What is the shape of galaxy clusters?
The 3D shape of galaxy clusters is ellipsoidal, according to a study conducted by an international team of astronomers, recently published in the journal The Astrophysical Journal Letters.
Led by Mauro Sereno, a researcher at the University of Bologna and at the National Institute for Astrophysics-INAF, the study involves other researchers at INAF and at research institutes in Taiwan, the United States, Spain and Israel.
Thanks to the method of analysis developed, the research group was able to determine the shape of as many as sixteen galaxy clusters.
Reconstructing the shape of galaxy clusters is no easy task. Galaxies are, in fact, the largest objects in the Universe, having masses millions of billions times the solar mass. Held together by the force of gravity, galaxy clusters are containers full of heterogeneous elements, shaped by an evolution that lasted billions of years, marked by catastrophic mergers between smaller clusters and by a slow accretion of neighbouring matter.
Furthermore, 80% of the mass of clusters is made up of dark matter: the invisible component of matter that, together with dark energy, which speeds up the expansion of the Universe, regulates the formation of cosmic structures.
The shape of these very complex objects is difficult to determine also because most of the instruments that observe the sky obtain two-dimensional images.
“We have been able to verify that an ellipsoidal shape describes well the distribution of matter and hot plasma of these clusters. Our result reinforces the most recent theoretical predictions about the structure of our Universe and the gravitational effect of dark matter”, explained Mauro Sereno.
The researchers considered various elements, such as the signals emitted by these objects in the visible spectrum or the so-called ‘Sunyaev-Zel’dovich effect’. This signal is produced by the interaction between the large amounts of high-temperature intergalactic gas of protons and electrons and the photons of the cosmic background radiation, the remains of the Big Bang. In addition, there are the effects produced by dark matter that keeps the cluster together, which can act as a lens, bending the light emitted by background galaxies.
“All these effects act in different ways depending on the shape of the cluster. In particular, they can reveal how much the cluster is elongated towards us or flattened in the the plane of the sky”, concluded Mauro Sereno.