A Cosmic Magnifying Glass with an Invisible Handle ✛
The light from a distant galaxy split into two, then three, then four, and in the center a fifth ghost appeared. That is HerS-3: a young galaxy full of dust and newborn stars, whose light passed through a gravitational lens, a massive object that bends rays like a magnifying glass bends light. Usually a lens produces arcs or rings, but here we get an Einstein cross: four images of a single source arranged around an invisible center. And that center is not empty.
Astronomers aimed three instruments at the system: NOEMA, ALMA, and the VLA. The spectra of all five images matched down to the details, meaning the light indeed came from a single galaxy. But when the team tried to explain the cross's shape using the gravity of the four visible galaxies, the model fell apart. Discrepancies reached 0.2 arcseconds, a chasm in the lensing world. So they added an invisible fifth component to the model: a massive dark matter halo, a spherical cloud of unseen matter that reveals itself only through gravity. And the picture came together with an accuracy better than 0.02 arcseconds.
That is how the cosmic magnifying glass revealed its invisible handle. Previously, Einstein crosses were sought in optical and near-infrared light, but none had been found in the submillimeter. HerS-3 is the first. And it is also the first case where the rare lens geometry directly points to a massive dark halo in a galaxy group. The halo's mass ranges from $10^{12}$.2 to $10^{13}$ solar masses, and the system's total magnification is 17 to 19 times. In the source plane, the galaxy is seen almost edge-on: a disk measuring 2.3 by 0.9 kiloparsecs, with a powerful gas outflow moving at least 350 km/s.
This system is a ready-made laboratory. Further observations with ALMA and JWST will resolve the source structure down to scales below 100 parsecs and test how the outflow crosses caustics, the lines where light amplification is maximal. Monitoring variability will yield time delays between images, from which the Hubble constant can be measured. Thus a single distant galaxy, multiplied by gravity, becomes a key to dark matter and the expansion of the universe.
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.