The Dark Energy Survey has discovered six new ‘strong gravitational lenses’, extreme examples of Albert Einstein’s space-warping Theory of General Relativity.
Figure 1: On its way to Earth, light from a distant galaxy is deflected by intervening massive objects that act as lenses.
Einstein discovered that matter tells space how to curve, while space tells matter how to move. Anything that has mass, from galaxies to galoshes, makes an imprint in space—like a bowling ball on a trampoline. Like light traveling through your eyeglasses, the paths of particles traveling through this part of space are deflected.
Strong gravitational lenses represent one of the greatest examples of this phenomenon: as the light of a distant “source” galaxy travels to our eyes, its path is bent by deformations in space-time caused by a “lensing” galaxy between us and the distant source (see Figure 1). This distorted path results in warped shapes of galaxies: for example, a simple blue spiral galaxy appears as an extended arc or ring around the lensing galaxy (see Figure 2). To capture the appreciable warping of space-time in an image, one needs a very massive galaxy to sit between the observer and the more distant warped galaxy, and this configuration is pretty rare.
Figure 2: The blue arcs that are around the large galaxies in the center are actually very distant galaxies, not to the side of these large galaxies, but behind them. Image from the Hubble Space Telescope.
Finding arrangements of galaxies like this is notoriously difficult. First, there is the difficulty of finding a lensing event in an image. An arc caused by lensing can be mistaken as a simple unlensed galaxy, or vice-versa. Trained humans are still better than any computer algorithm at searching images for the telltale signs of strong lensing, but it takes a long time to search these images by eye.
Second, we need more information than what’s in an image: we have to make sure that the lensing galaxy is actually between us (the observer) and the source galaxy whose image is warped. If the warped galaxy is actually closer than the lensing galaxy, then it can’t be a lensing event! We use other telescopes, such as Gemini South in Chile, to precisely measure the distances to the lensing and source galaxies.
We visually scanned huge swaths of DES imaging and identified several candidates. Then, we measured distances at Gemini South, confirming six new gravitational lenses. “These are the first of many, many lenses that we’ll discover in the Dark Energy Survey, ushering in a new era for the study of these objects,” said Liz Buckley-Geer, one of the lead scientists on the discovery.
Figure 3: The six lenses discovered by DES in early data. These are different from that of Figure 2, because HST is a space-based telescope and produces cleaner images than DES.
We can learn a great deal about the cosmos from strong lenses. For example, the amount of warping of an object’s image is related to how much mass is in the lensing galaxy. So, we can measure the amount of dark matter in a galaxy by how much an image is warped: this is the only method physicists have for directly measuring how much dark matter surrounds a galaxy. We can also learn about dark energy and galaxy evolution in the early universe, and we’ll discuss how to do this in a future piece.
There have only been about 1000 strong gravitational lenses ever discovered. By its end, DES may discover twice as many, and it will produce one of the largest strong lensing datasets ever found.
You can read more about this analysis, Observation and Confirmation of Six Strong Lensing Systems in The Dark Energy Survey Science Verification Data, online on the arxiv.
About the Paper Author
Brian Nord is a post-doc at Fermi Accelerator National Laboratory. He did his graduate work on galaxy clusters and simulations of large-scale structure at the University of Michigan. He has been working with the DES strong lensing group for the past 3 years. Nord also does time lapse photography and has an encyclopedic memory of movies and television from the 1980’s.
About The DArchive Authors & Editors
Chicago. He works on various projects studying the large scale structure
of the Universe using the millions of galaxies DES observes. These
projects include galaxy clustering, correlations of structure with the
cosmic microwave background, and using the structure of the Universe to infer redshifts of galaxies. Ross is also an active science communicator, volunteering at Chicago’s Adler Planetarium as well as writing and editing for The Darchives. He also loves observing for DES in Chile, where he has observed more than 30 nights.