This is the classic social faux pas. You are in a happy clique, surrounded by all your friends – and one by one you subsume them, absorb them into you, until you are all alone, one grotesque agglomeration alone in what was once a crowded environment.
This appears to be what happened to a galaxy 9.2 billion years ago, scientists have determined. A galaxy in the relatively old Universe named 3C 297 is mysteriously all on its own – even though its surroundings suggest it should be part of a cluster of at least 100 galaxies, some the size of the Milky Way.
The fact that 3C 297 is on its own suggests that something else happened to all those other galaxies.
“It looks like we have a cluster of galaxies that is missing almost all of its galaxies,” says astronomer Valentina Missaglia from the University of Turin in Italy. “We expected to see at least a dozen Milky Way-sized galaxies, but we only see one.”
Data on the environment surrounding 3C 297 comes from the Chandra X-ray Observatory, which studies high-energy radiation from powerful sources across the cosmos. The galaxy itself is a source of this radiation; it hosts a quasar, an active galactic nucleus containing a supermassive black hole engulfing matter at such a frantic rate that it sparkles with one of the brightest lights in the Universe.
Quasars often emit beams of plasma from the polar regions of the supermassive black hole at their core, shooting jets of matter out into space at speeds approaching the speed of light in a vacuum. These are created from material swirling around the black hole’s event horizon, which is swept and accelerated along magnetic field lines towards the poles and launched into intergalactic space.
3C 297 has such jets, and this is where things around the galaxy get interesting. Data from Chandra and the Karl G. Jansky Very Large Array found several signs that the jets are passing through an intergalactic medium associated with a cluster of galaxies, known as the intracluster medium.
One of the jets is bent in a way that suggests it is interacting with gas in an intracluster medium. The other jet created an X-ray source 140,000 light-years from the galaxy, suggesting it hit gas, causing it to heat up and emit X-rays. Additionally, data from Chandra suggests it There are large amounts of hot gas in space around 3C 297.
These three features taken together suggest that there should be other galaxies gravitationally bound to 3C 297 as interacting clusters.
Indeed, there appear to be other galaxies in the same part of the sky as the distant quasar galaxy. So Missaglia and her colleagues turned to data from the Gemini optical and infrared observatory in Hawaii for a better understanding of the space around 3C 297.
These data revealed that the 19 galaxies are only close to 3C 297 in two dimensions; their distances from us are very different from that of 3C 297, and they do not belong to the same region of space. The strange lonely quasar galaxy is indeed all alone.
These clues suggest that 3C 297 is the result of a merger of giant clusters, making it what is called a “fossil group”; the remains of a cluster united into a single object.
“We think the gravitational pull of the single large galaxy combined with the interactions between the galaxies was too strong, and they merged with the large galaxy,” explains astronomer Juan Madrid from the University of Texas. “For these galaxies, resistance was seemingly futile.”
We have seen other clusters of galaxies in the process of these mergersand draw the gas filament “highways” they travel on their way to coalescence. We even saw other groups of fossils; however, the other groups of fossils identified so far have all been spotted closer to us, meaning we are seeing them later in the history of the Universe.
3C 297 is the first fossil cluster identified by astronomers so far, meaning these mergers may be happening much earlier in the lifespan of the Universe than previously thought.
This means that we may have to rethink how full galaxy cluster mergers unfold.
“It can be difficult to explain how the Universe can create this system only 4.6 billion years after the big Bang“, says astronomer Mischa Schirmer from the Max Planck Institute for Astronomy in Germany. “It doesn’t shatter our ideas about cosmology, but it does start to push the boundaries of how quickly galaxies and galaxy clusters must have formed.”
Given the a lot of things we discover in the early Universe that we the thought couldn’t be therehowever, maybe 3C 297 isn’t so quirky.
The research has been published in The Astrophysical Journal.