Ultra diffuse galaxy

Some ultra diffuse galaxies found in the Coma Cluster, about 330 million light years from Earth, have diameters of 60 kly (18 kpc) with 1% of the stars of the Milky Way Galaxy.[6] The distribution of ultra diffuse galaxies in the Coma Cluster is the same as luminous galaxies; this suggests that the cluster environment strips the gas from the galaxies, while allowing them to populate the cluster the same as more luminous galaxies. The similar distribution in the higher tidal force zones suggests a larger dark matter fraction to hold the galaxies together under the higher stress.[2]

Dragonfly 44, an ultra diffuse galaxy in the Coma Cluster, is one example.[3] Observations of its rotational speed suggest a mass of about one trillion solar masses, about the same as the mass of the Milky Way. This is also consistent with about 90 globular clusters observed around Dragonfly 44. However, the galaxy emits only 1% of the light emitted by the Milky Way.[7] On 25 August 2016, astronomers reported that Dragonfly 44 may be made almost entirely of dark matter.[8][4][9] However, later, spatially resolved kinematics measured a mass of about 160 billion solar mass, six times less than early mass measurements and 1 order of magnitude less than the Milky Way mass.[10] The most recent work found 20 globular clusters around the galaxy, which is consistent with the recent mass measurement.[11][12] The lack of X-ray emissions from the galaxy and surrounding area also show that the number of globular clusters can not be as many as was claimed before.[13]

In 2018, the same authors reported the discovery that the ultra diffuse galaxy NGC 1052-DF2[lower-alpha 2] is dark matter-free, based on velocity measurements of its ~10 globular cluster system.[15][5] They concluded that this may rule out some alternative gravity theories like modified Newtonian dynamics, but leaves other theories, such as the external field effect still possible. Detailed simulations in the framework of Modified Newtonian Dynamics confirm that NGC 1052-DF2 is quite consistent with theoretical expectations.[16]

In 2021, AGC 114905, an ultra-diffuse dwarf galaxy about 250 million light-years away, was reported to have almost no dark matter.[17] However, this conclusion relies heavily on the galaxy having a moderate inclination of 32° between disc and sky planes, which is estimated from the somewhat oval appearance. Using detailed simulations of AGC 114905 in the alternative gravity theory known as Modified Newtonian Dynamics, it was shown that a disc galaxy with its properties can appear slightly oval even if viewed face-on due to disc self-gravity, in which case the rotation curve could be much higher and the galaxy could be quite consistent with theoretical expectations.[18] An overestimated inclination is unlikely if galaxies are dominated by dark matter because then the disc is not self-gravitating, so it should be close to circular when viewed face-on.[19]

• Dark galaxy – A hypothesized galaxy with no, or very few, stars
• Low-surface-brightness galaxy, also known as LSBG – Galaxy which is less bright than the ambient night skyPages displaying short descriptions of redirect targets
• Type-cD galaxy – Galaxy morphology classification or c-Diffuse galaxy type
• Type-D galaxy – System for categorizing galaxies based on appearancePages displaying short descriptions of redirect targets or Diffuse-type galaxy

• Beasley, Michael A.; Romanowsky, Aaron J.; Pota, Vincenzo; Navarro, Ignacio Martin; Delgado, David Martinez; Neyer, Fabian; Deich, Aaron (1 February 2016). "An overmassive dark halo around an ultra-diffuse galaxy in the Virgo cluster". The Astrophysical Journal Letters (published 1 March 2016). 819 (2): L20. arXiv:1602.04002. Bibcode:2016arXiv160204002V. doi:10.3847/2041-8205/819/2/L20. S2CID 7783759.
• van Dokkum, Pieter G.; Romanowsky, Aaron J.; Abraham, Roberto; Brodie, Jean P.; Conroy, Charlie; Geha, Marla; Merritt, Allison; Villaume, Alexa; Zhang, Jielai (6 February 2015). "Spectroscopic confirmation of the existence of large, diffuse galaxies in the Coma cluster". The Astrophysical Journal Letters (published 1 May 2015). 804 (1): L26. arXiv:1504.03320. Bibcode:2015ApJ...804L..26V. doi:10.1088/2041-8205/804/1/L26. S2CID 54664026.
• Koda, Jin; Yagi, Masafumi; Yamanoi, Hitomi; Komiyama, Yutaka (4 June 2015). "Approximately a thousand ultra diffuse galaxies in the Coma cluster". The Astrophysical Journal Letters. 807 (1): L2. arXiv:1506.01712. Bibcode:2015ApJ...807L...2K. doi:10.1088/2041-8205/807/1/L2. S2CID 35812655.