Reticulum II

Short description: Dwarf galaxy

Coordinates:  02h 35m 42.14s, −54° 2′ 57.1″

Reticulum II

Observation data (J2000 epoch)
Constellation| Reticulum
Right ascension|  03h 35m 42.14s[1]
Declination| −54° 2′ 57.1″[1]
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.[2] Like other dwarf spheroidal galaxies, its stellar population is old: the galaxy was quenched before 11.5 billion years ago.[2]

Reticulum II is elongated, having a 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.[3] 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.[3] 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]

It has been named as a "globular-cluster-like dwarf".[10]

## References

1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 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. doi:10.1088/0004-637X/805/2/130. Bibcode: 2015ApJ...805..130K.
2. ↑ 2.0 2.1 Sacchi, Elena; Richstein, Hannah; Kallivayalil, Nitya; Van Der Marel, Roeland; Libralato, Mattia; Zivick, Paul; Besla, Gurtina; Brown, Thomas M. et al. (2021). "Star Formation Histories of Ultra-faint Dwarf Galaxies: Environmental Differences between Magellanic and Non-Magellanic Satellites?". The Astrophysical Journal Letters 920 (1): L19. doi:10.3847/2041-8213/ac2aa3. Bibcode: 2021ApJ...920L..19S.
3. ↑ 3.0 3.1 Cite error: Invalid `<ref>` tag; no text was provided for refs named `Simon`
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. doi:10.1088/0004-637X/807/1/50. Bibcode: 2015ApJ...807...50B.
5. ↑ Ji, Alex (22 March 2016). "The Origin of the Cosmos' Heaviest Elements". http://phys.org/news/2016-03-cosmos-heaviest-elements.html.
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. doi:10.1038/nature17425. PMID 27001693. Bibcode: 2016Natur.531..610J.
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. doi:10.1088/1475-7516/2015/09/016. Bibcode: 2015JCAP...09..016H.
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). doi:10.1103/PhysRevLett.115.081101. PMID 26340176. Bibcode: 2015PhRvL.115h1101G.
9. ↑ Overbye, Dennis (10 March 2015). "Gamma Rays May Be Clue on Dark Matter". The New York Times. https://www.nytimes.com/2015/03/11/us/gamma-rays-may-be-clue-on-dark-matter.html?_r=0.
10. ↑ Taylor, Ethan D.; Read, Justin I.; Orkney, Matthew D. A.; Kim, Stacy Y.; Pontzen, Andrew; Agertz, Oscar; Rey, Martin P.; Andersson, Eric P. et al. (11 September 2025). "The emergence of globular clusters and globular-cluster-like dwarfs". Nature 645 (8080): 327–331. doi:10.1038/s41586-025-09494-x. PMID 40931154.

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