Ghost Light from Dead Galaxies Sheds Light on Ancient Galaxy Interactions

Hubble Detects Long-gone Galaxies

The massive galaxy cluster Abell 2744, nicknamed Pandora's Cluster, takes on a ghostly look in this Hubble Space Telescope view where the total starlight from the cluster has been artificially colored blue. This plot reveals that not all the starlight is contained within the cities of stars — the galaxies — which appear as bright blue-white blobs. A fraction of the starlight is dispersed throughout the cluster, as seen in the darker blue regions. This light comes from "dead" galaxies. NASA/ESA/M. Montes (IAC), and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

Did you know that astronomers can learn about galaxies that died long ago? That's part of the story of the cosmos that the deep cosmos-gazing Hubble Space Telescope was built to tell. Along with other telescopes on the ground and on orbit, it fills in the story of the universe as it peers out at distant objects. Some of its most fascinating objects are galaxies, including some that formed in the infancy of the universe and are now long gone from the cosmic scene. What stories do they tell? 

What Hubble Found

Studying long-dead galaxies sounds like it would be impossible. In a way, it is. They're no longer around, but it turns out, some of their stars are. To learn more about early galaxies that no longer exist, Hubble observed dim light from orphaned stars that lie some 4 billion light-years away from us. They were born billions of years ago and somehow were ejected at high speed from their original galaxies, which themselves are long gone. It turns out some kind of galactic mayhem sent these stars reeling across space. They belonged to a galaxies in a massive galaxy called "Pandora's Cluster". The light from those far-flung stars provided clues to a crime scene of truly galactic proportions: as many as six galaxies were somehow torn to pieces within the cluster. How could this happen?

Gravity Explains a Lot

Each galaxy has a gravitational pull. It's the combined gravity of all the stars, clouds of gas and dust, black holes, and dark matter that exist in the galaxy. In a cluster, you get the combined gravitational pull of all the galaxies, and that affects all of the members of the cluster. That gravity is pretty strong. In addition, galaxies tend to move around within their clusters, which affects the motions and interacts of their cluster-mates. Add those two effects together and you set the scene for the destruction of some not-so-lucky small galaxies that happen to get caught in the action. They get stuck in a squeeze play between their larger neighbors as they travel, Eventually, the strong gravity of the bigger galaxies pulls the smaller ones apart. 

Astronomers found clues to this destructive shredding of galaxies by studying the light from stars scattered by the action.That light would be detectable long after the galaxies were destroyed. However, this predicted "intracluster" glow of stars is very faint and is quite a challenge to observe.These are extremely faint stars and they are brightest in infrared wavelengths of light.

This is where Hubble comes in. It has very sensitive detectors to capture that faint glow from the stars. Its observations helped scientists study the combined light of about 200 billion stars that were cast out from interacting galaxies.

Its measurements showed that the scattered stars are rich in heavier elements like oxygen, carbon, and nitrogen. This means they are not the first-ever stars formed. The first stars consisted mainly of hydrogen and helium, and forged heavier elements in their cores. When those earliest ones died, all the elements were cast into space and into nebulae of gas and dust. Later generations of stars formed from those clouds and show higher concentrations of heavy elements. It is the enriched stars that Hubble studied in an effort to track what happened to their galactic homes. 

Future Studies Zero in on More Orphan Stars

There's still a lot to figure out about the earliest, most distant galaxies and their interactions. Everywhere Hubble looks, it finds more and more distant galaxies. The farther out it peers, the further back in time it looks. Each time it makes a "deep field" observation, this telescope shows astronomers fascinating things about the earliest times in the cosmos. That's all part of the study of cosmology, the origin and evolution of the universe.