National Aeronautics and Space Administration Detailed Dark Matter Map Taken from: Hubble 2010: Science Year in Review Produced by NASA Goddard Space Flight Center and the Space Telescope Science Institute. The full contents of this book include 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
Detailed Dark Matter Map Dark matter is an exotic substance that cannot be seen or otherwise directly detected but is now generally accepted by astronomers as the primary constituent of the universe s mass. Astronomers detect its existence indirectly through its gravitational effect on visible matter. In a recent study using Hubble data, astronomers have made one of the most detailed maps to date of the inferred distribution of dark matter in a massive galaxy cluster. Immense galaxy clusters are thought to have formed around dense clumps of dark matter. Such clusters may contain hundreds or thousands of galaxies, each with billions of stars. These galaxies consist of normal matter the familiar atomic species that appear in the periodic table of elements. The combined gravitational influence of the normal matter and dark matter acts as a giant magnifying glass on the light from more distant galaxies located behind a cluster. The cluster s gravity bends and amplifies the light from these distant galaxies like the view in a fun-house mirror. This phenomenon, called gravitational lensing, produces multiple, warped, and greatly magnified images of each background galaxy. The amount of lensing is dependent on the total strength of the gravitational field present. Dark matter indirectly reveals its presence since the lensing distortions observed are much stronger than those predicted to be produced by the gravitational influence of the visible, normal-matter galaxies alone. The lensed images are then somewhat like a giant jigsaw puzzle for astronomers. As individual pieces, they provide critical evidence of a much larger picture. A team led by astronomer Dan Coe analyzed the observed degree of lensing in images from Hubble s Advanced Camera for Surveys (ACS). They studied the massive galaxy cluster Abell 1689, located 2.2 billion light-years away. The team mathematically determined for the first time a way to arrange the distributed mass of Abell 1689 in such a way that it bends all of the identifiable background galaxies precisely to their observed positions. As a result, they produced a higherresolution map of the cluster s dark matter distribution than was possible before and likely the highest-resolution map of any galaxy cluster to date. This image shows the calculated distribution of dark matter (tinted blue) found in the center of the giant galaxy cluster Abell 1689. It is superimposed over a Hubble visible-light image of the cluster that contains about 1,000 galaxies and hence trillions of stars. 127
Left: A Hubble visible-light image of galaxy cluster Abell 1689, one of the most massive structures known in the universe. The giant cluster s mass bends space and acts as a gravitational lens, magnifying and distorting the images of more distant galaxies located behind it. Right: A dark-matter map, shown in blue, is superimposed over the visible-light image of the cluster. The dark-matter concentrations are based on an analysis of the amount of invisible matter needed to produce the gravitational lensing observed in the visible-light image. The space-based imaging of Hubble was essential for this analysis. Abell 1689 is one of the most powerful gravitational lenses in the sky, but ground-based imaging revealed only a single strongly lensed galaxy. Over a period of six years, Hubble s high-resolution ACS images have revealed through Coe s study and others that there are 42 lensed galaxies. These appear in 135 multiple images, each adding information used to map the dark matter in more detail. Many of these galaxies are lensed into faint, thin arcs, difficult or impossible to detect through the blurring effects of our atmosphere. Twenty of the lensed images arising from eight different galaxies were identified in this most recent work. Based on their detailed dark-matter map, Coe and his team confirmed previous results indicating that the core of Abell 1689 is much denser in dark matter than expected for a cluster of its size. This implies to astronomers that Abell 1689 formed billions of years earlier than expected, when the universe was smaller and denser. Abell 1689 joins a handful of other well-studied clusters found to have overly dense cores. But if all clusters formed earlier than assumed, astronomers would expect to see many more of them in our universe today than they actually see. One possible resolution to this quandary is that the early universe had more dark energy than currently believed. Dark energy is a mysterious property of space that fights against the gravitational pull of dark matter. It pushes galaxies apart from one another by stretching the space between them. Additional dark energy at early times would have stunted the growth of galaxy clusters to produce the number astronomers observe today. 128
Additional research is needed to test this speculation, however. Alternatively, there may be a selection effect at work. The well-studied clusters that appear to have overly dense cores are also the ones that produce stronger gravitational lenses, which astronomers have selected so far as the most interesting to study in detail. Coe will continue this work on a team with a new three-year Hubble telescope program designed to shed more light on the interplay between dark matter and dark energy. The program is called CLASH, the Cluster Lensing and Supernova survey with Hubble, and will observe 25 galaxy clusters and map their dark matter in detail. The CLASH researchers did not select the clusters based on their gravitational lensing capabilities, but rather on the strength of their X-ray emission an indicator of the amount of hot gas present between the galaxies. Astronomers expect this larger and less biased sample to yield more conclusive evidence as to whether or not clusters formed early, perhaps due to early dark energy. Further Reading Coe, D., et al. A High-resolution Mass Map of Galaxy Cluster Substructure: LensPerfect Analysis of A1689. The Astrophysical Journal 723, no. 2 (November 10, 2010): 1678 1702. Dorminey, B., What Galaxy Superclusters Tell Us About the Universe. Astronomy 38, no. 1 (January 2010): 28 33. Jullo, E., et al., Cosmological Constraints from Strong Gravitational Lensing in Clusters of Galaxies. Science 329, no. 5994 (August 20, 2010): 924 927. Kruesi, L., What Do We Really Know About Dark Matter? Astronomy 37, no. 11 (November 2009): 28 33. Moskowitz, C., Chubby Galaxy Cluster Suggests Dark Energy Was Stronger Long Ago. Space.com, November 12, 2010. http://www.space.com/9523-chubby-galaxy-cluster-suggests-dark-energy-stronger-long.html, (accessed January 25, 2011). Dr. Dan Coe has been working on Hubble Space Telescope images of galaxy clusters since 2000 when he began his graduate studies at Johns Hopkins University. Analysis of these images has yielded detailed maps of dark matter, the mysteriously invisible predominant form of matter in the universe. Raised in Alexandria, Virginia, Dr. Coe earned a BS in applied and engineering physics from Cornell University in 1999, and a PhD in astronomy from Johns Hopkins in 2007. He then worked as a Caltech postdoctoral scholar at the NASA Jet Propulsion Laboratory before joining the Space Telescope Science Institute in Baltimore, Maryland in 2010 to begin his second postdoctoral work. He is currently working on CLASH, the Cluster Lensing and Supernova survey with Hubble, a large three-year project to study 25 galaxy clusters. 129