Free Astronomy Magazine November-December 2025

13 NOVEMBER-DECEMBER 2025 ASTRO PUBLISHING all matter in the universe, but it does not emit or absorb light. Astronomers can only detect it through its gravitational effects. The HerS-3 Einstein Cross offers a unique laboratory for studying how dark matter influences the formation of T he left panel shows the galaxy HerS-3, which 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 asso- ciated with the group. The right panel displays the detailed morphology of each of the five images of the Einstein cross as revealed by ALMA. [P. Cox et al/ALMA (ESO/NAOJ/NRAO)/NOEMA] [N. Lira, Cox et al. - ALMA (ESO/NAOJ/NRAO)] Array (NOEMA) in France, the Karl G. Jansky Very Large Array (VLA) in the USA, and the NASA/ESA Hubble Space Telescope. The findings are published in The Astrophysical Jour- nal . The galaxy, known as HerS-3, lies 11.6 billion light-years away and appears multiplied into five images by a massive group of galaxies lo- cated 7.8 billion light-years from Earth. This striking lensing effect, called an Einstein Cross, is scarce, and in this case, even more extraor- dinary because of the presence of a bright fifth image at the center of the cross. The light from HerS-3 is bent by four massive foreground galaxies that sit at the core of a larger group containing at least ten more galaxies. However, detailed lensing models showed that the vis- ible galaxies alone could not ac- count for the exact arrangement of the five images. “The only way to reproduce the remarkable configu- ration we observed was to add an invisible, massive component: a dark matter halo at the center of the galaxy group,” explains Pierre Cox, from the Institut d’Astrophysique de Paris and lead author of the study. “This halo weighs several trillion times the mass of our Sun.” Dark matter makes up about 80% of galaxies in the early universe. Be- cause of the magnification caused by lensing, the team was able to study HerS-3 in unprecedented de- tail. The galaxy appears as a lumi- nous starburst, with an inclined rotating disk and strong outflows of gas from its center. “HerS-3 formed when the universe was just two bil- lion years old, during the peak of cosmic star formation,” says Hugo Messias, co-author of the study and astronomer at the ALMA Observa- tory. “Thanks to this natural tele- scope, we can zoom into regions 10 times smaller than the Milky Way, almost 12 billion light-years away, and in the process infer hidden mat- ter in the light-of-sight.” This is the first detection of an Einstein Cross at submillimeter and radio wave- lengths—a milestone for facilities like ALMA that probe the cold gas and dust fueling the birth of stars in galaxies in the early universe. !