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The Canis Major Overdensity (CMa Overdensity) or Canis Major Dwarf Galaxy (CMa Dwarf) is a disputed dwarf irregular galaxy in the Local Group, located in the same part of the sky as the constellation Canis Major.

Canis Major Overdensity
Observation data (J2000 epoch)
ConstellationCanis Major
Right ascension07h 12m 35.0s[1]
Declination−27° 40′ 00″[1]
Distance25,000 ly
Characteristics
TypeIrr
Number of stars1 billion (1×109)
Apparent size (V)12 degrees × 12 degrees
Other designations
CMa Dwarf,[1] PGC 5065047

The supposed small galaxy contains a relatively high percentage of red giants and is thought to contain an estimated one billion stars in all.

The Canis Major Dwarf Galaxy is classified as an irregular galaxy and is now thought to be the closest neighboring galaxy to the Earth's location in the Milky Way, being located about 25,000 light-years (7.7 kiloparsecs) away from the Solar System[2] and 42,000 ly (13 kpc) from the Galactic Center. It has a roughly elliptical shape and is thought to contain as many stars as the Sagittarius Dwarf Elliptical Galaxy, the previous contender for closest galaxy to the Earth's location in the Milky Way.


Discovery


The existence of a strong elliptical-shaped stellar overdensity was reported in November 2003 by an international team of French, Italian, British and Australian astronomers, who claimed their study pointed to a newly discovered dwarf galaxy: the Canis Major Dwarf Galaxy.[3] This structure is located closer to the Sun than the center of this galaxy, at approximately 7.7 kpc (25,000 ly) from the Sun.

The team of astronomers that discovered it was collaborating on analysis of data from the Two-Micron All Sky Survey (2MASS), a comprehensive survey of the sky in infrared light, which is not blocked by gas and dust as severely as visible light. Because of this technique, scientists were able to detect a very significant overdensity of class M giant stars in a part of the sky occupied by the Canis Major constellation, along with several other related structures composed of this type of star, two of which form broad, faint arcs.


Characteristics


Some astronomers believe that the CMa overdensity is a dwarf galaxy in the process of being pulled apart by the gravitational field of the more massive Milky Way galaxy. The main body of the CMa is extremely degraded. Tidal disruption causes a filament of stars to trail behind it as it orbits the Milky Way, forming a complex ringlike structure, sometimes referred to as the Monoceros Ring, which wraps around the Milky Way three times.[4] The stream of stars was discovered in the early 21st century by astronomers conducting the Sloan Digital Sky Survey. It was in the course of investigating this ring of stars, and a closely spaced group of globular clusters similar to those associated with the Sagittarius Dwarf Elliptical Galaxy, that the CMa was discovered.

Globular clusters thought to be associated with the CMa include NGC 1851, NGC 1904, NGC 2298 and NGC 2808, all of which may have been part of the galaxy's globular cluster system before accreting into the Milky Way. NGC 1261 is another nearby cluster, but its velocity differs enough to make its relationship to the system unclear. Additionally, Dolidze 25 and H18 are open clusters that may have formed when the dwarf galaxy perturbed material in the galactic disk, stimulating star formation.[3]

The discovery of the CMa and subsequent analysis of the stars associated with it has provided some support for the current theory that galaxies may grow in size by swallowing their smaller neighbors. Martin et al.[3] believe that the preponderance of evidence points to the accretion of a small satellite galaxy of the Milky Way which was orbiting roughly in the plane of the galactic disk.


Dispute


Several studies cast doubts on the true nature of this overdensity.[5] Some research suggests that the trail of stars is actually part of the warped galactic thin disk and thick disk population and not a result of the collision of the Milky Way with a dwarf spheroidal galaxy.[6] Investigation of the area in 2009 yielded only ten RR Lyrae variable stars which is consistent with the Milky Way's halo and thick disk populations rather than a separate dwarf spheroidal galaxy.[7]


In fiction


British science fiction writer Stephen Baxter included the Canis Major Overdensity in his novella "Mayflower II" as the destination of the eponymous starship.


