A cosmic show-down – The interaction between dense galaxy clusters and large-scale cosmic structures



By studying the growth and dynamics
of galaxy clusters, researchers from the RIKEN Advanced Science Institute, Wako,
and the Academia Sinica Institute of Astronomy and Astrophysics, Taiwan, have
provided valuable clues on the evolution of the universe.
Huge numbers of stars are not the only distinctive feature of galaxy clusters. Another
important component is the intracluster medium (ICM), a hot plasma consisting of electrons
and protons, that has a greater mass than the galaxies and extends throughout the
vast intergalactical space of the cluster
The researchers focused their study on the ICM of the galaxy cluster known as A1689. They
analyzed x-ray observations made by the Japanese satellite Suzaku; its high sensitivity
for x-ray radiation enabled the observation of A1689’s ICM to very large distances away
from the center. The researchers also analyzed gravitational lensing effects, where—following
Einstein’s theory of relativity—they estimated the total mass of the cluster by the way light from
distant galaxies bent around different regions of A1689
“From the gravitational lensing analysis, the mass distribution of A1689 is precisely known,” notes
Madoka Kawaharada from the research team. “Therefore, by adding x-ray information … to the
cluster outskirts, we [could] compare the gas dynamics directly with the mass distribution.
Kawaharada and colleagues found significant interactions between the ICM and the large-scale
structure of galaxies, sometimes called the ’cosmic web’ that extends throughout the universe. At
the region where the A1689 cluster meets the large-scale structure, its ICM gets even hotter than

its usual 20 megakelvin, with temperatures reaching 60 megakelvin. This suggests a heating effect by
the shock wave that develops where the hot ICM plasma meets ‘colder’ gas from the large-scale structure.
In addition, the gravitational lensing data suggest that the ICM in the shock wave region is static, whereas
it is moving elsewhere, which supports it against the strong gravitational force of the cluster.
These results provide a valuable insight into the dynamics of these huge cosmic structures, particularly if they
can be confirmed for other galaxy clusters, says Kawaharada. “If they behave similarly, it will be evidence
that galaxy clusters do interact with the large-scale structure, confirming that they are a continuously
evolving product of the structure formation in the universe.”