Reticulum II

Reticulum II (or Reticulum 2) is an old dwarf galaxy in the Local Group. Reticulum II was discovered in 2015 by analysing images from the Dark Energy Survey. It is elongated, having an 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. Magnitude Mv of the galaxy is -2.7. The distance from Earth is 30 kpc. 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][2] It has an unusual enhancement of r-process elements; meaning that gold and europium are extra common in the brightest stars in the galaxy.[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.[4]

Reticulum II
Observation data (J2000 epoch)
ConstellationReticulum
Right ascension 03h 35m 42.14s[1]
Declination-54° 2′ 57.1″[1]
Distance (comoving)30 ± 2 kpc (97.8 ± 6.5 kly) [1]
Distance97.8 ± 6.5 kly (30 ± 2 kpc) [1]
Apparent magnitude (V)17.4[1]
Characteristics
Apparent size (V)3.37′ × 3.64′[1]
Other designations
Reticulum II, Reticulum 2[1]

Gamma rays mostly with energies between 2 and 10 GeV have been detected by the Fermi satellite.[5] The radiation from Reticulum II is more significant than that of other dwarf galaxy emissions.[6] However this finding has been contested.[7]

References
  1. 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: 130. arXiv:1503.02079. Bibcode:2015ApJ...805..130K. doi:10.1088/0004-637X/805/2/130.
  2. DES Collaboration (10 March 2015). "Eight New Milky Way Companions Discovered in First-Year Dark Energy Survey Data". The Astrophysical Journal. 807: 50. arXiv:1503.02584. Bibcode:2015ApJ...807...50B. doi:10.1088/0004-637X/807/1/50.
  3. Ji, Alex (22 March 2016). "The Origin of the Cosmos' Heaviest Elements". Phys.org. Retrieved 24 March 2016.
  4. 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: 610–613. arXiv:1512.01558. Bibcode:2016Natur.531..610J. doi:10.1038/nature17425. PMID 27001693.
  5. 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 (09): 016–016. arXiv:1503.06209. Bibcode:2015JCAP...09..016H. doi:10.1088/1475-7516/2015/09/016.
  6. 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.
  7. 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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