Hubble spies the ghost light of long-dead galaxies

The massive galaxy cluster Abell 2744, nicknamed Pandora's Cluster, where the starlight from the cluster has been coloured blue. The galaxies that are not colored blue lie either in the foreground or background and are not part of the cluster.

NASA’s Hubble Space Telescope has picked up the faint, ghostly glow of stars ejected from ancient galaxies that were gravitationally ripped apart several billion years ago.

As many as six galaxies were torn apart inside Abell 2744, an immense collection of nearly 500 galaxies nicknamed “Pandora’s Cluster", some 4 billion light-years away. The scattered stars are no longer bound to any one galaxy, and drift between galaxies in the cluster.

Hubble astronomers believe the six galaxies were torn apart over a period of 6 billion years.

Computer modeling of the gravitational dynamics among galaxies in a cluster suggests that galaxies as big as our Milky Way are the likely candidates as the source of the stars.

The doomed galaxies might have been destroyed as they plunged through the centre of the galaxy cluster where gravitational tidal forces are strongest.

Astronomers have long hypothesised that the light from scattered stars should be detectable after such galaxies are disassembled. However, the predicted “intracluster” glow of stars is very faint and was therefore a challenge to identify.

“The Hubble data revealing the ghost light are important steps forward in understanding the evolution of galaxy clusters,” said Ignacio Trujillo of the Instituto de Astrofisica de Canarias (IAC), La Laguna, Tenerife, Spain, one of the researchers involved in the study. “It is also amazingly beautiful in that we found the telltale glow by utilizing Hubble’s unique capabilities.”

The phantom stars are rich in heavier elements such as oxygen, carbon, and nitrogen. This means the scattered stars must be second- or third-generation stars that were enriched with the elements forged in the hearts of the universe’s first-generation stars.

With the mass of 4 trillion suns, Abell 2744 is a target in the Frontier Fields three-year program looking at select massive galaxy clusters to help astronomers probe the remote universe.

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