportunity.
Cluster smash-ups
Gravity sometimes draws two galaxy clusters together, and they collide. How should the various components of matter
behave in a smash-up? How should the stars, gas, and dark matter interact?
Ordinary matter exhibits certain basic behaviors in the process of forming structures, such as galaxies and clusters, and
objects, such as stars and planets. Ordinary matter undergoes collisions, sheds energy by emission of light, and forms
molecular bonds. These actions are possible because of the electromagnetic forces on charged particles of ordinary matter,
such as electrons and nuclei, and because of the atomic structure of ordinary matter.
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A Hubble image of 1E 0657–56, the aftermath of a collision of two clusters of galaxies. The green contours show the distribution of total mass as determined from gravitational distortions in the images of distant background galaxies. This technique is known as “weak gravitational lensing.” (See figure on page 80 in article by Scoville,
and sidebar on page 93.)
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During a collision of galaxy clusters, we would expect the two clouds of hot intergalactic gas to pass through each other,
but the friction of electromagnetic forces should slow their movement. The galaxies themselves are so compact and massive
that they should pass through without slowing down. (The gas has about the same non-effect on the galaxies as a fog bank
has on a brick tossed through it.) Furthermore, individual stars make up the galaxies, with lots of space between the stars,
so even in the unlikely event that two galaxies collide, very few stars would collide, and the galaxies should continue on,
relatively unscathed.
We expect dark matter to behave differently in cluster collisions than ordinary matter, for at least two reasons. First, because
dark matter does not produce light, clouds of it cannot cool and shrink to form dense, compact objects, but they remain
diffuse. Light is electromagnetic energy, so if dark matter cannot interact with light, then it likely does not react either to
electromagnetic forces, which are the basis for everyday collisions involving ordinary matter. For this reason, the two diffuse
dark-matter clouds would not slow down as they pass through each other, as the gas clouds do.
Second, we expect that dark matter interacts only weakly—just by gravity—with ordinary matter. The best evidence is the
rotation of the flattened disk of our Milky Way galaxy. The form of this rotation requires a diffuse, cloud-like shape for the
distribution of dark matter, which dominates gravitationally. If the dark matter could interact with the ordinary matter as
strongly as ordinary matter interacts with itself, via electromagnetic forces, then the flattened shape of the disk of ordinary
matter would have been destroyed long ago.
For these reasons, when dark matter passes through any other structure—whether normal or dark matter—the dark matter
should not slow down more than the small amount dictated by gravitational forces alone.
If this picture were correct, we would expect the clouds of hot gas to lag behind the galaxies in the aftermath of a collision
of clusters. Dark matter should track with the galaxies, separate from the gas clouds. If there were no dark matter, we would
expect to find most of the mass of the clusters in the lagging gas clouds. If dark matter exists, we should still find most of the
mass around the galaxies. By measuring the distribution of mass after a cluster collision, we can test these predictions.
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Weak Gravitational Lensing
The General Theory of Relativity, published by Albert Einstein in 1915, predicts that gravity will bend the path of
light as it passes a massive object. Astronomers call this
phenomenon “gravitational lensing,” by analogy to the
familiar bending of light rays by glass lenses.
Gravitational Lensing
Weak lens
Strong lens
Today, astronomers use different types of gravitational
lensing to estimate the masses of objects over a huge
range—planets, stars, galaxies, and clusters of galaxies.
Even though the principle is the same—the observed redirection of light coming from a source located behind
the lens—the measured effects and terminology differ.
Astronomers use “weak lensing” to measure large-scale
mass distributions, such as in clusters of galaxies. In
this case, the lens is the cluster, and the distant sources
are myriad random galaxies that have been sprinkled by
chance over the background. The gravity of the cluster
slightly and uniquely distorts each galaxy image, depending on the path its light took through the distribution of
the mass in the cluster. The effect is small on any single
galaxy image. In addition, astronomers do not know the
undistorted appearance of a galaxy, except in a general
sense. They must, therefore, analyze large ensembles of
images on a statistical basis, taking into account a distribution of galaxy shapes and orientations. The ensemble
reveals a faint underlying pattern, from which the position
and strength of the mass in the cluster is estimated.
