Ultra diffuse galaxy

An ultra diffuse galaxy (UDG) is an extremely low luminosity galaxy, the first example of which was discovered in the nearby Virgo Cluster by Allan Sandage and Bruno Binggeli in 1984.[lower-alpha 1] These galaxies have been studied for many years prior to their renaming in 2015. Their lack of luminosity is due to the lack of star-forming gas, which results in these galaxies being reservoirs of very old stellar populations.[2][3]

NGC 1052-DF2, an ultra diffuse galaxy.

Based on discoveries confirmed in 2018, this class of galaxies includes both extremes of dark matter content: Some UDGs consist almost entirely of dark matter (such a galaxy may have the same size and mass as the Milky Way but a visible star count of only 1%),[4] while other UDGs are almost entirely free of dark matter.[5]

Examples

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 emission form the galaxy and surrounding 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 alternate gravity theories like modified Newtonian dynamics, but leaves other theories such as the external field effect still possible.

See also

Footnotes
  1. UDGs are a subset of dwarf spheroidals and dwarf ellipticals and as such are a redundant name for an already well studied galaxy type.[1]
  2. NGC 1052-DF2 had previously been identified on photographic plates by Karachentsev et al. in 2000.[14]

References
  1. Conselice, Christopher J. (March 2018). "Ultra-diffuse galaxies are a subset of cluster dwarf elliptical/spheroidal galaxies". Research Notes of the AAS. 2 (1): 43. arXiv:1803.06927. Bibcode:2018RNAAS...2...43C. doi:10.3847/2515-5172/aab7f6. ISSN 2515-5172.
  2. "Astronomers discover 854 ultra-dark galaxies in the famous Coma Cluster". Science Daily. 22 June 2015.
  3. "Scientists discover the fluffiest galaxies". phys.org. 14 May 2015.
  4. Hall, Shannon (25 August 2016). "Ghost galaxy is 99.99 per cent dark matter with almost no stars". New Scientist. Retrieved 27 August 2016.
  5. van Dokkum; et al. (2018). "A galaxy lacking dark matter". Nature. 555 (7698): 629–632. arXiv:1803.10237. doi:10.1038/nature25767. PMID 29595770.
  6. "Scientists at Keck discover the fluffiest galaxies". Space Daily. 18 May 2015.
  7. Crosswell, Ken (26 July 2016). "The Milky Way's dark twin revealed". Nature News. Retrieved 30 July 2016.
  8. van Dokkum, Pieter; et al. (25 August 2016). "A high stellar velocity dispersion and ~100 globular clusters for the ultra-diffuse galaxy Dragonfly 44". The Astrophysical Journal Letters. 828 (1): L6. arXiv:1606.06291. Bibcode:2016ApJ...828L...6V. doi:10.3847/2041-8205/828/1/L6.
  9. Feltman, Rachael (26 August 2016). "A new class of galaxy has been discovered, one made almost entirely of dark matter". The Washington Post. Retrieved 26 August 2016.
  10. "Spatially Resolved Stellar Kinematics of the Ultra-diffuse Galaxy Dragonfly 44. I. Observations, Kinematics, and Cold Dark Matter Halo Fits, Peter van Dokkum et al. 2019". The Astrophysical Journal. doi:10.3847/1538-4357/ab2914/pdf.
  11. ""The number of globular clusters around the iconic UDG DF44 is as expected for dwarf galaxies", Saifollahi et al. 2020". Monthly Notices of the Royal Astronomical Society.
  12. "The puzzle of the strange galaxy made of 99.99% dark matter is solved".
  13. ""The American Astronomical Society, find out more The Institute of Physics, find out more The Archetypal Ultra-diffuse Galaxy, Dragonfly 44, Is not a Dark Milky Way", Bogdan 2020". The Astrophysical Journal Letters.
  14. Karachentsev, I.D.; Karachentseva, V.E.; Suchkov, A.A.; Grebel, E.K. (2000). "Dwarf galaxy candidates found on the SERC EJ sky survey". Astron. Astrophys. Suppl. Ser. 145 (3): 415–423. doi:10.1051/aas:2000249.
  15. Parks, Jake (21 October 2019). "Hubble reveals new evidence for controversial galaxies without dark matter". Discover Magazine.

Further reading
  • 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.
  • 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.
  • 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.
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