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CLASH of the Galaxy Clusters

The views expressed are those of the author and are not necessarily those of Scientific American.


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Galaxies do not usually exist alone. They tend to bunch together in small groups, like the Local Group of galaxies in which the Milky Way sits, or larger groups called clusters. This is useful for cosmologists, as it gives them a chance to study one of the most elusive substances in the universe: dark matter.

Galaxy cluster MACS 1206 as seen by the Hubble Space Telescope (click for a bigger version). Credit: NASA, ESA, M. Postman (STScI), and the CLASH Team

Dark matter makes up 23% of the universe but we know very little about it. A multi-wavelength survey called the Cluster Lensing And Supernova survey with Hubble (words which together make the obviously completely unintentional acronym CLASH) hopes to change that, by observing 25 clusters of galaxies in greater detail than ever before. The CLASH team have already completed observations of six clusters and plan to finish another five before the year is out.

MACS 1206 is a galaxy cluster that was recently surveyed by CLASH. It lies 4.5 billion light-years from Earth in the constellation Virgo in our night sky. The image of MACS 1206, above, was taken between April and July 2011 with Hubble’s Advanced Camera for Surveys and the Wide Field Camera 3. The observations, along with observations of other galaxy clusters to be surveyed by CLASH, will help astronomers construct detailed maps of dark matter in galaxy clusters.

Galaxy clusters are the perfect test site for dark matter’s gravitational effects because they are the biggest gravitationally bound objects in the universe. There are three defining features of a cluster: they contain hundreds of galaxies (at least — sometimes thousands); between the galaxies there are huge clouds of hot gas (we’re talking a hundred million degrees); finally, they contain dark matter, and lots of it.

We can’t see this dark matter — that’s where the ‘dark’ bit of its name comes from. We can, however, work out how much of it a galaxy cluster harbours using an effect known as gravitational lensing. Because of their huge masses, galaxy clusters act like giant ‘gravitational lenses’, bending and magnifying light that passes through them. This includes light from galaxies that lie beyond the cluster in our line of sight.

Gravitational lensing can cause the same galaxy to show up twice or more in different parts of an image, and can change how that galaxy looks in the image too.

The amount a galaxy is distorted depends on the total amount of mass in the cluster. The total mass includes that from the galaxies, the gas between the galaxies, and the dark matter. Astronomers are able to use the distortion of galaxies that lie behind galaxy clusters in our line of sight to measure the mass of the intervening galaxy cluster.

They can measure the amount of ‘normal’ matter in a galaxy cluster, so they know how much mass a cluster should contain if there were no dark matter.

Then, by using the amount of distortion to measure the total mass of the galaxy cluster, astronomers can work out how much dark matter exists within a particular cluster and how that dark matter is distributed. The distribution of dark matter in a cluster can give them some clues about how and when that galaxy cluster formed.

When Hubble surveyed MACS 1206 it found 47 multiple images of 12 galaxies that lie behind the cluster. A lot of the galaxies in the Hubble image of MACS 1206 look like they have been smeared slightly. If you look closely, you’ll notice that they are mostly smeared so that their longest edge is facing the centre of the image, making it look like they are in some kind of whirlpool. They are facing the centre of the galaxy cluster. By measuring how much the galaxies are smeared out, and in what way, astronomers hope to gain one more piece in the puzzle that is dark matter.

Kelly Oakes About the Author: Kelly Oakes has a master's in science communication and a physics degree, both from Imperial College London. Now she spends her days writing about science. Follow on Twitter @kahoakes.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Wifirex 5:29 am 03/6/2012

    Thank you for sharing that Kelly. I read a lot about this particular topic, I hear a lot about scientists still chipping away at dark matter theories and interpreting observations, and I can’t help but think…what if we’re looking at “dark matter” entirely the wrong way. Let me give an example based on my “little big theory”:

    What is ‘little big theory’? In a nutshell, it is the theory that the universes’ largest possible structure is based upon its smallest structure or vice-versa. This could repair several halts in observed and theoretical perspectives within the scientific community. The catch is however, this theory cannot be observed entirely.

    If it was possible to observe the theory, the following is what you could expect to see:

    The universal structure would look and behave similarly to an atom. It would be capable of absorbing energy and mass baring material, into its structure while simultaneously expelling and redirecting these resources in a variable number of ways, again in a linear paradigm to atomic behaviour.

    One of these behaviours in particular I think can explain a number of boggled ‘dark matter’ theories. Picture a “super electron” (SE), in valence with the universe at its core or nucleus. Then imagine this SE in orbit with the universe, passing over vast areas of space, pulling everything made of mass in its wake away from the centre, and proton equivalents doing the exact opposite of this in other places, resulting in a constant displacement of all galaxies/clusters/matter to new points within the universe – kneading would be the best way to describe it. Observations consistent with this theory (although described backwards in article “Dark Matter Furrows Brows”, John Matson, Scientific American, March 5, 2012), expressed that dark matter has been observed moving in “clumps” throughout the Milky Way. It is true that anything what resides within a planar fabric is capable of movement, although provided it is made of mass and/or energy; regarding “dark matter” as a “matter” will only keep the brightest minds confused for a very long time (there is one exception to this rule – black holes, but I will drop my bomb theories on singularities in a relevant article).

    So, ‘little big theory’ depicts that “dark matter” (my term – planar fabric), is incapable of moving or condensing, instead it is the residuals (matter), embedded in this fabric that move and change in composition forming heavy/dense areas of space. The planar fabric merely offers a body in which mass based materials can reside collectively (like a cup and water). Thus, at an ultra slow pace all galaxies and systems are moved all over the universe at all times – this could support/replace the theory of ‘universal expansion and contraction’ and theories related to the centre of the universe.

    I know you probably don’t comment on your own material, but just for arguments sake, if you haven’t heard anything similar or equal to in theory to mine, can you run this by some of your academic peers and see what they think? Oh and if anyone would care to comment, be my guest!

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