Today’s space spectacular is brought to you by a team of astronomers at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. They used instruments on board the Hubble Space Telescope to create one of the most detailed maps of dark matter yet attained, by focusing a giant cosmic lens on the distant galaxy cluster known as Abell 1689, housed in the constellation Virgo.
Gravitational lensing is a phenomenon first observed in 1979, when astronomers noticed that a large galaxy (or cluster of galaxies) could interrupt the line of sight to a distant galaxy and warp the resulting image. Per the tenets of general relativity, matter bends space-time and as light travels through this bent space-time, the light’s path will be deflected.
This deflection can be directly observed. Like a glass lens being placed in front of a light bulb, the light will distort from our viewpoint — the heavier the mass, the greater the distortion. So it produces an image of a distant quasar (or another galaxy) that is magnified and split by the foreground galaxy. Gravitational lensing has become an important tool for astrophysicists keen on measuring the mass of distant galaxies — like Abell 1689.
That’s what Dan Coe and his colleagues at JPL have done in their study of Abell 1689. The cluster’s mass bent the surrounding space time and thus the light from even more distant galaxies behind it produced warped and greatly magnified images of those galaxies. Coe and company were able to analyze those distorted images and estimate how much dark matter should be present within the cluster. And the image definitely provides further evidence that dark matter does, indeed, exist; if the gravitational effects were only due to the visible matter, the lensing effect would be much weaker, with less distortion.
Previous work with gravitational lensing has been based on making educated guesses to determine the best model for dark matter distribution, comparing those approximate models to observational data and tweaking them accordingly. The JPL team worked closely with mathematician Edward Fuselier to help them “crack the code of gravitational lensing,” according to Coe. “We can obtain, directly from the data, a mass map that gives a perfect fit.” That data incorporates 135 multiple images of 42 background galaxies, painstakingly assembled like a giant jigsaw puzzle.
The result is the best image yet showing the distribution of dark matter in the galaxy cluster. It shows up in the image as an ethereal haze. Sure, it’s aesthetically pleasing, but Sean at Cosmic Variance points out that looks aren’t everything, either:
One surprising implication of that higher density is that these galaxy clusters mostly likely started forming billions of years earlier than astronomers had assumed, back when the universe was smaller and more dense. Otherwise, dark energy would have had a stronger effect, pushing matter further apart and stunting the growth of such large galaxy clusters like Abell 1689.
Plans are already afoot to study even more galaxy clusters under the Cluster Lensing and Supernova survey with Hubble (CLASH), too, which will gather data from 25 clusters over the next three years. This should give us even better maps of the mysterious dark matter, and shed insight on how these clusters may have formed — and maybe offer a few more tantalizing hints about the role of dark energy in the early universe.
Image credit: NASA
