Helmi stream
The Helmi Stream is a stellar stream of the Milky Way galaxy. It started as a dwarf galaxy, now absorbed by the Milky Way as a stream. It was discovered in 1999, and is formed of old stars deficient in heavy elements, and has a mass of 10 to 100 million solar masses. It was absorbed by the Milky Way some 6 to 9 billion years ago. The stream was named after Amina Helmi, who discovered this stellar stream after noticing this group of stars all moving at the same speed and in the same direction. The Helmi Stream discovery affirmed theories that the merging of galaxies played a significant role in creating the giant structures of the Milky Way galaxy. In 2024 a subdwarf of spectral type sdT4 was identified as a possible member of the Helmi stream. The brown dwarf is called CWISE J155349.96+693355.2 has Vtan > 300 km/s and a poorly constrained radial velocity of +110±90 km/s.
The Helmi Stream is a stellar stream of the Milky Way galaxy. It started as a dwarf galaxy, now absorbed by the Milky Way as a stream. It was discovered in 1999, and is formed of old stars deficient in heavy elements, and has a mass of 10 to 100 million solar masses. It was absorbed by the Milky Way some 6 to 9 billion years ago.[1]
The stream was named after Amina Helmi, who discovered this stellar stream after noticing this group of stars all moving at the same speed and in the same direction.[2][3] The Helmi Stream discovery affirmed theories that the merging of galaxies played a significant role in creating the giant structures of the Milky Way galaxy.[2]
In 2024 a subdwarf of spectral type sdT4 was identified as a possible member of the Helmi stream. The brown dwarf is called CWISE J155349.96+693355.2 has Vtan > 300 km/s and a poorly constrained radial velocity of +110±90 km/s.[4]
Ultra-metal-poor star
[edit]In 2026 an ultra-metal-poor star was discovered in the Helmi stream dubbed HE 0144−4657. This star is believed to have formed in a gas cloud enriched by a Population III star explosion of ~50 solar masses before the dwarf galaxy collided with the Milky Way.[5]
Extragalactic planet
[edit]The Helmi stream was briefly believed to be home to the first discovered planet purportedly of extragalactic origin, orbiting the star HIP 13044.[1] However, further analysis of radial velocity data failed to confirm the discovery.[6]
See also
[edit]References
[edit]- ^ a b "Extragalactic Expat: Newfound Exoplanet Likely Came from Another Galaxy", John Matson, Scientific American. 18 November 2010
- ^ a b "Amina Helmi, the "archeologist of the Milky Way," explains how our own galaxy could unlock the mystery of dark matter". FBBVA. 12 December 2017. Retrieved 7 November 2019.
The idea had already been mooted that the merging of small galaxies could have played a major part in forming today's giant structures. Helmi developed the tools that were able to test this hypothesis and confirm that it held true, at least for the Milky Way.
- ^ Skibba, Ramin (10 June 2021). "A galactic archaeologist digs into the Milky Way's history". Knowable Magazine. doi:10.1146/knowable-060921-1. S2CID 236290725. Retrieved 4 August 2022.
- ^ Burgasser, Adam J.; Schneider, Adam C.; Meisner, Aaron M.; Caselden, Dan; Hsu, Chih-Chun; Gerasimov, Roman; Aganze, Christian; Softich, Emma; Karpoor, Preethi; Theissen, Christopher A.; Brooks, Hunter; Bickle, Thomas P.; Gagné, Jonathan; Artigau, Étienne; Marsset, Michaël; Rothermich, Austin; Faherty, Jacqueline K.; Kirkpatrick, J. Davy; Kuchner, Marc J.; Andersen, Nikolaj Stevnbak; Beaulieu, Paul; Colin, Guillaume; Gantier, Jean Marc; Gramaize, Leopold; Hamlet, Les; Hinckley, Ken; Kabatnik, Martin; Kiwy, Frank; Martin, David W.; Massat, Diego H.; Pendrill, William; Sainio, Arttu; Schümann, Jörg; Thévenot, Melina; Walla, Jim; Wędracki, Zbigniew (2 November 2024). "New Cold Subdwarf Discoveries from Backyard Worlds and a Metallicity Classification System for T Subdwarfs". The Astrophysical Journal. 982 (2): 79. arXiv:2411.01378. Bibcode:2025ApJ...982...79B. doi:10.3847/1538-4357/adb39f.
- ^ Placco, Vinicius; Limberg, Guilherme; Kennedy, Catherine; Christlieb, Norbert (20 March 2026). "HE 0144–4657: A Carbon-enhanced Ultra-metal-poor Star ([Fe/H] ∼ −4.1) from the Helmi Stream Disrupted Dwarf Galaxy*". The Astrophysical Journal Letters. 1000: L34. doi:10.3847/2041-8213/ae4dea.
- ^ Jones, M. I.; Jenkins, J. S. (2014). "No evidence of the planet orbiting the extremely metal-poor extragalactic star HIP 13044". Astronomy & Astrophysics. 562: id.A129. arXiv:1401.0517. Bibcode:2014A&A...562A.129J. doi:10.1051/0004-6361/201322132. S2CID 55365608.