Rare Einstein Cross discovered showing an anomaly
The light emitted by distant galaxies can travel billions of light-years before reaching Earth. If, during this journey, the light passes near a massive object, called a lens, it is affected by gravitational influence. The image of the distant galaxy is distorted and magnified. Unlike common optical lenses, gravitational lenses - which are also galaxies or groups of galaxies - do not concentrate light into a single image but form multiple images of the same light source. A particularly rare configuration of these lenses is one where the distorted source forms four images arranged in a cross. In this case, it is called an "Einstein Cross."
In a study led by Dr. P. Cox (Sorbonne University) and recently published in The Astrophysical Journal, an exceptional case of an Einstein Cross was presented where a fifth central image is present. After examining all available observations, it was discovered that the background light source is a gas-rich galaxy 11.6 billion light-years away from Earth. The lens is a group of four large galaxies and several smaller ones, located 7.8 billion light-years from Earth.
Thanks to the contribution of Dr. E. Borsato and Prof. E. M. Corsini (University of Padua), the galaxies in the group were characterized, and their luminous mass was estimated. And it is precisely by analyzing the distribution of matter in the lens that it was possible to explain the presence of the fifth image. The presence of luminous matter alone is not sufficient to explain the positions and shape of the cross images. It was therefore necessary to introduce an additional, massive component of "dark matter" of about 3 billion solar masses, representing an extended halo around the individual members of the group. This component does not emit light, so it cannot be observed directly. Nevertheless, since light is affected by the gravitational influence of all matter present, visible and not, cases like this are fundamental for investigating the nature of dark matter.
