This image of Abell 2744 is the first to come from Hubble’s Frontier Fields observing program, which is using the magnifying power of enormous galaxy clusters to peer deep into the distant Universe. Abell 2744, nicknamed Pandora’s Cluster, is thought to have a very violent history, having formed from a cosmic pile-up of multiple galaxy clusters.
Astronomers previously observed Abell 2744 with the NASA/ESA Hubble Space Telescope back in 2011, exploring the cluster’s history. They found that at least four galaxy clusters had crashed into one another to form Abell 2744, causing some weird and wonderful effects. This mix of cosmic phenomena, some of which had never been seen before, led to the nickname of Pandora’s Cluster (www.spacetelescope.org/news/heic1111).
A mix of hazy elliptical galaxies and colorful spirals can be seen clumping together in the center of this image. The effects of the cluster’s gravity can be seen in the blue arcs and distorted shapes that are scattered across the frame, including galaxies that seem to be bleeding into the surrounding space. The arcs are actually the distorted images of galaxies far in the distance.
Abell 2744 is the first of six targets for an observing program known as Frontier Fields. This three-year, 840-orbit program will yield our deepest views of the universe to date, using the power of Hubble to explore more distant regions of space than could otherwise be seen, by observing gravitational lensing effects around six different galaxy clusters.
Gravitational lensing is a phenomenon caused by an object’s influence on the space-time around it. All objects that have mass affect the space around them in this way, but huge clumps of mass like galaxy clusters do so more strongly. This causes light from more distant objects hidden behind this makeshift “lens” to be deflected and bent, leading to a bizarre array of optical effects—for example, by creating mirror images of one galaxy, as well as smearing galaxies out into arcs, and creating multiple images of individual objects. This effect is one of the predictions of Albert Einstein’s general theory of relativity.
Although Orest Chwolson (1924) and Frantisek Klin (1936) are sometimes credited as being the first ones to discuss this effect in print, it is more commonly associated with Einstein, who published a famous article on the subject in 1936. Swiss astronomer Fritz Zwicky, who worked at Mount Wilson Observatory, followed this up by suggesting in 1937 that the effect could allow galaxy clusters to act as gravitational lenses. It was not until 1979 that this effect was actually confirmed by observation.
As well as creating weird shapes, gravitational lensing also magnifies the images so that astronomers can see more detail. This means that distant objects that otherwise would be too distant and faint to be seen become visible—something that the Frontier Fields program aims to exploit over the coming years.
Some results from this program are already starting to emerge, with Abell 2744 as the first target. In a paper submitted to The Astrophysical Journal last November, a group of astronomers detected a large number of distant, gravitationally lensed galaxy candidates—all viewed through Abell 2744. These deep surveys using massive galaxy clusters show that looking through cosmic lenses can be an effective and useful way to study the distant Universe.
For more information on gravitational lensing, see Hubblecast 70: “Peering around cosmic corners” (www.spacetelescope.org/videos/hubblecast70a).