In the phenomenon known as gravitational lensing, the images of background galaxies
are distorted by the presence of a large intervening mass. In the strong lensing case,
the background galaxy’s image is obviously contorted, and appears as multiple and
distinct arc-shaped objects. In weak lensing, a smaller intervening mass only slightly
changes the background galaxy’s shape, typically flattening it in a given direction. See
the additional graphic on page 80.
Some 22,631 background galaxies were analyzed to obtain the mass distribution in 1E 0657–56.
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These particular predictions are powerful, because they do not depend on assumptions about how gravity operates over
cosmic distances. Since Zwicky’s original observations, astronomers have found many astronomical objects that demand
far more gravity than the observed normal matter and the conventional law of gravity can explain. The separation of gas from
galaxies and any dark matter during cluster collisions provides us, for the first time, a method of directly testing whether dark
matter exists, even if the “universal” law of gravity isn’t so universal.
Cluster collisions are relatively rare events, but we found one ideally aligned to observe the distribution of mass in its aftermath. The grouping of galaxies, named 1E 0657–56, was detected as an x-ray source by the Einstein X-ray Observatory in
the early 1990s. It has two clouds of gas and two distinct clusters of galaxies. Although the intergalactic gas in 1E 0657–56
is hotter than most clusters, no one realized how truly special this cluster is until Maxim Markevitch observed it using the
Chandra X-ray Observatory in 2000. The Chandra images show that the smaller of the two gas clouds has a bow shock—a
region of intense turbulence—on the side away from the larger cloud, and that both gas clouds lie between the two clusters
of galaxies.
The only reasonable explanation for these observations is that the smaller cluster has passed through the larger cluster, and
the two systems are moving away from each other. We are observing the collision roughly 100 million years after it took
place, so the galaxies have had time to move away from the gas clouds. The gas clouds are lagging behind, as expected.
Observing the invisible
One technique for locating the mass in colliding clusters (both seen and unseen matter) is known as “weak gravitational
lensing.” This technique measures how the gravity of the clusters alters the shapes of galaxies in the distant background, far
beyond the clusters. We used this technique on pictures of the colliding galaxy clusters 1E 0657–56 from various sources:
the European Southern Observatory’s Very Large Telescope and Wide Field Imager, the Magellan Observatory in Chile, and
the Hubble Space Telescope. We found that the majority of the mass in the colliding clusters is located around the galaxies,
is invisible, and is well separated from the lagging clouds of hot gas that are visible through the Chandra X-ray Observatory.
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Let’s review the facts. The mass of the gas is many times that of the stars. The mass of the stars and gas combined is far less
than necessary to bind a cluster. The stars and gas roughly account for all the ordinary mass created in the Big Bang, so any
other mass in clusters must be non-ordinary dark matter. The darkness of dark matter suggests it does not interact except
by gravity. In a collision of clusters, we would expect the dark matter to stay with the stars, which also do not interact. The
cluster collision within 1E 0657–56 confirms this expectation and dramatically demonstrates the reality of Fritz Zwicky’s dark
matter—a still mysterious, invisible material that gravitates, but does not collide—regardless of whether gravity works as
we think it should over cosmic distances.
Douglas Clowe was born in St. Louis, Missouri, and grew up in Boulder, Colorado, and Euless, Texas. He attended the California Institute of Technology as an undergraduate, getting a B.S. in physics in 1993. In 1998,
he earned a Ph.D. in astronomy from the University of Hawaii. His research interests are mainly in observational cosmology, and focus on using gravitational lensing and clusters of galaxies to learn about how the
universe operates, including studies about dark matter, dark energy, and how galaxies evolve over time.
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