An international team of astronomers has observed a galaxy in the early universe that is gravitationally lensed into the rare form of an “Einstein Cross”, including a central  fifth image. Using observations from the Northern Extended Millimeter 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 researchers showed that the configuration of these five images can only be explained by the presence of a massive dark matter halo associated with the foreground group of galaxies that is lensing the source. This discovery provides a unique laboratory to probe dark matter and to better understand the evolution of galaxies in the early Universe. The findings are now published in the Astrophysical Journal.

Strong gravitational lensing occurs when light from a distant galaxy passes by a massive galaxy (or a group of galaxies) that distorts spacetime. This causes the path of light of the background source to bend, making it appear brighter and producing multiple images, arcs, or a ring. The resulting amplification enables one to study galaxies in the early universe in great detail.

Such systems are also powerful astrophysical laboratories for constraining the properties of dark matter in galaxies, groups, and clusters. Current theories suggest that dark matter, which accounts for about 80 percent of the mass of the Universe, is made of as-yet-unidentified particles that don’t interact with visible light. However, due to its large mass, dark matter can be identified by its gravitational influence.

When NOEMA observed the galaxy, known as HerS-3, which is 11.6 billion light-years away, it appeared multiplied into five images producing a nearly perfect cross. All five images were detected in various molecular emission lines with NOEMA, unequivocally indicating that they share the same distance and are therefore the lensed images of HerS-3. This striking lensing effect, called an Einstein Cross, is a rare phenomenon, and in this case, even more extraordinary because of the presence of a bright fifth image at the center of the cross.

The ten times higher angular resolution observations using ALMA revealed the detailed morphology of each image. The NOEMA and ALMA data are complemented with the VLA imaging data tracing the radio waves. The remarkable lensing configuration of HerS-3 represents the first detection of an Einstein cross at sub-millimeter and radio wavelengths.

The light from HerS-3 is bent by four massive foreground galaxies that are at the core of a larger group containing at least ten more galaxies, located 7.8 billion light-years from Earth and which have been identified in the near-infrared with the HST.

Figure: The left panel shows the galaxy HerS-3, that is gravitationally amplified in an Einstein cross with a bright fifth central image, as observed with NOEMA in the millimeter continuum (yellow contours), superimposed on the HST near-infrared image, identifying the four galaxies (G1 to G4) of the lensing galaxy group. The yellow star indicates the position of the dark matter (DM) halo associated with the group. The right panel displays the detailed morphology of each of the five images of the Einstein cross as revealed by ALMA. Adapted from Cox et al. 2025 (Figures 1, 3)

In order to recover the intrinsic properties of the distant galaxy HerS-3 and to explore the characteristics of the foreground lensing group, a state-of-the-art lensing model, that simulates how gravity bends the light of galaxies, was applied. Taking only into account the four visible massive galaxies that are close to HerS-3 and are located at the center of the galaxy group, the lensing models were unable to reproduce the exact arrangement of the five images of the Einstein cross.

The fact that there is no other visible galaxy at the same distance in the nearby field around the lensing group, opened up the possibility of a large, unseen component: a concentration of dark matter associated with the group of galaxies. Only by adding this massive component, which is constrained to lie at the center of mass of the group, does the source reconstruction precisely match the properties of the five images. The estimated mass of the dark matter halo amounts to a few trillion solar masses.

The HerS-3 system with its exceptional fifth central image Einstein cross is a unique astrophysical laboratory set up by the universe itself.  It allows us to zoom on the intricate structure of a galaxy during the most active phase of cosmic evolution while, at the same time, revealing the presence of dark matter, the hidden mass that shapes galaxies and sculpts the universe. Studying systems like this may bring us closer to uncover the properties of dark matter and understand how it influenced the earliest stages of cosmic evolution.

This research is presented in a paper entitled “HerS-3: An Exceptional Einstein Cross Reveals a Massive Dark Matter Halo” and has been published in the Astrophysical Journal.
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