Einstein Cross with a Fifth Image: Dark Matter in a Galaxy Group ✛
Context
Gravitational lensing is the bending of light by massive objects, turning distant galaxies into arcs, rings, or crosses. Einstein crosses are rare: among hundreds of known lensed galaxies, only about a dozen are confirmed cases, and all were found in the optical or near-infrared. In the submillimeter, no Einstein cross has been seen until now, even though lensed dusty starburst galaxies are discovered by the dozen. The study of HerS-3 fills this gap and adds an unprecedented detail: a fifth image at the center of the cross. Nearby in the archive are studies of gravitational lenses as tools for finding dark matter, but this work stands apart: it is the first to link a rare lens geometry to a direct indication of a massive dark halo in a galaxy group.
Methods
Observations were made with three instruments: NOEMA (1.1 mm, resolution 0.92x0.52 arcseconds) provided spectra of molecular lines, ALMA (1 mm, resolution 0.12x0.10 arcseconds) gave a detailed map of the dust continuum, and VLA (6 GHz) produced a radio image. Spectra of the five images showed identical CO(9-8) and H2O line profiles, confirming their common origin. The lens model was built in two stages: first, the positions of the images were fitted as point sources, then the extended source structure was reconstructed using a pixel-based method. The model included the four visible galaxies of the group with their photometric properties, and then a fifth component was added: a spherical halo with a cored isothermal sphere profile or NFW.
Results
All five images of the cross have the same redshift z=3.0607 and similar spectra, unambiguously confirming their nature. A model with only the four visible galaxies fails to reproduce both the positions and orientations of the images, with discrepancies reaching 0.2 arcseconds. Adding a dark halo located 3-7 arcseconds southeast of the brightest group galaxy yields agreement to better than 0.02 arcseconds. The mass of this halo within the image radius lies between $10^{12}$.2 and $10^{13}$ solar masses, and the total magnification of the system is 17-19 times. In the source plane, HerS-3 appears as a nearly edge-on galaxy with a disk about 2.3x0.9 kiloparsecs in size and a powerful gas outflow with a velocity of at least 350 km/s.
Implications
This work differs from neighboring lens studies in that it is the first to demonstrate an Einstein cross with a central fifth image in the submillimeter and directly link it to a massive dark halo in a galaxy group. Neighboring works use lenses to search for dark matter substructure or measure the Hubble constant, but here the very existence of the halo is inferred from the impossibility of explaining the lens geometry with visible matter alone. This makes HerS-3 a unique laboratory for studying the distribution of dark matter in galaxy groups on scales of tens of kiloparsecs.
Future development
Further observations with ALMA and JWST will resolve the source structure to scales below 100 parsecs and test model predictions, such as the appearance of an additional arc from the outflow crossing caustics. This will refine the density profile of the dark halo and possibly reveal substructure. Monitoring the source variability will yield time delays between images, turning the system into a tool for measuring the Hubble constant.
Impact
The results will impact cosmology and dark matter physics, as well as our understanding of galaxy evolution in the early universe.
Next steps
Obtain deep images of the galaxy group with JWST and conduct long-term monitoring of HerS-3 to measure time delays.
Key open problems
For galaxy astrophysics, this work addresses the unresolved question of dark matter distribution in galaxy groups and its role in the evolution of such systems. The absence of a visible component at the halo location raises questions about the nature of dark matter and its clustering on group scales.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The paper presents the first confirmed Einstein cross with a fifth central image in the submillimeter regime. Observations with NOEMA, ALMA, and VLA confirm a common origin for all five images based on identical redshifts and spectra. A lens model with four visible galaxies fails to reproduce the image configuration, but adding a dark halo with a mass of 10^12.2–10^13 M⊙ yields an exact match to the positions.
Where its strength is
A unique combination of rare lens geometry and direct evidence for a massive dark halo in a galaxy group. ALMA's high resolution allowed reconstructing the source structure and revealing a powerful gas outflow.
Where it could go next
- It may be interesting to compare the properties of the dark halo in this system with constraints on dark matter substructure obtained from studies of strongly magnified stars in lensed galaxies.based on: Super-magnified stars: How ripples on the light ocean reveal dark matter's secrets arXiv:2607.14034
- The rarity of Einstein crosses in the submillimeter band can be quantified by comparing with the detection rate of lensed dusty galaxies in large surveys such as the one where HerS-3 was found.based on: More Than a Hundred Distant Stars in a Single Galaxy arXiv:2604.22702
- The fifth image at the center of the cross is sensitive to the mass distribution in the lens core, offering a way to test dark matter models that predict a cored or cuspy profile, similar to methods applied to lensed quasars.based on: Red Eyes: Cosmic Lens Reveals a Pair of Black Holes arXiv:2601.06015v1
Worth trying
- Investigate whether the relative brightnesses of the five images can be used to constrain substructure in the dark matter halo.
- Compare the dark halo mass derived from lensing with independent mass estimates of the group based on the dynamics of visible galaxies.
- Conduct radio monitoring of the system to search for variability caused by microlensing by stars in the lensing group.
Why this may matter
This system offers a rare opportunity to study the dark matter distribution in a galaxy group with high precision and could serve as a benchmark for future discoveries of Einstein crosses in the submillimeter band.
Nearby works in our archive
- Hunting the Mysterious Light at the Galaxy's Centersimilarity 0.51
- Cosmic Lens Amplified a Star's Explosion a Hundredfoldsimilarity 0.51
- Super-magnified stars: How ripples on the light ocean reveal dark matter's secretssimilarity 0.51
- More Than a Hundred Distant Stars in a Single Galaxysimilarity 0.51
- The Smallest Cosmic Lenssimilarity 0.50
- Red Eyes: Cosmic Lens Reveals a Pair of Black Holessimilarity 0.50
The archive is sparse around this work: the closest review is only 0.51, against a usual 0.64. It looks like few have drilled nearby.
An Einstein cross with a fifth image is such a rare configuration that this is the first case ever observed in the submillimeter. The separation between the outermost images reaches 7.5 arcseconds, which is like spotting a coin from a distance of several kilometers.
In Greg Egan's novel "The Light of Distant Stars," gravitational lenses are used to observe distant civilizations. HerS-3 is a real example of how a lens allows us to peer into the early universe and "see" the invisible.
Key numbers
- source redshift: 3.0607
- lens redshift: ~1.0
- separation of outermost images: 7.5 arcseconds
- dark halo mass: 1012.2-1013 solar masses
- total magnification: 17-19 times