Everyone is aware of that galaxies are massive buildings fabricated from stars. That is a easy definition, and ignores the truth that galaxies additionally comprise fuel, mud, planets, moons, comets, asteroids, and so forth., and naturally, darkish matter. However one sort of galaxy is generally fabricated from darkish matter, and so they’re troublesome to detect.
They’re referred to as darkish galaxies, and so they comprise no stars, or solely only a few stars. Scientists have lengthy theorized about their existence, which has remained hypothetical; they’ve discovered galaxies with low floor brightness, and so they’ve discovered darkish galaxy candidates. However new analysis has discovered the strongest candidate but.
The analysis is “Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster” and it is revealed in The Astrophysical Journal Letters. The lead writer is Dayi (David) Li, a post-doctoral fellow in statistics and astrophysics on the College of Toronto.
The candidate galaxy has been dubbed CDG-2, for Candidate Darkish Galaxy 2. (CDG-1 is explained here.) CDG-2 is within the Perseus galaxy cluster about 300 million light-years away. The plain query is, if it is so darkish how was it detected?
It comes all the way down to globular clusters (GC). Most galaxies have GCs. They’re spherical teams of stars which can be sure collectively gravitationally and may comprise thousands and thousands of stars. Round spiral galaxies like ours, they’re largely discovered within the galactic halo. Their origins are unclear, as is the position they play within the evolution of galaxies.
On this work, the researchers used the Hubble, the ESA’s Euclid house telescope, and Japan’s Subaru telescope. They looked for tight groupings of GCs that might point out the presence of a galaxy. The Hubble discovered 4 closely-connected GCs within the Perseus cluster. The researchers then utilized superior statistical strategies on information from the three telescopes that exposed a faint glow across the GCs. This glow is a powerful indication that there is an underlying galaxy whose particular person stars are too dim to resolve.
“That is the primary galaxy detected solely by way of its globular cluster inhabitants,” lead writer Li stated in a press release. “Underneath conservative assumptions, the 4 clusters characterize your complete globular cluster inhabitants of CDG-2.”
*This determine reveals the spatial distributions of GC candidates within the Hubble’s F814W photos V12-ACS (left) and V14-ACS (proper). The Hubble’s Superior Digital camera for Surveys captured each photos. The pink circles and blue diamonds are GC candidates from two completely different prior surveys. “The GC candidates that represent CDG-2 from the 2 photos are enlarged and annotated on the aspect,” the authors write. Picture Credit score: Li et al. 2026. ApJL*
If the idea that the 4 GCs are the galaxy’s whole inhabitants of GCs is appropriate, then the researchers say that they comprise 16% of its seen content material. Additionally they say that CDG-2 is roughly as luminous as six million Solar-like stars. “Our outcomes point out that CDG-2 is without doubt one of the faintest galaxies having related GCs, whereas no less than ∼16.6% of its gentle is contained in its GC inhabitants,” they write of their paper.
“Given the excessive statistical significance that CDG-2 shouldn’t be a random grouping of 4 GCs, we subsequently stacked the 2 photos of V12-ACS and V14-ACS,” they clarify. The center picture within the determine under illustrates the results of that motion. It reveals extraordinarily diffuse emissions across the 4 GCs, in line with the researchers.
*This determine is made up of cutout photos from Euclid’s VIS and the Hubble’s Superior Digital camera for Surveys. The central half stacks each photos from the Hubble’s ACS and reveals extraordinarily diffuse emissions round CDG-2s’s 4 GCs. Euclid information confirms it. Picture Credit score: Li et al. 2026. ApJL*
“The morphology of the diffuse emission in each the HST and Euclid information is sort of an identical,” the researchers write. They conclude that its presence cannot be as a consequence of any potential imaging artifacts in both survey.
CDG-2’s affirmation instantly brings CDG-1 again into the highlight. CDG-2’s diffuse emissions may present some constraints on the identical from CDG-1. “Though earlier observations didn’t reveal detectable diffuse emission round CDG-1, the extremity of CDG-2 begs the query as as to whether CDG-1 may very well be an much more excessive “twin” of CDG-2 with hardly any stars fashioned outdoors of its GCs or that the GC populations have been barely dissolved,” the authors write.
“Alongside the identical line of thought, additional and higher-quality observations of CDG-1 are crucial since CDG-1 can become a galaxy that’s much more excessive than CDG-2,” the authors write. Additionally they say that CDG-1 may very well be the very first instance of a galaxy that’s solely a darkish matter halo with none stars, except for its GCs.
As for its origins, a possible situation is that interactions with different galaxies within the Perseus cluster stripped away CDG-2’s star-forming fuel, abandoning darkish matter. Since GCs are so tightly sure, they’ll resist tidal forces higher. They could be all that stay of the galaxy’s preliminary stellar inhabitants.
Whereas the origin of GCs, and their position in galactic evolution, are nonetheless unclear, they clearly have astronomical utility. This research reveals that GCs may very well be a dependable indicator of darkish galaxies.
