Astronomy:CDG-2

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CDG-2
Image of Candidate Dark Galaxy 2 (CDG-2) taken by Hubble Space Telescope, with the four globular clusters in it highlighted.
Observation data
ConstellationPerseus
Right ascension 03h 17m 12.61s
Declination+41° 20′ 52.64″
Distancec. 245 million ly
Group or clusterPerseus
Notable featuresCandidate for a dark galaxy

Candidate Dark Galaxy 2 (CDG-2) is a candidate for a dark galaxy, composed predominantly of dark matter, along with four globular clusters, in the Perseus cluster.[1] It'd be one of the most dark matter dominated galaxies probably detected, estimated to be between 99.94% and 99.98% dark matter.[2] It has been not confirmed yet if it's a true dark galaxy, more studies are required for a definitive conclusion.


Discovery

CDG-2 was discovered by a Hubble Space Telescope survey (the PIPER survey) looking for larger than expected densities of globular clusters in the Perseus cluster. These higher densities of globular clusters could correspond to ultra-diffuse galaxies. In March 2025, researchers used a new statistical technique, a Poisson cluster process (adapted from the Neyman–Scott process), to perform the search and identified CDG-2 as a possible galaxy.[3] In June 2025 the Hubble Space Telescope images, images from the Euclid survey, and images from the Subaru telescope were examined[4] and diffuse emission around the globular clusters in CDG-2 was found, providing strong evidence that CDG-2 was indeed a galaxy. It is the first galaxy to be detected through the presence of globular clusters.[5]

Contents

Unlike most ultra-diffuse galaxies that have been discovered so far, CDG-2 is believed to only have the four globular clusters originally identified, though there were a few possible additional globular clusters that required further investigation. CDG-2 is estimated at having a dark matter halo mass of approximately 5.7×1010M⊙, meaning that it is a highly dark matter dominated galaxy, with a halo mass fraction of 99.94 to 99.98%.[5] The existence of this galaxy may be a useful test for various theories of galaxy formation.[6] Since dark galaxies are a prediction of the Lambda-CDM model, the prevalence of dark galaxies is an important test of the theory, and simulations of their prevalence have drawn upon CDG-2 as a reference, along with Cloud-9.[7]

See also

References

  1. ↑ "Dark Galaxy CDG-2 Near Perseus Cluster - NASA Science" (in en-US). 2026-02-18. https://science.nasa.gov/asset/hubble/dark-galaxy-cdg-2-near-perseus-cluster/. 
  2. ↑ Lea, Robert (2026-02-19). "Hubble telescope discovers rare galaxy that is 99% dark matter" (in en). https://www.space.com/astronomy/dark-universe/hubble-telescope-discovers-rare-galaxy-that-is-99-percent-dark-matter. 
  3. ↑ Li, Dayi; Stringer, Alex; Eadie, Gwendolyn; Abraham, Roberto (March 2025). "Poisson cluster process models for detecting ultra-diffuse galaxies". Annals of Applied Statistics 19 (1): 261–285. doi:10.1214/24-AOAS1958. https://projecteuclid.org/journals/annals-of-applied-statistics/volume-19/issue-1/Poisson-cluster-process-models-for-detecting-ultra-diffuse-galaxies/10.1214/24-AOAS1958.full. 
  4. ↑ information@eso.org. "Hubble identifies one of darkest known galaxies - The elusive object dubbed CDG-2 may be composed of 99% dark matter" (in en). https://www.esahubble.org/news/heic2605/. 
  5. ↑ 5.0 5.1 Li 李, Dayi (David) 大一; Liu 刘, Qing 青; Eadie, Gwendolyn M.; Abraham, Roberto G.; Marleau, Francine R.; Harris, William E.; van Dokkum, Pieter; Romanowsky, Aaron J. et al. (2025-06-16). "Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster". The Astrophysical Journal Letters 986 (2): L18. doi:10.3847/2041-8213/adddab. ISSN 2041-8205. Bibcode: 2025ApJ...986L..18L. 
  6. ↑ Garay, Jorge. "A Galaxy Composed Almost Entirely of Dark Matter Has Been Confirmed" (in en-US). Wired. ISSN 1059-1028. https://www.wired.com/story/a-galaxy-composed-almost-entirely-of-dark-matter-has-been-confirmed/. Retrieved 2026-02-21. 
  7. ↑ García-Bethencourt, Guacimara; Cintio, Arianna Di; Comerón, Sébastien; Arjona-Gálvez, Elena; Contreras-Santos, Ana; Cardona-Barrero, Salvador; Brook, Chris B. A.; Negri, Andrea; Libeskind, Noam I. (2026-01-07). "HI-bearing dark galaxies predictions from constrained Local Group simulations: how many and where to find them". arXiv:2601.04024 [astro-ph.GA].



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