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An Einstein cross is a phenomenon where a distant object appears as multiple images arranged in a cross-like pattern around a foreground galaxy. A fifth image in the newly discovered cross reveals the presence of dark matter / publication in “The Astophysical Journal”
An international team of astronomers has observed a galaxy in the early universe that has the rare shape of an Einstein cross. The Einstein cross is an astronomical phenomenon in which the light of a very distant galaxy is bent by the gravitational force of an intervening galaxy in front of it. This makes the distant object appear to us as if it were several, usually four, images arranged in a cross shape around the galaxy in front of it – an effect known as gravitational lensing. The Einstein cross that has now been discovered also has a fifth central image. The researchers showed that the arrangement of these five images can only be explained by the presence of a dark matter halo associated with the foreground group of galaxies. The results have been published under the title ‘HerS-3: An Exceptional Einstein Cross Reveals a Massive Dark Matter Halo’ in The Astrophysical Journal.
The researchers used observations from the Northern Extended Millimetre Array (NOEMA, Institut de Radioastronomie Millimétrique – IRAM, France), the Atacama Large Millimeter/submillimeter Array (ALMA, Chile), the Karl G. Jansky Very Large Array (VLA, USA) and the Hubble Space Telescope (HST, NASA/ESA). The discovery offers new insights into dark matter and the early universe.
So-called gravitational lensing effects such as the Einstein cross occur when the gravity of a very massive galaxy (or group of galaxies) bends the light from even more distant objects. To us, these distant objects appear distorted, multiplied or stretched into arcs and rings in the sky. This results in an optical magnification of the objects, enabling an in-depth analysis of galaxies in the early universe.
Objects such as the Einstein Cross are also used to determine the properties of dark matter in galaxies and star groups and clusters. Current theories suggest that dark matter, which accounts for approximately 80 per cent of the mass of the universe, consists of particles that have yet to be identified and do not interact with visible light. Although dark matter is invisible, its existence can be inferred from its gravitational effects.
In this study, the images from NOEMA showed the galaxy HerS-3, which is 11.6 billion light years away, multiplied into five images and forming a nearly perfect cross. An Einstein cross is a rare phenomenon, but this one is particularly unusual due to the bright, central fifth image. The five images of HerS-3 all show the same molecular emission lines, indicating that they are multiple images of the same galaxy.
The observations through the ALMA telescope revealed the detailed structures of the individual images. The NOEMA and ALMA data are complemented by data from the Very Large Array tracing the radio waves and the cold molecular gas. The latter data were analysed by Prachi Prajapati, doctoral candidate in the research group of Professor Dr Dominik Riechers at the University of Cologne’s Institute for Astrophysics, and show the five images of the galaxy in cold molecular gas, which is essential for star formation. This is the first time that an Einstein cross is detected at submillimetre and radio wavelengths.
The light from HerS-3 is bent by four massive foreground galaxies. These form the core of a larger group of at least ten other galaxies that are 7.8 billion light years away from Earth and were identified in the near-infrared with the Hubble Space Telescope. In order to determine the properties of the distant galaxy HerS-3 and to explore the galaxy group in the foreground, the researchers simulated how gravity bends the light of galaxies. It turned out that the exact arrangement of the five images of the Einstein cross cannot be reproduced in the simulation if only the four visible massive galaxies located near HerS-3 and located at the centre of the galaxy group are taken into account.
The fact that there is no other visible galaxy at the same distance close to the foreground galaxy group therefore indicates the existence of a large, invisible component: a concentration of dark matter associated with the galaxy group. Only by adding this massive component, which lies at the centre of mass of the group, does the reconstruction exactly match the properties of the five images. The estimated mass of the dark matter halo amounts to several trillion solar masses.
The HerS-3 system with its fifth central image in the form of the Einstein cross makes it possible to observe the complex structure of a galaxy during the most active phase of the universe’s development. Studying such systems could help to uncover the properties of dark matter and understand how it influenced the earliest stages of cosmic evolution.
In addition to researchers from the University of Cologne, scientists from the Institut d’Astrophysique de Paris of the Sorbonne Université in Paris and the Institut de Radioastronomie Millimétrique (Institute for Radio Astronomy in the Millimetre Range, IRAM) also contributed to the publication.
Press and Communications Team:
Jan Voelkel
+49 221 470 2356
j.voelkel@verw.uni-koeln.de
Press Spokesperson: Dr Elisabeth Hoffmann – e.hoffmann@verw.uni-koeln.de
Media Contact:
Professor Dr Dominik A. Riechers
Institute for Astrophysics
+49 221 470 76027
riechers@ph1.uni-koeln.de
Prachi Prajapati
Institute for Astrophysics
+49 221 470 7787
prajapati@ph1.uni-koeln.de
Publication:
http://doi.org/ 10.3847/1538-4357/adf204
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