See also



References


  1. "NASA/IPAC Extragalactic Database". Results for Canis Major Dwarf. Retrieved 16 March 2007.
  2. "Astronomers find nearest galaxy to the Milky Way". Archived from the original on 27 May 2008. Retrieved 24 September 2009.
  3. N. F. Martin; R. A. Ibata; M. Bellazzini; M. J. Irwin; G. F. Lewis; W. Dehnen (2004). "A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion onto the Milky Way". Monthly Notices of the Royal Astronomical Society. 348 (12): 12. arXiv:astro-ph/0311010. Bibcode:2004MNRAS.348...12M. doi:10.1111/j.1365-2966.2004.07331.x. S2CID 18383992.
  4. Maggie Masetti (14 April 2011). "The Nearest Galaxies". The Cosmic Distance Scale. NASA. Archived from the original on 26 November 2011. Retrieved 26 November 2011. See section "The Canis Major Dwarf".
  5. Lopez-Corredoira, M.; Moitinho, A.; Zaggia, S.; Momany, Y.; Carraro, G.; Hammersley, P. L.; Cabrera-Lavers, A.; Vazquez, R. A. (July 2012). "Comments on the "Monoceros" affair". arXiv:1207.2749 [astro-ph.GA].
  6. Momany, Y.; Zaggia, S. R.; Bonifacio, P.; Piotto, G.; De Angeli, F.; Bedin, L. R.; Carraro, G. (July 2004). "Probing the Canis Major stellar over-density as due to the Galactic warp". Astronomy and Astrophysics. 421 (2): L29. arXiv:astro-ph/0405526. Bibcode:2004A&A...421L..29M. doi:10.1051/0004-6361:20040183. S2CID 6371010.
  7. Mateu, Cecilia; Vivas, A. Katherina; Zinn, Robert; Miller, Lissa R.; Abad, Carlos (2009). "No Excess of RR Lyrae Stars in the Canis Major Overdensity". The Astronomical Journal. 37 (5): 4412–23. arXiv:0903.0376. Bibcode:2009AJ....137.4412M. doi:10.1088/0004-6256/137/5/4412. S2CID 18967866.



На других языках

[de] Canis-Major-Zwerggalaxie

Die Canis-Major-Zwerggalaxie im Sternbild Großer Hund (lateinisch Canis Major) ist nach dem heutigen Stand der Forschung die nächste Nachbargalaxie der Milchstraße. Sie befindet sich in einem Abstand von 42.000 Lichtjahren vom Zentrum der Milchstraße; das sind nur etwa 80 Prozent des Abstandes der Sagittarius-Zwerggalaxie, die vor der Entdeckung der Canis-Major-Galaxie als nächster Nachbar der Milchstraße galt.
- [en] Canis Major Overdensity

[es] Enana del Can Mayor

La galaxia enana Can Mayor se encuentra en la constelación Canis Maior. La galaxia contiene un total de aproximadamente mil millones de estrellas, de las cuales un alto porcentaje son gigantes rojas.

[ru] Карликовая галактика в Большом Псе

Карликовая галактика в Большом Псе — гипотетическая[3] галактика Местной группы, находящаяся в созвездии Большого Пса. Галактика содержит относительно большой процент звёзд красных гигантов, и предположительно содержит один миллиард звёзд. Классифицируется как неправильная галактика и на данный момент является ближайшей к нашему положению в Млечном Пути среди соседних галактик: 25 000 световых лет от Солнечной системы и 42 000 световых лет от центра нашей галактики. Имеет форму неровного эллипса и предположительно содержит примерно столько же звёзд, сколько и Карликовая эллиптическая галактика в Стрельце, ранее считавшаяся ближайшей к нам галактикой. Существование Карлика в Большом Псе в настоящее время оспаривается рядом исследователей[3].


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