Gravitational Lensing Probes Dark Energy

Abell 1689 is one of the most massive clusters of galaxies known, making it a superb venue for the study of dark matter. That’s because the cluster, some 2.2 billion light years away, creates gravitational lensing that magnifies and distorts the light from galaxies far beyond it. Astronomers used Abell 1689 in 2008 to identify one of the youngest and brightest galaxies ever seen, a galaxy in existence a mere 700 million years after the beginning of the universe. That find, A1689-zD1, turned out to be ablaze with star formation in an era when stars were only beginning to emerge.

New Hubble studies have now used Abell 1689 yet again to make some of the most detailed maps yet of dark matter. The idea is this: The cluster’s gravitational lensing bends and amplifies the light of objects beyond it. The researchers, led by JPL’s Dan Coe, go to work on the distorted images that result, figuring out the mass it would take to produce them. If the galaxies we see in the cluster were the sole source of gravity, the distortions would be much weaker. To straighten out the images, then, requires a great deal of dark matter within the cluster.

Image: Compass and Scale Image for Abell 1689 Dark Matter Map. Credit: NASA, ESA, D. Coe (NASA, Jet Propulsion Laboratory/California Institute of Technology, and Space Telescope Science Institute), N. Benitez (Institute of Astrophysics of Andulusia, Spain), T. Broadhurst (University of the Basque Country, Spain), and H. Ford (Johns Hopkins University).

The lensing effect is powerful, with the Coe team finding 135 multiple images of 42 background galaxies at distances ranging from 7 to 12 billion light years. The map of dark matter distribution that results from this work, if verified, would represent the highest resolution depiction of a galactic cluster’s dark matter distribution yet produced. It’s a particularly interesting result because the effects of dark energy, pushing against the gravitational pull of dark matter, should have had a disruptive effect on the growth of the cluster. The results parallel studies of other galactic clusters with dense cores, leading Coe to this conclusion:

> “Galaxy clusters, therefore, would had to have started forming billions of years earlier in order to build up to the numbers we see today. At earlier times, the universe was smaller and more densely packed with dark matter. Abell 1689 appears to have been well fed at birth by the dense matter surrounding it in the early universe. The cluster has carried this bulk with it through its adult life to appear as we observe it today.”

Galaxy clusters, in other words, probably formed earlier than previously thought, before dark energy could go to work to inhibit their growth. Coe’s work with mathematician Edward Fuselier has produced new techniques for calculating the dark energy map, a feat the scientist likens to ‘cracking the code’ of gravitational lensing. Adds Coe:

> “Other methods are based on making a series of guesses as to what the mass map is, and then astronomers find the one that best fits the data. Using our method, we can obtain, directly from the data, a mass map that gives a perfect fit.”

The analysis method in play is called LensPerfect, described this way in the paper on this work:

> LensPerfect is a novel approach to gravitational lens mass map reconstruction. The 100+ SL features produced by A1689 present us with a large puzzle. We must produce a mass model of A1689 with the correct amounts of mass in all the right places to deflect light from 30+ background galaxies into multiple paths such that they arrive at the 100+ positions observed. > > Most SL [strong gravitational lensing] analysis methods construct many possible models and then iterate to find that which best matches the data. LensPerfect instead uses direct matrix inversion to find perfect solutions to the input data. Using LensPerfect, we may, for the first time, obtain a mass map solution which perfectly12 reproduces the input positions of all 100+ multiple images observed in A1689.

Gravitational lens work, then, involves reconstructing the actual mass distribution based on the highly magnified and distorted images produced by the lensing. It’s no small feat, but dark matter and dark energy are among the highest priority targets for modern science. Sharpening our tools for understanding what lensing is telling us is a step toward understanding both. This work studies dark energy through matter which, though dark, is increasingly within the grasp of study because of its profound effects on spacetime at the galactic cluster scale.

More clusters are to be studied in the same way through the Cluster Lensing and Supernova survey with Hubble (CLASH) program, which will examine 25 clusters over the course of the next three years. Conclusive evidence of early cluster formation may help us put some boundaries on dark energy in the early universe. Let’s hope so, for a universe of which we see and understand a mere four percent (the rest being dark matter and dark energy) is a challenge that energizes the very heart of physics.

The paper is Coe et al., “A High-Resolution Mass Map of Galaxy Cluster Substructure: LensPerfect Analysis of A1689,” The Astrophysical Journal 722 (2010), pp. 1-25 (abstract).


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