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Galactic dark matter survey reveals the invisible
Posted: 21 October 2011

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Dark matter might be elusive, but that has not stopped astronomers attempting to edge that touch closer in trying to uncover the characteristics of this mysteriously invisible material. Studies of galaxy cluster MACS J1206.2-0847, imaged by NASA’s Hubble Space Telescope, have kicked off a dark matter census project as part of a multiwavelength survey called the Cluster Lensing and Supernova survey with Hubble (CLASH) and is set to allow astronomers to construct the most detailed map of dark matter in galaxy clusters ever made.

Galaxy cluster MACS 1206 is one of the first targets in the CLASH survey and massive galaxies like these are perfect laboratories for studying dark matter due to their size. Lensing effects can produce multiple images of the same distant object, as evident in this Hubble image. Image: NASA/ESA/M.Postman (STScI) and the CLASH Team.

Dark matter is believed to make up the majority of matter in the Universe, but despite this it is tricky to detect since it neither emits nor scatters light, and interacts with normal matter only by gravity. This is how the CLASH team are able to map something that is seemingly invisible, in the effects of gravitational lensing that causes hefty galaxy clusters rich in dark matter to act as giant magnifying glasses, bending and distorting any light that passes through them. And this is exactly what the team revealed when turning Hubble towards the galaxy cluster MACS J1206.2-0847 or MACS 1206 for short, which lies around 4.5 billion light years away.

“Matter causes space itself to distort,” says Marc Postman of the Space Telescope Science Institute, Baltimore, Maryland. “This is a prediction of Einstein’s theory of relativity. Since light travels in a path that follows space, when light encounters a region of space that is bent it will travel in a bent path.” The bending of light brings with it distorted shapes of distant galaxies, but often the size of the distortion is larger than can be explained by the gravity of just normal matter. The extra lensing effect is coming from dark matter.

The maps that the CLASH team have generated serve as tests of previous results that suggest dark matter is more densely packed inside galaxy clusters than some other models have predicted, thus leading to the theory that MACS 1206’s assembly began earlier than most commonly thought. “Theories suggest that the central density of dark matter in a cluster is dependent on the density of the Universe when the dark matter first aggregated into the region it now occupies,” says Postman. “As the density of the Universe is decreasing with time, if one finds clusters with unusually high concentrations of dark matter that could be explained, in part, by saying the cluster’s dark matter first formed when the Universe was denser at an earlier epoch in time than expected. Precisely how much earlier is matter of debate.” He adds; “The epoch of first cluster formation has to be at least 10 billion years ago and may well be as much as 12 billion years ago.”

NASA’s Hubble Space Telescope is part of a survey to investigate the distribution of dark matter in large galaxy clusters. Image: NASA.

With six hefty galaxy cluster observations completed and probing with unprecedented precision, the team have their eyes set on mapping a further 19 massive clusters to uncover their dark matter distributions. And it seems that the mass of these galaxies is key to this survey. “The more massive a cluster is, the larger the area of distortion,” says Postman. “Hence, the more massive clusters will generally show more lensed objects than a less massive cluster and a more massive cluster will also show strongly distorted objects at larger distances from the cluster centre in a less massive cluster.”

With its keen ultraviolet to near-infrared vision and unique capabilities compared to any other space telescope, Hubble has supported the CLASH astronomers in their discovery of 12 newly identified distant galaxies in the lensed image, throwing 47 multiple images into the bargain and helping astronomers to build on earlier work produced by the space telescope and its ground-based cousins. With the diverse range of wavelengths, astronomers are able to estimate the distances to these galaxies, which to any Earth-based telescope are otherwise four times fainter than they are able to detect.

“We observe the clusters in 16 separate filters and we also determine the dark matter distribution by finding cases of objects that have multiple images of themselves on the sky,” says Postman. “An object can appear in multiple spots behind the cluster when the object's light passes close to the cluster core. By counting up the number of such multiple images, and comparing their precise locations with models, we can estimate where the dark matter must be located to account for the observed distorted objects. The 16 different colors come into play because they provide an estimate of the object’s actual distance from Earth. If we have a fairly precise estimate of how far away each distorted object is then our estimates of how much dark matter there is and how that dark matter is distributed become much more accurate.”

If the CLASH team find that most of the clusters in their survey host excessively high accumulations of dark matter in their central cores, then it could possibly provide us with new clues to the early stages in the origin of the Universe’s structure – new leads that will be left to the powerful cameras of future telescopes such as NASA’s James Webb Space Telescope to follow up, which is set to study the chemical composition of large galaxy clusters such as MACS 1206.

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