Dark matter mapped in 3D for first time

 作者:贺兰夫     |      日期:2019-02-26 02:14:00
By David Shiga, Seattle (Image: NASA/ESA/R Massey/Caltech) (Image: NASA/ESA/A Koekemoer/STScI) The distribution of dark matter has been mapped in 3D for the first time, revealing how the mysterious substance has evolved over the lifetime of the universe. The results confirm that dark matter provided the scaffolding that allowed ordinary matter to clump together to form galaxies and clusters of galaxies. Dark matter is an invisible substance that betrays its presence through the gravitational tug it exerts on ordinary matter. It is six times more abundant than ordinary matter and is thought to have seeded the first distinct structures in the universe, which began as a very uniform soup of matter. Computer simulations suggest that the formation of dark matter clumps attracted surrounding gas, which then condensed to form galaxies and galaxy clusters. But this dark matter clumping process had never been confirmed observationally. Now, astronomers have mapped the changing distribution of both dark matter and ordinary matter over time. Nick Scoville, of Caltech in Pasadena, US, led the Cosmic Evolution Survey (COSMOS), which combined data from three of the world’s leading observatories to produce the map. The key to determining the dark matter distribution is an effect called gravitational lensing, by which light rays from a distant object such as a galaxy are bent by the gravity of an intervening concentration of matter. Although dark matter cannot be seen directly, its presence can be inferred by the way its gravity distorts the images of galaxies behind it. The Hubble Space Telescope (HST) mapped out these distortions over a patch of sky equivalent to the width of four Full Moons in the largest survey it has ever performed. It devoted 10% of its time over two years to complete the survey. The Subaru telescope on Mauna Kea, Hawaii, US, and the Very Large Telescope in Paranal, Chile, measured the spectrum of light from galaxies seen by Hubble, which allowed the galaxies’ distances to be calculated. The XMM-Newton X-ray satellite contributed by mapping gas within galaxies and galaxy clusters – the most abundant form of ordinary matter in those objects. Combining all of this data allowed the astronomers to chart the bending of light that could not be attributed to the observed ordinary matter alone, thus revealing the distribution of dark matter in the patch of sky surveyed. By carefully measuring the distance of various distorted background galaxies, the team was also able to determine the distance to the dark matter responsible for the distortion. The result is a 3D map of the distribution of both ordinary and dark matter. Although 3D maps of ordinary matter have been produced before, the 3D dark matter map is a first. Watch an animation showing the dark matter map from different angles. Because it takes light more time to travel from farther away, the more distant slices of this 3D map represent earlier eras in the history of the universe. This allowed the team to trace changes in the distribution of dark matter over a period ranging from about 6.5 billion to 3.5 billion years ago. The results provide a reassuring confirmation of standard theories of how structures such as galaxies formed and grew over billions of years, says COSMOS team member Richard Ellis of Caltech. “We’ve seen that the dark matter has grown in clumpiness over time,” he told New Scientist. “As the dark matter becomes clumpier, the ordinary material that makes up you and me flows into it.” This flow of ordinary matter creates voids and clumps together to form galaxies and clusters of galaxies, Ellis says. The COSMOS results are a very important step forward, says Eric Linder of the University of California in Berkeley, US, who is not a member of the team. He is especially interested in some areas in the map that appear to show discrepancies between the distribution of dark matter and ordinary matter. Some areas show clumps of dark matter that do not have galaxies in them, which he says could be the result of pressure from supernovae clearing ordinary matter out of some regions. On large scales, dark matter only responds to the force of gravity, so does not feel this pressure. Other areas show concentrations of ordinary matter with no corresponding dark matter clumps. “That’s more of a puzzle,” he says. “I don’t have a good explanation about how you would do that.” But Scoville, who led the survey, says it is too soon to say for sure if these discrepancies are real, since they are at the limit of the survey’s resolution. The COSMOS team is collecting more data that should help resolve the matter, he says. The results were presented on Sunday at a meeting of the American Astronomical Society in Seattle, Washington, US. Journal reference: Nature (DOI: