Fornax Dwarf Spheroidal
The Fornax dwarf galaxy
Observation data (J2000 epoch)
ConstellationFornax
Right ascension02h 39m 59.3s[1]
Declination−34° 26′ 57″[1]
Redshift53 ± 9 km/s[1]
Distance466 ± 10 kly (143 ± 3 kpc)[2]
Apparent magnitude (V)9.3[1]
Characteristics
TypedE0[1]
Apparent size (V)17.0′ × 12.6′[1]
Notable featureshas 6 globular clusters
Other designations
Fornax dSph,[3] Fornax Dwarf Elliptical,[3] Fornax Dwarf Galaxy,[3] Fornax dE,[3] PGC 10074 / 10093,[1] ESO 356-4

The Fornax Dwarf Spheroidal (formerly known as the Fornax System) is an elliptical dwarf galaxy in the constellation Fornax that was discovered in 1938 by Harlow Shapley. He discovered it while he was in South Africa on photographic plates taken by the 24 inch (61 cm) Bruce refractor at Boyden Observatory, shortly after he discovered the Sculptor Dwarf Galaxy.[4]

The galaxy is a satellite of the Milky Way and contains six globular clusters, an unusually high number for its size;[5] the largest, NGC 1049, was discovered before the galaxy itself. The galaxy is also receding from the Milky Way at 53 km/s. It mostly contains population II stars, but also has populations of young and intermediate age.[2]

Globular clusters

Using the Hubble Space Telescope, scientists derived a color-magnitude diagram for Fornax 4, a globular cluster within this galaxy. Unlike the globular clusters Fornax 1, 2, 3, and 5, which have horizontal branches across a wide range of colors and include RR Lyrae variables, Fornax 4 is found to have only red in its horizontal branch. Fornax 4 is also ~3 Gyr younger than the other globular clusters. The color-magnitude diagram of Fornax 4 has a strong similarity to "young" galactic globular Ruprecht 106.[6]

It has been debated for some time whether the globular cluster Fornax 6 is a true member of the Fornax Dwarf, or merely a chance alignment of stars and/or galaxies.[5] A 2021 study found it to be a true cluster and a member, but it is notably more metal-rich and therefore likely younger than the other clusters. It is estimated to be about 2 billion years old.[5]

It is not known why spheroidals allow the formation of globular clusters like Fornax 4 and Terzan 7 (of Sagittarius dwarf) long after globular clusters ceased to form in the main body of the Galactic halo. Another possibility is that "young" globular clusters of the outer halo like Ruprecht 106 were originally formed in now defunct dwarf spheroidals.[7]

References

  1. 1 2 3 4 5 6 7 "NASA/IPAC Extragalactic Database". Results for Fornax Dwarf Spheroidal. Retrieved 2006-11-29.
  2. 1 2 Oakes, Elias K.; Hoyt, Taylor J.; Freedman, Wendy L.; Madore, Barry F.; Tran, Quang H.; Cerny, William; Beaton, Rachael L.; Seibert, Mark (2022). "Distances to Local Group Galaxies via Population II, Stellar Distance Indicators. II. The Fornax Dwarf Spheroidal". The Astrophysical Journal. 929 (2): 116. arXiv:2204.09699. Bibcode:2022ApJ...929..116O. doi:10.3847/1538-4357/ac5b07. S2CID 248260222.
  3. 1 2 3 4 "NAME Fornax Dwarf Spheroidal". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2006-11-29.
  4. ↑ Shapley H (1938). "Two Stellar Systems of a New Kind". Nature. 142 (3598): 715–6. Bibcode:1938Natur.142..715S. doi:10.1038/142715b0. S2CID 4071472.
  5. 1 2 3 Pace, Andrew B.; Walker, Matthew G.; Koposov, Sergey E.; Caldwell, Nelson; Mateo, Mario; Olszewski, Edward W.; Bailey Iii, John I.; Wang, Mei-Yu (2021). "Spectroscopic Confirmation of the Sixth Globular Cluster in the Fornax Dwarf Spheroidal Galaxy". The Astrophysical Journal. 923 (1): 77. arXiv:2105.00064. Bibcode:2021ApJ...923...77P. doi:10.3847/1538-4357/ac2cd2. S2CID 233481757.
  6. ↑ Buonanno, R.; et al. (1 October 1999). "HST photometry of the fornax dSph galaxy: cluster 4 and its field". The Astronomical Journal. 118 (4): 1671–1683. arXiv:astro-ph/9907073. Bibcode:1999astro.ph..7073B. doi:10.1086/301034. S2CID 117753848.
  7. ↑ van den Bergh, Sidney (April 2000). "Updated Information on the Local Group". The Publications of the Astronomical Society of the Pacific. 112 (770): 529–536. arXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. doi:10.1086/316548. S2CID 1805423.


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