Abell 2744 is a giant galaxy cluster, situated in the Sculptor constellation, that was derived from the simultaneous collision of multiple smaller galaxy clusters – an incident that transpired over a span of 350 million years. Nicknamed Pandora’s Cluster, the giant galaxy cluster is thought to comprise of, at the very least, four entirely separate clusters, making up less than five percent of its mass. The gas within the cluster is approximated to constitute around 20 percent of the mass, as observed using NASA’s Chandra X-ray Observatory, and remains so hot that it only shines in X-rays.
The remaining 75 percent of the cluster’s mass is said to constitute invisible dark matter, and was observed by gravitational lensing – the arcing of light rays, from distant galaxies, as they travel through the gravitational field of the cluster. Massive objects, like galaxy clusters, are capable of warping and distorting space-time around it. As a result, astronomers were able to map the location of the mass – and, therefore, the location of the dark matter – by looking at the way the light from galaxies in the background had become distorted by the makeshift lens.
Abell 2744 Lens Reveals Distant Galaxies
Meanwhile, for the very first time, an image of the galaxy clusters of Abell 2744 has been obtained by Hubble’s Frontier Fields observing program. The image shows elliptical galaxies and spirals concentrated at the center of the image. The influence of the cluster’s gravity can be witnessed in the indistinct and distorted shapes that fill the image, which include galaxies that appear to bleed into space. The arcs are the distorted images of galaxies that remain far in the distance; the blue arcs are distant galaxies, shown as they appeared 12 billion years ago, soon after the Big Bang.
The image was unveiled at the 223rd meeting of the American Astronomical Society, in Washington DC.
Abell 2744 is one of six targets selected for observation, as part of the Frontier Fields program. Lasting a duration of three years, it is hoped the program can harness the power of Hubble to explore more distant galaxy clusters, by mapping the effects of gravitational lensing.
Gravitational lensing can also magnify and brighten the image, enabling astronomers to see distant objects in greater detail; this phenomenon is something the Frontier Fields program is hoping to continue to exploit, in the future.
Indeed, a recent study, performed by Hakim Atek and colleagues, used Hubble and Spitzer observations of Abell 2744 to scour the region for gravitationally lensed high-redshift galaxies. Published in the Nov. 29 issue of The Astrophysical Journal, the group found multiple galaxy candidates; the candidates were all viewed through Abell 2744, with the giant galaxy cluster acting as a powerful lens.
Abell 2744 was originally observed by NASA and the European Space Agency’s Hubble Space Telescope and the European Southern Observatory’s Very Large Telescope, back in 2011. As indicated by Renato Dupke – one of the original members of the team that first studied Abell 2744 in great detail – astronomers coined the giant galaxy cluster Pandora’s Cluster after observing strange effects that had never been characterized before. Describing the distribution of the different types of matter as “complicated and uneven,” Dan Coe of the Space Telescope Science Institute, in Baltimore, explained that Abell 2744 was extremely unusual.
Seemingly, the collision of multiple galaxy clusters has culminated in the separation of the hot gas and dark matter. At the heart of the cluster lies a “bullet,” at which the gas of one cluster impacted with the gas of another; this created a shock wave, with the dark matter, reportedly, passing through the collision entirely unaffected.
At the outer parts of the region, one spot contains plenty of dark matter, but no luminous galaxies or hot gas. Curiously, a separate clump of gas has been ejected, preceding the associated dark matter. Astronomers have yet to fathom this phenomenon, but believe it could prove key to understanding interactions between dark matter and the other ingredients of the Universe.
The galaxies discovered are likely to be prime targets for the James Webb Space Telescope to explore – an infrared observatory that is slated for a 2018 launch. Designed to peer back over 13 billion years ago to witness the first stars and galaxies forming, it is thought that JWST will allow scientists to see inside dust clouds with greater resolution, where stars and planets are emerging to life nearby, and could yield exciting new information regarding Earth-like planets, dark energy and even the origins of life.
By James Fenner