An excess of small-scale gravitational lenses observed in galaxy clusters

Nowadays, a number of observational evidence consistently support the Standard Model of Cosmology: about 70% of the content of the Universe is provided by an elusive form of Dark Energy, about 25% by Dark Matter whose nature is still unknown and only 5% by “normal” atoms. Despite the nature of these dark components is unknown at present, this cosmological model allows us to make detailed predictions on the physical properties of the cosmic structures that we observe. The most sophisticated tools to work out such predictions are represented by numerical simulations which make optimal use of high-performance computing infrastructures.

In this context, clusters of galaxies are the most massive cosmic structures in the Universe: with a mass of about a million of billions of masses of the sun and a dimension of several millions of light years, clusters of galaxies also acts as powerful gravitational lenses, which deform the images of far away galaxies, much like optical lenses distort the image of background objects.

In this article, to be soon published in the journal “Science” a team of Italian astronomers, in collaboration with colleagues from US and The Netherlands and led by Massimo Meneghetti (INAF-OAS)  presents a detailed study of the gravitational lensing properties of clusters of galaxies observed with the NASA’s Hubble Space Telescope and with the Very Large Telescope of the European Southern Observatory (ESO), located in Chile. The comparison between these observations and numerical simulations carried out in a collaboration between Stefano Borgani (Department of Physics, University of Trieste) and Elena Rasia (INAF-Astronomical Observatory of Trieste) provided a quite unexpected result: the standard cosmological model predicts a structure of clusters of galaxies that is much less “granular” than showed by observations. This result could suggest that simulations of cosmic structures still lack some fundamental ingredient. The other, possibly more attractive, implication is that our understanding of the nature of dark matter and dark energy is incomplete, if not wrong.

Stefano Borgani: “The level of details on the structure of galaxy clusters obtainable from the most advanced telescopes now available is really impressive. This quality of observational data needs to be paralleled by a comparable quality of the model predictions obtainable from advanced numerical simulations. This is mandatory for us to be confident about the profound implications of our results on the current understanding of the Universe and of the fundamental laws that determine its evolution.

Elena Rasia: "This work constitutes a leap forward on increasing our understanding of the formation of structures in the Universe and presents a challenge for cosmological models. Until recently one of the big problems in numerical cosmology was the misrepresentation of satellite galaxies around our Galaxy,being more numerous in simulations than observed numbers. Solved that complication by including and refining the modelization of baryon physics, we are now discovering the opposite mis-match in the cluster central regions.  The puzzling enigma that has to be faced is how to reconcile the contrasting trends in the Galaxy environment and in the galaxy cluster cores."

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Color-composite image of the central region of the galaxy cluster MACSJ1206. (A to D) The image combines HST observations in the filters F105W, F110W, F125W, F140W, F160W (red channel), F606W,
F625W, F775W, F814W, F850LP (green channel), and F435W and F475W (blue channel). The dashed and solid lines in (A) show the critical lines of the lenses at source redshifts of 1 and 7, respectively. Panels (B), (C), and (D) zoom into three galaxy-galaxy-strong-lensing events enclosing sources at redshifts 1.425, 4.996, and 3.753, respectively. The white lines in those panels show the critical lines of the lenses at the corresponding source redshifts. In (A) and (B), the sources of the background lenses are bluer than those of the foreground lenses.
In (C), the lensed source is not visible in the HST image but is detected in an observation with the Multi-Unit Spectroscopic Explorer (MUSE) spectrograph on the VLT. The source is detected at a wavelength of âˆ¼7289 Ã…, corresponding to the redshifted Lyman-a spectral line of hydrogen, at locations indicated by the cyan contours. The white crosses indicate the positions of four multiple images of the source.