Galaxy clusters are the most massive structures in the universe that are held together by gravity, generally thought to have formed when smaller groups of galaxies smashed into each other in ever-bigger cosmic collisions. Most are between 6.5 and 32 million light years across and can contain up to 1,000 galaxies, along with hot intergalactic gas all bound together primarily by the gravity of dark matter. Recall that we’re still clueless about what dark matter is even though we know it makes up all but 4% of all the matter in the universe.
Rich galaxy clusters make for some of astronomers’ favorite hunting grounds for dark matter and other exotica. The Hubble Space Telescope recently peered across 6 billion light years of space to resolve extremely faint features of the galaxy cluster Abell 370 that have never been seen before. Shown here in stunning detail, Abell 370 is part of the Frontier Fields program which uses massive galaxy clusters to study the mysteries of dark matter and the very early universe.
Located in the fall-winter constellation Cetus the Sea Monster, Abell 370 is made up of hundreds of galaxies. The curious blue arcs in the photo are also galaxies but located in the distant background and warped into view by the gravitational might of the foreground cluster. Astronomers describe them as gravitationally lensed galaxies. They belong to yet another cluster that’s twice as distant or some 12 billion light years away. We’d probably never know about this remote cluster were it not for Abell 370 acting as a “lens” to focus and amplify its light.
Albert Einstein, currently featured on the excellent National Geographic TV channel’s series Genius, predicted more than a century ago that gravity could bend light. Astronomers used the 1919 total solar eclipse to prove Einstein right, when they observed that stars near the eclipsed sun were shifted from their positions by the sun’s gravity in exactly the amount the wild-haired physicist predicted.
Abell 370’s enormous gravitational influence warps the shape of spacetime around it, causing the light of background galaxies to spread out along multiple paths and appear both distorted and magnified. We see this as a series of streaks and arcs curving around the center of the image. The effect even gives the core of Abell 370 a rounded, lens-like appearance. Massive galaxy clusters act like natural telescopes, giving astronomers a close-up view of the very distant galaxies behind the cluster. In this case, we catch a glimpse of the universe in its infancy, only a few hundred million years after the Big Bang.
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The most interesting of the warped galaxies might be the big, luminous arc below and left of center. It’s actually two distorted images of a single spiral galaxy that lies 5 billion light years behind the cluster. Simply amazing.
Abell 370 was the last of the Frontier Fields program, which occupied 630 hours of Hubble observing time and over 560 orbits of the Earth. Six clusters of galaxies were imaged in exquisite detail, including Abell 370, the last one to be finished.
Studying massive galaxy clusters like Abell 370 has helped astronomers measure both its normal matter and the dark stuff. By studying its lensing properties, astronomers now know that the cluster contains two large, separate clumps of dark matter, contributing to the evidence that this massive galaxy cluster is actually the result of two smaller clusters merging together.
Now that the observations for the Frontier Fields program are complete, astronomers can use the full data set to explore the clusters, their gravitational lensing effects and the magnified galaxies from the early universe hiding out behind the cluster. If this whole enterprise reminds you of one of those Russian matryoshka or nesting dolls, I agree!