CDG-2

CDG-2
CDG-2
	哈伯太空望遠鏡拍攝的CDG-2圖像，其中突顯了該星系內的四個球狀星團
觀測資料
星座	英仙座
赤經	03h 17m 12.61s
赤緯	+41° 20′ 52.64″
距離	約2.45億ly
屬於	英仙座星系團
其他	為暗星系
	參考: https://science.nasa.gov/asset/hubble/dark-galaxy-cdg-2-near-perseus-cluster/
	參見：星系、星系列表

候選暗星系2號（英語：Candidate Dark Galaxy 2，CDG-2）位於英仙座星系團，主要由暗物質構成，另有四個球狀星團。^[1]它是迄今發現暗物質占比最高的星系之一，估計暗物質逾99%。^[2]

發現

CDG-2由哈伯太空望遠鏡英仙座星系團巡天計畫PIPER發現。該計畫原為尋找球狀星團密度異常偏高的區域，因為這類區域可能對應超稀疏星系。2025年3月，研究人員採用一種新統計方法，即泊松簇過程（由內曼—斯科特過程改造而來）展開搜尋，並將CDG-2識別為一顆可能存在的星系。^[3]2025年6月，研究人員檢視哈伯太空望遠鏡、歐幾里得太空望遠鏡與昴星團望遠鏡圖像後^[4]，在CDG-2球狀星團周圍發現彌散輻射，因而有力證明其為星系。它也是首個憑藉球狀星團蹤跡發現的星系。^[5]

概述

與迄今發現的大多數超瀰漫星系不同，CDG-2據信只有最初識別出的四個球狀星團，另有少數可能的球狀星團仍待進一步查證。其暗物質暈質量估計約為 $5.7\times 10^{10}M_{\odot }$ ，可見這是一座暗物質占絕對優勢的星系，暈質量占比達99.94%至99.98%。^[5]這座星系的存在，或可成為檢驗多種星系形成理論的有力例證。^[6]暗星系是ΛCDM模型所預言的一類天體，其多寡因而成為檢驗這一理論的重要依據。相關豐度模擬也常以CDG-2和Cloud-9為參照。^[7]

參考文獻

^ Dark Galaxy CDG-2 Near Perseus Cluster - NASA Science. 2026-02-18 [2026-02-21] （美國英語）.
^ Lea, Robert. Hubble telescope discovers rare galaxy that is 99% dark matter. Space. 2026-02-19 [2026-02-21] （英語）.
^ Li, Dayi; Stringer, Alex; Eadie, Gwendolyn; Abraham, Roberto. Poisson cluster process models for detecting ultra-diffuse galaxies. Annals of Applied Statistics. March 2025, 19 (1): 261–285. doi:10.1214/24-AOAS1958 –透過Project Euclid.
^ information@eso.org. Hubble identifies one of darkest known galaxies - The elusive object dubbed CDG-2 may be composed of 99% dark matter. www.esahubble.org. [2026-02-21] （英語）.
^ ^5.0 ^5.1 Li 李, Dayi (David) 大一; Liu 劉, Qing 青; Eadie, Gwendolyn M.; Abraham, Roberto G.; Marleau, Francine R.; Harris, William E.; van Dokkum, Pieter; Romanowsky, Aaron J.; Danieli, Shany; Brown, Patrick E.; Stringer, Alex. Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster. The Astrophysical Journal Letters. 2025-06-16, 986 (2): L18. Bibcode:2025ApJ...986L..18L. ISSN 2041-8205. arXiv:2506.15644 . doi:10.3847/2041-8213/adddab .
^ Garay, Jorge. A Galaxy Composed Almost Entirely of Dark Matter Has Been Confirmed. Wired. [2026-02-21]. ISSN 1059-1028 （美國英語）.
^ 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. HI-bearing dark galaxies predictions from constrained Local Group simulations: how many and where to find them. 2026-01-07. arXiv:2601.04024  [astro-ph.GA].

[1] Dark Galaxy CDG-2 Near Perseus Cluster - NASA Science. 2026-02-18 [2026-02-21] （美國英語）.

[2] Lea, Robert. Hubble telescope discovers rare galaxy that is 99% dark matter. Space. 2026-02-19 [2026-02-21] （英語）.

[3] Li, Dayi; Stringer, Alex; Eadie, Gwendolyn; Abraham, Roberto. Poisson cluster process models for detecting ultra-diffuse galaxies. Annals of Applied Statistics. March 2025, 19 (1): 261–285. doi:10.1214/24-AOAS1958 –透過Project Euclid.

[4] rmation@eso.org. Hubble identifies one of darkest known galaxies - The elusive object dubbed CDG-2 may be composed of 99% dark matter. www.esahubble.org. [2026-02-21] （英語）.

[:0-5] 5.0 ^5.1 Li 李, Dayi (David) 大一; Liu 劉, Qing 青; Eadie, Gwendolyn M.; Abraham, Roberto G.; Marleau, Francine R.; Harris, William E.; van Dokkum, Pieter; Romanowsky, Aaron J.; Danieli, Shany; Brown, Patrick E.; Stringer, Alex. Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster. The Astrophysical Journal Letters. 2025-06-16, 986 (2): L18. Bibcode:2025ApJ...986L..18L. ISSN 2041-8205. arXiv:2506.15644 . doi:10.3847/2041-8213/adddab .

[6] Garay, Jorge. A Galaxy Composed Almost Entirely of Dark Matter Has Been Confirmed. Wired. [2026-02-21]. ISSN 1059-1028 （美國英語）.

[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. HI-bearing dark galaxies predictions from constrained Local Group simulations: how many and where to find them. 2026-01-07. arXiv:2601.04024  [astro-ph.GA].

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