Instead of being evenly distributed throughout the universe, galaxies - driven by the force of gravity - are grouped by the thousands in gigantic formations. Researchers have now discovered atypical structures of mysterious origin around such a cluster of galaxies.
The research report by Joydeep Bagchi and his colleagues reads like a detective story. It's just not about a mysterious death or a we althy lady's missing gold. The crime scene is almost 50 million light-years away at a galaxy cluster called Abell 3376. Unlike their colleagues who deal with earthly cases, Bagchi and Co. can only look through a telescope to clear up the matter.
But first things first. First of all, the scientists from the Inter-University Center for Astronomy and Astrophysics in Pune, India, had discovered an object that they were increasingly suspicious of: Using the VLA's antennas, they observed two diffuse arc-like structures that were emitting radio radiation. Projected onto the celestial plane, they conform to an elliptical shape approximately seven times five million light-years in size.
Bagchi and colleagues looked for optical galaxies that they could associate with these radio sources. But they were unsuccessful. The researchers considered it unlikely that the arcuate structures are accumulations of radio galaxies – i.e. galaxies that radiate primarily in this spectral range. A random overlay of radio sources in the background was also ruled out, since these are too rare to produce such a phenomenon. So where did the energy come from that created such things in the cosmos?
The concave shape of the gigantic arcs indicated a connection with the galaxy cluster to Bagchi and colleagues. To follow the trail, they consulted earlier observations in the X-ray range. These showed a thread-like structure in the cluster, galaxies are distributed in several smaller groups along one axis. The gas movements seemed to be restless here. Possibly a high-energy event took place in the cluster as a result of formation processes. Other indications from the old data also point to this.
During structure formation, gravitational collapse of smaller mass components occurs again and again. For example, intergalactic matter is pulled into the cluster by gravity and falls into the outskirts of the cluster. In addition, mixing processes and collisions of smaller galaxy groups occur in the center of the cluster. According to scientists, the energy released during such processes should generate shock waves, which then propagate through the intergalactic medium.
At the front of this wave, electrons contained in the thin gas condense and are accelerated to almost the speed of light, emitting what is known as synchrotron radiation. This electromagnetic radiation is created when the charged particles are deflected by a magnetic field. Bagchi and his team therefore detect the synchrotron radiation emitted in the radio range in their telescopes.
Simulations show that under the scenarios run through, shock waves do indeed appear very plausible. They spread far enough and would also be long-lived and energetic enough to accelerate electrons to energies high enough to emit the observed radio emission.
However, the exact mechanism that leads to the acceleration of the electrons has yet to be discovered – as does the origin of the cosmic magnetic fields that deflect the electrons in the intergalactic medium. So Bagchi and his colleagues have enough starting points to stay on the trail of the mystery. Perhaps the new look at the scene, planned for other spectral ranges, will help the researchers to finally close the case.