Galaxy clusters are the largest and most recently formed cosmic structures in the universe, of which their content is expected to reflect the overall matter composition. They are dominated by the elusive substance that we call dark matter, but they also contain a smaller fraction of matter in the form of gas and stars. Although only these last components can be directly observed with detectors capable of capturing their electro-magnetic emission covering a broad range of frequencies, from the X-ray to the radio domains, nature helps us to detect the presence of the dark matter too. In fact, due to their overwhelming masses (which is of the order of millions of billions of stars like our sun), galaxy clusters are the most efficient gravitational lenses observable in the sky.
Einstein’s theory of general relativity describes gravity in terms of mass and energy assemblies curving space time. The light emitted by distant galaxies, located way farther than the clusters themselves, occasionally happens to travel through the space-time curved by these massive structures. As a result, the light is deflected, like if it was passing through a convex lens. The images of the source galaxies appear distorted. The distortions are particularly evident near the centers of galaxy clusters, where the local matter density reaches the highest peaks. Here, large gravitational arcs can be formed, like the one shown in the figure on the right. These features are among the clearest evidence for the presence of dark matter in galaxy clusters.
Astrophysicists measure the amplitude of these distortions to understand how dark matter is distributed in cosmic structures. At the same time, they use supercomputers to simulate the formation of galaxy clusters in the context of a given cosmological model, making assumptions on the nature of dark matter. Based on observations of the universe on scales much larger than those probed by galaxy clusters, like the observations of the cosmic-microwave-background or the spatial clustering of galaxies, dark matter particles are believed to interact only through gravity. Under these circumstances, the cluster models emerging from numerical simulations are characterized by a well established density profile: the mass per unit volume varies as a function of radius following a nearly universal law, independent of the cluster size. In addition, cluster halos are found to keep memory of the density of the universe at the epoch of their gravitational collapse, being the most recently formed less concentrated than those formed earlier. The validity of this theoretical result was yet to be tested with an extensive observational campaign. A number of studies addressing individual clusters had casted doubt about the consistency of the theoretical expectations with observations and even suggested the existence of a tension between them.
The Cluster Lensing And Supernova survey with Hubble (CLASH) has recently set a milestone in this debate. The CLASH team, led by Prof. M. Postman at the Space Telescope Science Institute (STScI, Baltimore) and composed by researchers from several institutes across the world (including a large number of INAF members), used the Hubble Space Telescope in combination with some of the most powerful ground-based telescopes to obtain deep, high resolution, multi-color observations of a sample of 25 massive galaxy clusters. Their gravitational lensing effects have been detected from the centers to the outskirts, allowing to measure precisely the shape of their density profiles. Following a novel approach involving state-of-the-art simulation techniques, the results of these measurements have been compared to a set of high resolution numerical simulations tailored to mimic all the criteria used to select the CLASH sample. By doing so, the CLASH team has found a remarkable agreement between observations and expectations in the framework of the standard cosmological model.
Meneghetti et al. (2014): The MUSIC of CLASH: predictions on the concentration-mass relation
Merten et al. (2014): CLASH: The Concentration-Mass Relation of Galaxy Clusters
Umetsu et al. (2014): CLASH: Weak-Lensing Shear-and-Magnification Analysis of 20 Galaxy Clusters