Hubble Captures Rare Image Of A Galaxy Made Almost Entirely Of Invisible Dark Matter

Scientists using a trio of some of the world’s most powerful observatories, the Hubble Space Telescope, ESA’s Euclid, and the Subaru Telescope, have pulled back the curtain on a ghost galaxy that is nearly 99% dark matter, detected thanks solely to its four associate globular clusters.

When the team analyzed images of CDG-2, there was no apparent galaxy, although the researchers noted a tight-knit group of spherical, gravitationally bound hives of stars, i.e. globular clusters. While these clusters usually orbit within the busy centers of galaxies, the astronomers found four of them sitting in an area of space that appeared almost entirely empty. David Li, an astrostatistics researcher at the University of Toronto, stated that "this is the first galaxy ever detected solely through its globular cluster population." Because these clusters are so tightly bound, they are resistant to the gravitational forces that can shred more diffuse structures, acting like markers for hidden systems.

To confirm that these four lonely clusters weren't just drifting through space, researchers layered multiple high-resolution images from Hubble, Euclid, and Subaru. This technique, very similar to long-exposure photography, allowed them to capture an extremely faint, diffuse glow surrounding the clusters. This light confirmed the presence of a galaxy, but one that is starving for normal matter. The team believe that as CDG-2 traveled through the dense Perseus cluster, gravitational interactions with other galaxies stripped away most of its hydrogen gas.

However, what remains is a galaxy dominated by dark matter. Preliminary calculations suggest that 99% of CDG-2’s mass is dark matter. This discovery follows similar findings like Cloud-9, another starless relic identified by Hubble earlier this year, as well as Nube, an ultra-diffuse dwarf galaxy that may hold fuzzy dark matter.