HerS-3: An Exceptional Einstein Cross Reveals a Massive Dark Matter Halo
Abstract
We present a study of HerS-3, a dusty star-forming galaxy at zspec = 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 ALMA 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 HST F110W image reveals a foreground lensing group of four galaxies with a photometric redshift zphot~1.0. Lens models that only include the four visible galaxies are unable to reproduce the properties of HerS-3. By adding a fifth massive component, lying south-east 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, HerS-3 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.
Keywords
Cite
@article{arxiv.2509.14983,
title = {HerS-3: An Exceptional Einstein Cross Reveals a Massive Dark Matter Halo},
author = {P. Cox and K. M. Butler and C. R. Keeton and L. Eid and E. Borsato and T. J. L. C. Bakx and R. Neri and B. M. Jones and P. Prajapati and A. J. Baker and S. Berta and A. Cooray and E. M. Corsini and L. Marchetti and A. Omont and A. Beelen and R. Gavazzi and D. Ismail and R. J. Ivison and M. Krips and M. D. Lehnert and H. Messias and D. Riechers and C. Vlahakis and A. Weiß and P. van der Werf and C. Yang},
journal= {arXiv preprint arXiv:2509.14983},
year = {2025}
}
Comments
39 pages, 23 figures