Check Out This 'Ghost Light' From Horribly Mangled Dead Galaxies

A long time ago in a galaxy cluster far away, some gruesome gravitational dismemberment went down. The scene was Abell 2744, nicknamed "Pandora's Cluster." Located four billion light years from Earth, the cluster is enormous, containing about 500 galaxies with a combined mass equal to four trillion Suns.

It also contains the mangled remains of four to six Milky-Way-sized galaxies, which were ripped apart by the cluster's gravitational forces over the course of six billion years. These tortured galaxies spilled their stellar guts all over the intracluster void, resulting in the expulsion of about 200 billion stars from their gravitational confines.

The "ghost light" from these free-floating, outcast stars was recently picked up by the Hubble Space Telescope and analyzed by a team based out of the Instituto de Astrofísica de Canarias (IAC), in Tenerife, Spain. The team published its findings in the October issue of The Astrophysical Journal.

"This is an original piece of work which marks a 'before and after' in our understanding of the formation and evolution of galaxy clusters," lead author Mireia Montes said in a statement.

"Previous studies of the intracluster light had suffered from serious limitation in the depth and the spectral range of the observations, restricting the accuracy of the ages and metallicities derived for the stars that emit that light," she continued.

Indeed, Montes' study represents the deepest, most detailed images ever taken of these distant intracluster stars. Only a telescope with the capabilities of the Hubble could differentiate the light of the exiled stars from the glare of the galaxies surrounding them.

The team observed the ghost light in both the optical and infrared spectrums, and discovered that the outcast stars account for an impressive 10 percent of the cluster's overall light output. That they should be so numerous and bright is an interesting finding on its own, but the finding will also shed light on certain theories about the nature and development of mega-clusters like Abell 2744.

"The Hubble data revealing the ghost light are important steps forward in understanding the evolution of galaxy clusters," co-author Ignacio Trujillo said in a statement. "It is also amazingly beautiful in that we found the telltale glow by utilizing Hubble's unique capabilities."

Hubble's imaging of Abell 2744 was conducted as a part of the two-year-old Frontier Fields program, which aims to use these galaxy clusters as gravitational lenses to study even more remote regions of the universe. The lensing effect is caused by gravity distorting and amplifying background light, transforming the cluster itself into a handy magnifying glass for the objects behind it.

But Montes' study proves that there are bizarre and exciting findings to be gleaned from studying the foreground clusters on their own merits, including their eerie history of decapitating full galaxies and orphaning billions of stars. Along those lines, Montes and her colleagues intend to search for ghost light in the other four clusters targeted by Frontier Fields.

The universe already has its fair share of spooky phenomena, from zombie stars, to galactic cannibalism, to vampire suns, to monster hypergiants. But when to creepy space stuff, it's hard to top ghost light from the lonely stars that survived a six-billion-year-long galactic torture session. Right in time for Halloween, too. Well played, Pandora's Cluster.