Dragonfly 44

Dragonfly 44 is an ultra diffuse galaxy in the Coma Cluster.[2][3][1][4] This galaxy is well-known because observations of the velocity dispersion in 2016 suggested a mass of about one trillion solar masses, about the same as the Milky Way. This mass was also consistent with a count of about 90 and 70 globular clusters observed around Dragonfly 44 in two different studies.

Dragonfly 44
Hubble Space Telescope view of the galaxy Dragonfly 44
Observation data
ConstellationComa Berenices
Right ascension13h 00m 58.0s[1]
Declination+26° 58′ 35″[1]
Redshift~0.023
Helio radial velocity6280 ± 120 km/s
Distance~100 Mpc (~330 Mly)
Group or clusterComa Cluster
Apparent magnitude (V)21 mag; or 19.4 mag [1]
Apparent magnitude (B)22
Absolute magnitude (V)-16.1 mag [1]
Characteristics
Mass~1.6×1011 M☉
Apparent size (V)10 x 35 arcsec
Other designations
SDSS J130057.98+265839.6, SDSS J130058.17+265836.1, SDSS J130058.21+265829.3

Later, spatially resolved kinematics measured a mass of about 160 billion solar masses, six times less than early mass measurements and 1 order of magnitude less than the Milky Way’s mass.[5] The most recent work found 20 globular clusters around the galaxy, which is consistent with the recent mass measurement.[6][7] The lack of X-ray emission from the galaxy and surroundings also show that the number of globular clusters cannot be as many as was claimed before.[8]

The galaxy emits only 1% of the light emitted by the Milky Way.[9] The galaxy was discovered with the Dragonfly Telephoto Array.[10]

Early study

To determine the amount of dark matter in this galaxy, in 2016, astronomers used the DEIMOS instrument installed on Keck II to measure the velocities of stars for 33.5 hours over a period of six nights so they could determine the galaxy's mass. The scientists then used the Gemini Multi-Object Spectrograph on the 8-m Gemini North telescope to reveal a halo of spherical clusters of stars around the galaxy's core.[11] Following this observation, in August 2016, astronomers reported that this galaxy might be made almost entirely of dark matter.[12][13][14]

In 2020, a much lower estimate of the number of globular clusters (20 rather than 80) reduced the estimated dark matter to more normal proportions.[15][16]

See also

References
  1. Van Dokkum, Pieter; et al. (1 May 2015). "Spectroscopic confirmation of the existence of large, diffuse galaxies in the coma cluster". The Astrophysical Journal Letters. 804 (1): L26. arXiv:1504.03320. Bibcode:2015ApJ...804L..26V. doi:10.1088/2041-8205/804/1/L26. S2CID 54664026.
  2. "Scientists discover the fluffiest galaxies". phys.org. 14 May 2015.
  3. Van Dokkum, Pieter; et al. (7 January 2015). "Forty-seven milky way-sized, extremely diffuse galaxies in the Coma-Cluster". The Astrophysical Journal Letters. 798 (2): L45. arXiv:1410.8141. Bibcode:2015ApJ...798L..45V. doi:10.1088/2041-8205/798/2/L45. S2CID 119279968.
  4. Dragonfly 44: Ultra-Diffuse Galaxy Made Mostly of Dark Matter. Aug 2016
  5. Van Dokkum, Pieter; Wasserman, Asher; Danieli, Shany; Abraham, Roberto; Brodie, Jean; Conroy, Charlie; Forbes, Duncan A.; Martin, Christopher; Matuszewski, Matt; Romanowsky, Aaron J.; Villaume, Alexa (2019). "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. 880 (2): 91. doi:10.3847/1538-4357/ab2914. S2CID 119185644.
  6. ""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. arXiv:2006.14630. doi:10.1093/mnras/staa3016. S2CID 220127887.
  7. "The puzzle of the strange galaxy made of 99.99% dark matter is solved".
  8. ""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. arXiv:2009.07846. doi:10.3847/2041-8213/abb886. S2CID 221761669.
  9. Crosswell, Ken (26 July 2016). "The Milky Way's dark twin revealed". Nature News. Retrieved 30 July 2016.
  10. Rachel Feltman (25 August 2016). "A new class of galaxy has been discovered, one made almost entirely of dark matter". Washington Post.
  11. http://earthsky.org/space/dragonfly-44-dark-matter-galaxy-2016
  12. 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. S2CID 1275440.
  13. Hall, Shannon (25 August 2016). "Ghost galaxy is 99.99 per cent dark matter with almost no stars". New Scientist. Retrieved 27 August 2016.
  14. 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.
  15. The puzzle of the strange galaxy made almost entirely of dark matter Oct 2020
  16. The number of globular clusters around the iconic UDG DF44 is as expected for dwarf galaxies


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