Canis Major Overdensity

Canis Major Overdensity
Canis Major Overdensity
Observation data (J2000epoch)
Constellation	Canis Major
Right ascension	07h12m35.0s
Declination	−27° 40′ 00″
Distance	25,000ly
Characteristics
Type	Irr
Number of stars	1 billion (1×109)
Apparent size(V)	12degrees× 12 degrees
Other designations
	CMa Dwarf,PGC 5065047

TheCanis Major Overdensity(CMa Overdensity) orCanis Major Dwarf Galaxy(CMa Dwarf) is a disputeddwarf irregular galaxyin theLocal Group,located in the same part of the sky as theconstellation Canis Major.

The supposed smallgalaxycontains a relatively high percentage ofred giantsand is thought to contain an estimated one billion stars in all.

At the time of its announcement, the Canis Major Dwarf Galaxy is classified as an irregular galaxy and is thought to be the closest neighboring galaxy to the Earth's location in theMilky Way,being located about 25,000 light-years (7.7 kiloparsecs) away from theSolar System^[2]and 42,000ly(13kpc) from theGalactic Center.It has a roughly elliptical shape and is thought to contain as many stars as theSagittarius Dwarf Elliptical Galaxy,the previous contender for closest galaxy to Earth, though later studies disputed this conclusion.

Discovery

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

The team of astronomers that discovered it was collaborating on analysis of data from theTwo-Micron All Sky Survey(2MASS), a comprehensive survey of the sky ininfraredlight, 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 ofclass Mgiant 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 thegravitational fieldof the more massive Milky Way galaxy. The main body of the CMa is extremely degraded.Tidal disruptioncauses a filament of stars to trail behind it as it orbits the Milky Way, forming a complex ringlike structure, sometimes referred to as theMonoceros Ring,which wraps around the Milky Way three times,^[4]The stream of stars was discovered in the early 21st century by astronomers conducting theSloan Digital Sky Survey.The Monoceros Ring is similar to theVirgo Stellar Streamwhich is thought to be the result of theMilky Waytearing stars and gases from theSagittarius Dwarf Spheroidal Galaxyinto a Stellar Stream. It was in the course of investigating the Monoceros Ring, and a closely spaced group ofglobular clusterssimilar to those associated with the Sagittarius Dwarf Elliptical Galaxy, that the CMa Overdensity was discovered.

Globular clusters thought to be associated with the CMa includeNGC 1851,NGC 1904,NGC 2298andNGC 2808,all of which may have been part of the galaxy's globular cluster system beforeaccretinginto the Milky Way.NGC 1261is another nearby cluster, but its velocity differs enough to make its relationship to the system unclear. Additionally, Dolidze 25 andH18areopen clustersthat may have formed when the dwarf galaxy perturbed material in thegalactic disk,stimulatingstar 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 smallsatellite galaxyof 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 galacticthin diskandthick diskpopulation and not a result of the collision of theMilky Waywith a dwarf spheroidal galaxy.^[6]Investigation of the area in 2009 yielded only tenRR Lyrae variablestars which is consistent with the Milky Way'shaloand thick disk populations rather than a separate dwarf spheroidal galaxy.^[7]

In fiction

British science fiction writerStephen Baxterincluded the Canis Major Overdensity in his novella "Mayflower II" as the destination of the eponymous starship.

References

^^a ^b ^c"NASA/IPAC Extragalactic Database".Results for Canis Major Dwarf.Retrieved16 March2007.
^"Astronomers find nearest galaxy to the Milky Way".Archived fromthe originalon 27 May 2008.Retrieved24 September2009.
^^a ^b ^cN. 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.
^Maggie Masetti (14 April 2011)."The Nearest Galaxies".The Cosmic Distance Scale.NASA.Archived fromthe originalon 26 November 2011.Retrieved26 November2011.See section "The Canis Major Dwarf".
^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].
^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.
^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.

External links

"Canis Major Dwarf".SEDS Messier Database.
"NAME CMa Dwarf Galaxy".SIMBAD.Centre de données astronomiques de Strasbourg.

[ned-1] "NASA/IPAC Extragalactic Database".Results for Canis Major Dwarf.Retrieved16 March2007.

[Can-2] "Astronomers find nearest galaxy to the Milky Way".Archived fromthe originalon 27 May 2008.Retrieved24 September2009.

[Martin2003-3] 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.

[Masetti2011-4] Maggie Masetti (14 April 2011)."The Nearest Galaxies".The Cosmic Distance Scale.NASA.Archived fromthe originalon 26 November 2011.Retrieved26 November2011.See section "The Canis Major Dwarf".

[Lopez-Corredoira20012-5] 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].

[Momany2004-6] 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.

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