Reticulum II

Observation data (J2000 epoch)
Constellation| Reticulum
Right ascension| 03h 35m 42.14s[1]
Declination| −54° 2′ 57.1″[1]
Distance| 103 kly (31.6 kpc)[2]
Apparent magnitude (V)| 17.4[1]
Characteristics
Apparent size (V)| 3.37′ × 3.64′[1]
Notable features| Enriched in r-process elements
Other designations
Reticulum II, Reticulum 2[1]

Reticulum II (or Reticulum 2) is a dwarf galaxy in the Local Group. Reticulum II was discovered in 2015 by analysing images from the Dark Energy Survey. It is a satellite of the Magellanic Clouds and was probably captured relatively recently.[3] Like other dwarf spheroidal galaxies, its stellar population is old: the galaxy was quenched before 11.5 billion years ago.[3]

Reticulum II is elongated, having an major/minor axis ratio of 0.6. The size is given by a half-light radius of 15 parsecs (pc). This is too large for it to be a globular cluster. The absolute magnitude (MV) of the galaxy is −2.7. The distance from Earth is about 30 kpc.[2] The galaxy contains some blue horizontal branch stars. Other features visible are a main sequence, and a main sequence turn off, and a red giant branch.[1][4] It has an unusual enhancement of r-process elements, meaning that gold and europium are enriched in the brightest stars in the galaxy.[5] About 72% of its stars are enriched in r-process elements.[2] The implication of the unusual enrichment in elements heavier than zinc, is that the r-process is very rare, and only happened once in this galaxy, possibly by the collision of two neutron stars.[6]

Gamma rays mostly with energies between 2 and 10 GeV have been detected by the Fermi satellite.[7] The radiation from Reticulum II is more significant than that of other dwarf galaxy emissions.[8] However this finding has been contested.[9]

## References[edit]

1. ^ a b c d e f Sergey E. Koposov; Vasily Belokurov; Gabriel Torrealba; N. Wyn Evans (10 March 2015). "Beasts of the Southern Wild. Discovery of a large number of Ultra Faint satellites in the vicinity of the Magellanic Clouds". The Astrophysical Journal. 805 (2): 130. arXiv:1503.02079. Bibcode:2015ApJ...805..130K. doi:10.1088/0004-637X/805/2/130. S2CID 118267222.
2. ^ a b c Simon, Joshua D.; et al. (2023). "Timing the r-process Enrichment of the Ultra-faint Dwarf Galaxy Reticulum II". The Astrophysical Journal. 944 (1): 43. arXiv:2212.00810. Bibcode:2023ApJ...944...43S. doi:10.3847/1538-4357/aca9d1. S2CID 254221028.
3. ^ a b Sacchi, Elena; Richstein, Hannah; Kallivayalil, Nitya; Van Der Marel, Roeland; Libralato, Mattia; Zivick, Paul; Besla, Gurtina; Brown, Thomas M.; Choi, Yumi; Deason, Alis; Fritz, Tobias; Geha, Marla; Guhathakurta, Puragra; Jeon, Myoungwon; Kirby, Evan; Majewski, Steven R.; Patel, Ekta; Simon, Joshua D.; Tony Sohn, Sangmo; Tollerud, Erik; Wetzel, Andrew (2021). "Star Formation Histories of Ultra-faint Dwarf Galaxies: Environmental Differences between Magellanic and Non-Magellanic Satellites?". The Astrophysical Journal Letters. 920 (1): L19. arXiv:2108.04271. Bibcode:2021ApJ...920L..19S. doi:10.3847/2041-8213/ac2aa3. S2CID 236965965.
4. ^ DES Collaboration (10 March 2015). "Eight New Milky Way Companions Discovered in First-Year Dark Energy Survey Data". The Astrophysical Journal. 807 (1): 50. arXiv:1503.02584. Bibcode:2015ApJ...807...50B. doi:10.1088/0004-637X/807/1/50. S2CID 12011753.
5. ^ Ji, Alex (22 March 2016). "The Origin of the Cosmos' Heaviest Elements". Phys.org. Retrieved 24 March 2016.
6. ^ Ji, Alexander P.; Frebel, Anna; Chiti, Anirudh; Simon, Joshua D. (21 March 2016). "R-process enrichment from a single event in an ancient dwarf galaxy". Nature. 531 (7596): 610–613. arXiv:1512.01558. Bibcode:2016Natur.531..610J. doi:10.1038/nature17425. PMID 27001693. S2CID 205248401.
7. ^ Hooper, Dan; Linden, Tim (3 September 2015). "On The gamma-ray emission from Reticulum II and other dwarf galaxies". Journal of Cosmology and Astroparticle Physics. 2015 (9): 016. arXiv:1503.06209. Bibcode:2015JCAP...09..016H. doi:10.1088/1475-7516/2015/09/016. S2CID 118482641.
8. ^ Geringer-Sameth, Alex; Walker, Matthew G.; Koushiappas, Savvas M.; Koposov, Sergey E.; Belokurov, Vasily; Torrealba, Gabriel; Evans, N. Wyn (17 August 2015). "Indication of Gamma-Ray Emission from the Newly Discovered Dwarf Galaxy Reticulum II". Physical Review Letters. 115 (8): 081101. arXiv:1503.02320. Bibcode:2015PhRvL.115h1101G. doi:10.1103/PhysRevLett.115.081101. PMID 26340176. S2CID 16494335.
9. ^ Overbye, Dennis (10 March 2015). "Gamma Rays May Be Clue on Dark Matter". The New York Times. Retrieved 20 October 2016.

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