Without a doubt, galaxy clusters are the largest coherent structures in the known Universe. They are made up of hundreds of thousands of galaxies, all held together through the force of gravity. While it may be difficult to observe clusters at first, careful analysis of how seemingly-unrelated galaxies move soon hints at their presence. For many years, astronomers and astrophysicists have been wondering how is it that these giant structures are held together, and also what factors govern their growth. Now, for the first time, an international collaboration of scientists proposes an answer to this mystery.
Scientists from the Japan-based RIKEN institution, working together with colleagues from the Academia Sinica Institute of Astronomy and Astrophysics, managed to identify details pertaining to cluster growth and development. While doing so, they may have also gained new insight into how our Universe developed a short time after the Big Bang exploded everything into being. Their work was prompted by the fact that the actual mass of galaxies in a cluster only accounts for a fraction of the gravitational pull that exists in these regions.
There are only two other possible sources for this force, physicists hypothesize. One is the elusive dark matter, which, for now, remains invisible to detectors, but which is believed to interact with normal matter only through gravity. The other one is the intracluster medium (ICM), the stuff that resides within the cluster itself. Using telescopes capable of measuring the Universe in X-ray wavelengths, researchers found in previous studies that the ICM is made up of superheated plasma, which exits at temperatures of millions of degrees.
Using Japan's sensitive Suzaku X-ray satellite, the group looked at the ICM in the galaxy cluster Abell 1689, in order to understand its influence better. It was discovered that the medium there has a temperature of no less than 23 million degrees, on average. However, areas that are heated beyond this value exist. The warmest spot, the team reports, was found to be heated at about 60 million degrees. This hot region had a weird filamentary structures protruding from it, which researchers determined was made up of many galaxies.
The group writes in the April 9 issue of The Astrophysical Journal that this structure is most likely nothing more than a shock wave. They hypothesize that it was produced after the galaxy cluster that provides the heat for the ICM collided with the cold gas clouds the filament contains. Hence, the science team was able to determine that the large-scale structure of the surrounding Universe influenced the growth of the cluster itself. Gravitational lensing data supplied by the Japanese Subaru observatory, and the Hubble Space Telescope, confirmed the initial readings Suzaku gathered.