New Einstein's Cross discovery reveals giant dark matter halo
Astronomers using the Hubble Space Telescope have discovered a new Einstein's Cross, a gravitational lensing phenomenon that exposes a massive dark matter halo surrounding a distant galaxy. This rare quadruple image of a quasar offers fresh evidence on dark matter's extent. The research, led by scientists from Spain's Instituto de Astrofísica de Canarias, appears in Nature Astronomy.
The newly identified Einstein's Cross, a striking example of general relativity in action, was spotted through observations with NASA's Hubble Space Telescope. Named for Albert Einstein's prediction of light bending by gravity, this configuration shows a distant quasar appearing as four distinct images due to the gravitational pull of a foreground galaxy acting as a lens.
The lens galaxy, located at a redshift of 0.94, warps spacetime enough to multiply the image of the background quasar at redshift 1.73. Researchers found that this lensing reveals a dark matter halo around the galaxy far larger than expected—about seven times the size of a typical halo for such a system. "This is the first time we've seen such an extended dark matter halo in an Einstein Cross," said lead researcher Sherry Suyu of the Instituto de Astrofísica de Canarias.
The discovery builds on the original Einstein Cross found in 1985 in the constellation Pegasus. Unlike that one, this new variant provides clearer data on dark matter distribution because of its alignment and the quasar's brightness. The team combined Hubble imaging with ground-based spectroscopy to map the halo's structure, confirming its mass extends well beyond the visible stars and gas.
Dark matter, which makes up about 85% of the universe's matter, remains invisible but influences gravity on cosmic scales. This finding supports models where dark matter halos envelop galaxies, aiding their formation and stability. However, the halo's unusual size raises questions about how such structures evolve over billions of years.
The study highlights Hubble's enduring role in probing the unseen universe, even as newer telescopes like the James Webb Space Telescope prepare for deeper looks. Suyu noted, "These lenses are natural telescopes, magnifying distant objects and teaching us about invisible matter." The research involved international collaboration, including contributions from the Max Planck Institute for Astrophysics.
While the exact coordinates of the new Cross place it in the northern sky, accessible from observatories worldwide, the implications extend to broader cosmology. It underscores that dark matter halos may be more expansive in certain environments, potentially refining simulations of galaxy growth.