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Pisces II (Psc II) is a dwarf spheroidal galaxy situated in the Pisces constellation and discovered in 2010 in the data obtained by the Sloan Digital Sky Survey. The galaxy is located at the distance of about 180 kpc (kiloparsecs) from the Sun. It is classified as a dwarf spheroidal galaxy (dSph) meaning that it has an elongated shape with the half-light radius of about 60 pc and ratio of the axis of about 5:3.

Pisces II is one of the smallest and faintest satellites of the Milky Way—its integrated luminosity is about 10,000 times that of the Sun (absolute magnitude of about −5), which corresponds to the luminosity of an average globular cluster. The stellar population of Pisces II consists mainly of moderately old stars formed 10–12 billion years ago. The metallicity of these old stars is low at −2.3 < < −1.7, which means that the percentage of their mass that consists of "heavy metals" is no more than ⁠1/80⁠ of the corresponding percentage in the Sun.

In 2016, follow-up work on Pegasus III highlighted that both it and Pisces II lie relatively close to each other (within approximately 43 kpc) and share similar radial velocities in the Galactic standard of rest frame (note: this is not the same as the LSR). This suggests that these two satellite galaxies may actually be associated with one another, and there is also a possibility that there was an even closer passage about 1 billion years ago. However, further spectroscopic measurements are required to confirm their association.

Notes

1. ^ Andromeda II was also given the alias Pisces II by Martin et al. (2009), who also proposed aliases for several other satellite galaxies of the Andromeda Galaxy However, that name was later used by a different group unaware of these names, for this object.
2. In astronomy and physical cosmology, unlike other physical sciences, "heavy metals" refers to all elements except hydrogen and helium.

References

1. ^ Belokurov, V.; Walker, M. G.; Evans, N. W.; Gilmore, G.; Irwin, M. J.; Just, D.; Koposov, S.; Mateo, M.; Olszewski, E.; Watkins, L.; Wyrzykowski, L. (2010). "Big Fish, Little Fish: Two New Ultra-Faint Satellites of the Milky Way". The Astrophysical Journal. 712 (1): L103–L106. arXiv:1002.0504. Bibcode:2010ApJ...712L.103B. doi:10.1088/2041-8205/712/1/L103. S2CID 29195107.
2. ^ Richstein, Hannah; Patel, Ekta; Kallivayalil, Nitya; Simon, Joshua D.; Zivick, Paul; Tollerud, Erik; Fritz, Tobias; Warfield, Jack T.; Besla, Gurtina; Van Der Marel, Roeland P.; Wetzel, Andrew; Choi, Yumi; Deason, Alis; Geha, Marla; Guhathakurta, Puragra; Jeon, Myoungwon; Kirby, Evan N.; Libralato, Mattia; Sacchi, Elena; Sohn, Sangmo Tony (2022). "Structural Parameters and Possible Association of the Ultra-faint Dwarfs Pegasus III and Pisces II from Deep Hubble Space Telescope Photometry". The Astrophysical Journal. 933 (2): 217. arXiv:2204.01917. Bibcode:2022ApJ...933..217R. doi:10.3847/1538-4357/ac7226. S2CID 247957904.
3. "Object No. 1 - PISCES II". NASA/IPAC Extragalactic Database.
4. "NAME Pisces II -- Galaxy". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 24 March 2018.
5. Martin, Nicolas F.; McConnachie, Alan W.; Irwin, Mike; Widrow, Lawrence M.; Ferguson, Annette M. N.; Ibata, Rodrigo A.; Dubinski, John; Babul, Arif; et al. (1 November 2009). "PAndAS' CUBS: Discovery of Two New Dwarf Galaxies in the Surroundings of the Andromeda and Triangulum Galaxies". The Astrophysical Journal. 705 (1): 758–765. arXiv:0909.0399. Bibcode:2009ApJ...705..758M. doi:10.1088/0004-637X/705/1/758. S2CID 15277245.
6. Kim, Dongwon; Jerjen, Helmut; Geha, Marla; Chiti, Anirudh; Milone, Antonino P.; Mackey, Dougal; da Costa, Gary; Frebel, Anna; Conn, Blair (2016). "Portrait of a Dark Horse: A Photometric and Spectroscopic Study of the Ultra-Faint Milky Way Satellite Pegasus III". The Astrophysical Journal. 833 (1): 16. arXiv:1608.04934. Bibcode:2016ApJ...833...16K. doi:10.3847/0004-637X/833/1/16. S2CID 73677994.

Further reading

• Sand, David J; Strader, Jay; Willman, Beth; Zaritsky, Dennis; McLeod, Brian; Caldwell, Nelson; Seth, Anil; Olszewski, Edward (2012). "Tidal Signatures in the Faintest Milky Way Satellites: The Detailed Properties of Leo V, Pisces Ii, and Canes Venatici Ii". The Astrophysical Journal. 756 (1): 79. arXiv:1111.6608. Bibcode:2012ApJ...756...79S. doi:10.1088/0004-637X/756/1/79. S2CID 46030862.

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