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  • 21 Aug, 2019

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Willman 1
Willman 1 is an ultra low-luminosity dwarf galaxy or a star cluster. Willman 1 was discovered in 2004 and is located near Ursa Major in the night sky. It is named after Beth Willman of Haverford College, the lead author of a study based on the Sloan Digital Sky Survey data. The object is a satellite of the Milky Way, at ~120,000 light-years away. Willman 1 has an elliptical shape with the half-light radius of about 25 pc. Its heliocentric velocity is approximately −13 km/s.

As of 2007, it was declared the least massive galaxy known, opening up a new category of ultra-low-mass galaxies, lower than the then-theoretical minimum of 10 million solar masses thought to be needed to form a galaxy.

As of 2016, it is the third dimmest likely galaxy known, after Segue 1 and Virgo I, and is over ten million times less luminous than the Milky Way. It has an absolute magnitude of −2.7 ± 0.7. Observations indicate its mass is about 0.4 million solar masses, which means that Willman's 1 mass to light ratio is around 800. A high mass to light ratio implies that Willman 1 is dominated by dark matter. It is difficult, however, to estimate the mass of such faint objects because any mass estimate is based on an implicit assumption that an object is gravitationally bound, which may not be true if the object is in a process of disruption.

The stellar population of Willman 1 consists mainly of old stars formed more than 10 billion years ago. The metallicity of these stars is also very low at [Fe/H] ≈ −2.1, which means that they contain 110 times less heavy elements than the Sun.

Notes

a. 15.2 ± 0.4 apparent magnitude − 5 * (log10(38 ± 7 kpc distance) − 1) = −2.7 absolute magnitude

References

  1. ^ (Willman et al. 2005)
  2. ^ Martin, Nicolas F.; de Jong, Jelte T. A.; Rix, Hans-Walter (10 September 2008). "A Comprehensive Maximum Likelihood Analysis of the Structural Properties of Faint Milky Way Satellites". The Astrophysical Journal. 684 (2): 1075–1092. arXiv:0805.2945. Bibcode:2008ApJ...684.1075M. doi:10.1086/590336. ISSN 0004-637X.
  3. ^ Willman, Beth; Geha, Marla; Strader, Jay; Strigari, Louis E.; Simon, Joshua D.; Kirby, Evan; Ho, Nhung; Warres, Alex (1 October 2011). "WILLMAN 1—A PROBABLE DWARF GALAXY WITH AN IRREGULAR KINEMATIC DISTRIBUTION". The Astronomical Journal. 142 (4): 128. arXiv:1007.3499. Bibcode:2011AJ....142..128W. doi:10.1088/0004-6256/142/4/128. ISSN 0004-6256.
  4. ^ Britt, Robert Roy (25 October 2004). "Puzzling Milky Way Companion Found". Space.com. Retrieved 18 September 2024.
  5. ^ Zucker, D. B.; Belokurov, V.; Evans, N. W.; Kleyna, J. T.; Irwin, M. J.; Wilkinson, M. I.; Fellhauer, M.; Bramich, D. M.; Gilmore, G.; Newberg, H. J.; Yanny, B.; Smith, J. A.; Hewett, P. C.; Bell, E. F.; Rix, H.-W. (10 October 2006). "A Curious Milky Way Satellite in Ursa Major". The Astrophysical Journal. 650 (1): L41–L44. arXiv:astro-ph/0606633. Bibcode:2006ApJ...650L..41Z. doi:10.1086/508628. ISSN 0004-637X.
  6. ^ Kanipe, Jeff (2006). Chasing Hubble's shadows: the search for galaxies at the edge of time. New York: Hill and Wang. p. 153. ISBN 978-0-8090-3406-2. Retrieved 18 September 2024.
  7. ^ Willman, Beth; Blanton, Michael R.; West, Andrew A.; Dalcanton, Julianne J.; Hogg, David W.; Schneider, Donald P.; Wherry, Nicholas; Yanny, Brian; Brinkmann, Jon (2005). "A New Milky Way Companion: Unusual Globular Cluster or Extreme Dwarf Satellite?". The Astronomical Journal. 129 (6): 2692–2700. arXiv:astro-ph/0410416. Bibcode:2005AJ....129.2692W. doi:10.1086/430214. ISSN 0004-6256.
  8. ^ Shiga, David (4 June 2007). "Smallest galaxy hints at hidden population". New Scientist. Retrieved 18 September 2024.
  9. ^ Loewenstein, Michael; Kusenko, Alexander (1 June 2012). "DARK MATTER SEARCH USING XMM-NEWTON OBSERVATIONS OF WILLMAN 1". The Astrophysical Journal. 751 (2): 82. arXiv:1203.5229. Bibcode:2012ApJ...751...82L. doi:10.1088/0004-637X/751/2/82. ISSN 0004-637X.
  10. ^ McDaniel, Alex; Ajello, Marco; Karwin, Christopher M.; Di Mauro, Mattia; Drlica-Wagner, Alex; Sánchez-Conde, Miguel A. (19 March 2024). "Legacy analysis of dark matter annihilation from the Milky Way dwarf spheroidal galaxies with 14 years of Fermi -LAT data". Physical Review D. 109 (6): 063024. arXiv:2311.04982. Bibcode:2024PhRvD.109f3024M. doi:10.1103/PhysRevD.109.063024. ISSN 2470-0010.
  11. ^ "Seven. The Experimental Hunt for Dark Matter Particles", The Cosmic Cocktail: Three Parts Dark Matter, Princeton University Press, pp. 128–146, 31 December 2014, doi:10.1515/9781400850075-008, ISBN 978-1-4008-5007-5, retrieved 18 September 2024
Preceded by
 Least massive galaxy known
2007 – 2013
500,000MSun 		Succeeded by
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