The Universe is full of stray cosmic objects: rogue planets that have broken free from the gravitational pull of their host stars, comets, and asteroids that have been slingshot out of their planetary systems and brown dwarfs that have been cast away from their stellar nurseries, while left to wander endlessly into the dark depths of interstellar space. On a larger cosmic scale, the list of castaway objects includes the stars that have been ejected from their host galaxies during earlier epochs of the Universe, when the former were obliterated in head-on cataclysmic collisions inside galaxy clusters, leaving their stars to roam freely in the empty intergalactic void. Now, just in time for Halloween, scientists have announced that the Hubble Space Telescope has for the first time managed to directly observe the “ghost light” of these stray, orphaned stars inside the massive galaxy cluster Abell 2744, during a series of observations conducted as part of the Frontier Fields program.
As described in a previous AmericaSpace article, the Frontier Fields is an ambitious three-year-long observing campaign that utilises NASA’s Great Observatories—Hubble, Spitzer, and Chandra space telescopes—to create six new Deep Fields centered around six specific galaxy clusters, by taking advantage of the effects of gravitational lensing, where the light coming from distant background cosmic objects is bent from the huge mass of another object that happens to lie in between and is refocused toward our line of sight here on Earth. The Frontier Fields observations were initiated in September 2013 with Hubble imaging Abell 2744, a massive supercluster of galaxies (also known as the Pandora’s Cluster), which is the result of a collision between four smaller galaxy clusters, located at a distance of 3.5 billion light-years away in the direction of the southern constellation Sculptor. Superclusters are the largest of structures in the Universe, and Abell 2744 is no exception – astronomers have determined that it contains nearly 500 galaxies that are undergoing a major merger and packs a mass of more than 4 trillion Suns. The study of superclusters can provide more clues regarding the distribution of matter in the Universe on the largest of scales, while helping astronomers gain valuable insights about its overall evolution and eventual fate.
Video Credit: NASA/ESA and F. Summers, B. Lawton, M. Lussier, G. Bacon, and D. Coe (STScI)
These observations of Abell 2744, which lasted for a total of 157 Hubble orbits and were concluded this past summer, have already returned several important scientific results, like the discovery of thousands of previously unseen galaxies at distances ranging between 4 and 13 billion light-years away, some at the very edges of the observable Universe. Even though such findings were the main goal of the Frontier Fields program, Hubble’s observations have nevertheless proven extremely useful to other areas of astronomical research as well. As reported in a new study that was published in the Oct. 1 issue of The Astrophysical Journal, a team of Spanish astronomers has managed for the first time to study in great detail the diffuse light coming from the population of rogue stars inside Abell 2744 that had been ejected from their host galaxies when the latter were ripped apart in cataclysmic collisions during an earlier epoch in the cluster’s violent history. Even though this so-called “intracluster light” was known to be a contributor to Abell 2744’s overall luminosity, its detailed study had nevertheless been impossible up till now, due to its extremely faint nature compared to the much brighter light that was coming from the outer parts of the cluster’s galaxies and other background and foreground sources as well.
In order to overcome these challenges, the study’s research team analysed Hubble’s observations of Abell 2744, which have been the deepest and most detailed to date due to the fact that they were made in multiple wavelengths in visible and near-infrared light, using the superior observing capabilities of the space telescope’s Wide Field Camera 3 (WFC3) and Advanced Camera for Surveys (ACS). By utilizing Hubble’s various filters, the researchers were able to construct images of Abell 2744 in many different colors in visible and near-infrared wavelengths, which allowed them to detect the faint glow of the elusive intracluster light out to distances of 350 thousand light-years from the cluster and measure the age and metallicity of the stray stellar population that was emitting it. These photometric studies showed that the diffuse intracluster light was bluer compared to the light from the main galaxy members of Abell 2744, indicating that it was emitted from stars that were considerably younger—possibly 6 to 9 billion years younger than Abell 2744 itself. Further analysis showed that these stars were richer in heavier elements like oxygen, carbon, and nitrogen with metallicities equalling or surpassing that of the Sun, indicating that they definitely weren’t members of the Universe’s primordial, first generation stars, but have been formed at relatively recent cosmological times. According to the researchers, this could only mean that these stars were members of small spiral galaxies similar in structure and composition to the Milky Way, which had a high star formation rate up to the point of their cataclysmic destruction. “The result of this study shows that the galaxies which could in principle be the originators of the stars which emit this intra-cluster light are very similar to the Milky Way, and that they began to disintegrate in Abell 2744 some 9,000 million years ago,” says Dr. Ignacio Trujillo from the Astrophysics Institute of the Canaries in Spain and co-author of the study. “The quantity of mass which is observed as the source of this light is the equivalent of the destruction of 4 to 6 galaxies like ours.”
The researchers also estimate that the stray stellar population that was left after the cataclysmic destruction of these progenitor galaxies amounts to at least 6 percent of the total stellar mass of Abell 2744. “The results are in good agreement with what has been predicted to happen inside massive galaxy clusters,” says Mireia Montes from the University of La Laguna and lead author of the study. This agreement between theory and observation on such a challenging subject of astronomical study underscores the Hubble space telescope’s defining contributions and significant importance in our understanding of the Universe. “The Hubble data revealing the ghost light are important steps forward in understanding the evolution of galaxy clusters,” says Trujillo. “It is also amazingly beautiful in that we found the telltale glow by utilizing Hubble’s unique capabilities.”
Even before the highly productive observations of Abell 2744 were successfully completed, Hubble had already moved on the next target on its list: MACS J0416.1-2403, a galaxy cluster located approximately 4 billion light-years away in the direction of the constellation Eridanus. By the end of the three-year-long period of observations for the Frontier Fields, Hubble will have devoted 840 of its orbits to the project and nearly 2 million seconds of total exposure time—an amount of exposure time almost equal to that needed for Hubble’s eXtreme Deep Field image.
If Hubble’s past accomplishments are any indication, we can be sure that many more exciting and unexpected discoveries still await us, more than 24 years after the iconic space telescope first opened its electronic eyes in order to allow humanity to probe even deeper into the mysteries and infinite reaches of the Cosmos.
The author and the rest of the AmericaSpace team would like to extend their heartfelt condolences to everyone at Virgin Galactic and to the family and loved ones of the co-pilot who lost his life during the crash of SpaceShip Two yesterday at the Mojave desert and wish for a quick recovery of the second pilot who was badly injured during this utterly horrific accident. At the same time, we’d like to express our deepest sympathies and support to everyone at NASA and Orbital Sciences Corporation who have suffered a painful setback last Tuesday, due to the explosion of the Antares rocket during launch from NASA’s Wallops Flight Facility in Virginia. Per aspera ad astra!