HerS-3: An exceptional Einstein cross reveals a massive dark matter halo

We present a study of HerS3, a dusty star-forming galaxy at z_spec = 3.0607, which is gravitationally amplified into an Einstein cross with a fifth image of the background galaxy seen at the center of the cross. Detailed 1 mm spectroscopy and imaging with NOEMA and the Atacama Large Millimeter/submillimeter Array resolve the individual images and show that each of the five images display a series of molecular lines that have similar central velocities, unambiguously confirming that they have identical redshifts. The Hubble Space Telescope F110W image reveals a foreground lensing group of four galaxies with a photometric redshift z_phot ~ 1.0. Lens models that only include the four visible galaxies are unable to reproduce the properties of HerS3. By adding a fifth massive component, lying southeast of the brightest galaxy of the group, the source reconstruction is able to match the peak emission, shape, and orientation for each of the five images. The fact that no galaxy is detected near that position indicates the presence of a massive dark matter halo in the lensing galaxy group. In the source plane, HerS3 appears as an infrared luminous starburst galaxy seen nearly edge on. The serendipitous discovery of this exceptional Einstein cross offers a potential laboratory for exploring at small spatial scales a nuclear starburst at the peak of cosmic evolution and studying the properties of a massive dark matter halo associated with the lensing galaxy group.