The Coma Cluster, a cluster of galaxies 300 million light-years away, has a dark secret. Whenever a new galaxy is dragged into the cluster, it also tends to die. This is how scientists describe a galaxy when it stops creating new stars. The secret behind the cluster's deathly embrace seems to be explained by a staggering amount of dark matter.
Dark matter is one of the great mysteries of physics. It is a type of particle with mass, but it doesn't emit light so we can't actually see it. Dark matter still interacts with visible matter through gravity so we can observe the effects of its presence in galaxies and galaxy clusters. And according to new computer simulations, the Coma Cluster is predicted to have up to 100 times more dark matter than visible matter.
"[Computer simulations] found the galaxies could have fallen into the cluster as early as seven billion years ago, which, if our current theories of galaxies evolution are correct, suggests they must have lots of dark matter protecting the visible matter from being ripped apart by the cluster," said Cameron Yozin, a Ph.D. student based at the University of Western Australia, who led the study.
The galaxies in the Coma Cluster that Yozin studied were about the same size as the Milky Way – so far so good. However, these galaxies seemed to have a significant deficiency of stars: about 99% fewer stars than our galaxy, to be precise.
This lack of stars is due to the powerful gravitational attraction of the cluster. A healthy, growing galaxy contains stars and also the ingredients for new stars: dust and gas. "Galaxies originally form when large clouds of hydrogen gas collapse and are converted to stars – if you remove that gas, the galaxy cannot grow further," explained Yozin.
If a very young galaxy passes too close to the Coma Cluster, it succumbs to its gravitational tug. The galaxy is then pulled into the cluster, but most of its floating gas and dust gets blown out in the process. This means that the young galaxy no longer has the ingredients to form new stars, so it is classified as a "failed" galaxy by astrophysicists. This happened all over the Coma Cluster – the cluster filled with dead galaxies.
This process is known as "quenching," and Yozin and his team have shown that this process happened earlier than they previously thought. The process is beautifully described in this diagram:
Artist's impression of quenching shows how a normal blue (star-forming) galaxy loses its gas while falling into the Coma Cluster very early on in its formation. Cameron Yozin, ICRAR/UWA
"For the first time, my simulations have demonstrated that these galaxies could have been quenched by the cluster as early as seven billion years ago," said Yozin. The published results can be found in the Monthly Notices of the Royal Astronomical Society.
The relationship between the dark matter of the galaxy and the cluster is a complicated one. The parts of the galaxy that were protected have a dark, protective cushion: the dark matter within the galaxy. The galaxy is also stripped bare of its gas by the incredible quantity of dark matter in the Coma Cluster. The unseen matter both protects and destroys.
"They have however avoided being ripped apart completely in this environment," said Yozin, "because they fell in with enough dark matter to protect their visible matter."