View source for Canis Major Overdensity

{{Short description|Disputed galaxy located near the constellation Canis Major}}
{{Use dmy dates|date=May 2020}}
{{Infobox galaxy
| image = 
   {{Location mark
     | image=Canis Major constellation map.svg
     | float=center | width=240 | position=right
     | mark=Red circle.svg | mark_width=10 | mark_link= 
     | x%=17 | y%=75
   }}
| caption=Location of the Canis Major Overdensity (red circle)
 | name = Canis Major Overdensity
| epoch = [[Epoch (astronomy)#Julian years and J2000|J2000]]
| type = Irr
| ra = {{RA|07|12|35.0}}<ref name="ned">{{Cite web |title=NASA/IPAC Extragalactic Database |url=http://nedwww.ipac.caltech.edu/cgi-bin/nph-objsearch?objname=Canis+Major+Dwarf#ObjNo1 |website=Results for Canis Major Dwarf |access-date=16 March 2007}}</ref>
| dec = {{DEC|-27|40|00}}<ref name="ned" />
| dist_ly = {{val|25000|fmt=commas}} [[light-year|ly]]
| stars = 1 billion
| z = 
| appmag_v = 
| size_v = 12 [[degree (angle)|degrees]] × 12 degrees
| constellation name = [[Canis Major]]
| names = CMa Dwarf,<ref name="ned" /> PGC 5065047
}}

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

The supposed small [[galaxy]] contains a relatively high percentage of [[red giant]]s and is thought to include an estimated one billion stars.

At the time of its announcement, the Canis Major Dwarf Galaxy was classified as an irregular galaxy and was thought to be the closest neighboring galaxy to the Earth's location in the [[Milky Way]], located about {{convert|25000|ly|kpc|abbr=off|lk=off}} away from the [[Solar System]]<ref name="Can">{{Cite web |title=Astronomers find nearest galaxy to the Milky Way |url=http://astro.u-strasbg.fr/images_ri/canm-e.html |archive-url=https://web.archive.org/web/20080527020725/http://astro.u-strasbg.fr/images_ri/canm-e.html |archive-date=27 May 2008 |access-date=24 September 2009}}</ref> and {{convert|42000|ly|kpc|abbr=on|lk=on}} 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 Earth, though later studies disputed this conclusion.

==Discovery==

An international team of French, Italian, British, and Australian [[Astronomer|astronomers]] reported the existence of a strong elliptical-shaped stellar overdensity in November 2003. They claimed their study pointed to a newly discovered dwarf galaxy: the Canis Major Dwarf Galaxy.<ref name="Martin2003">{{Cite journal |last1=N. F. Martin |last2=R. A. Ibata |last3=M. Bellazzini |last4=M. J. Irwin |last5=G. F. Lewis |last6=W. Dehnen |year=2004 |title=A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion onto the Milky Way |journal=Monthly Notices of the Royal Astronomical Society |volume=348 |issue=12 |pages=12 |arxiv=astro-ph/0311010 |bibcode=2004MNRAS.348...12M |doi=10.1111/j.1365-2966.2004.07331.x|doi-access=free |s2cid=18383992 }}</ref> This structure is located closer to the [[Sun]] than the center of this galaxy, at approximately {{convert|7.7|kpc|ly|abbr=on|lk=on}} from the Sun.

The team of astronomers that discovered it was collaborating on the analysis of data from the [[2MASS|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 [[Stellar classification|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 force|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,<ref name="Masetti2011">{{Cite web |title=The Nearest Galaxies |url=http://heasarc.nasa.gov/docs/cosmic/nearest_galaxy_info.html |last=Maggie Masetti |date=14 April 2011 |website=The Cosmic Distance Scale |publisher=[[NASA]] |access-date=26 November 2011 |archive-date=26 November 2011 |archive-url=https://web.archive.org/web/20111126202215/http://heasarc.nasa.gov/docs/cosmic/nearest_galaxy_info.html |url-status=dead }} See section "The Canis Major Dwarf".</ref> The stream of stars was discovered in the early 21st century by astronomers conducting the [[Sloan Digital Sky Survey]]. The Monoceros Ring is similar to the [[Virgo Stellar Stream]], which is thought to be the result of the [[Milky Way]] tearing stars and gases from the [[Sagittarius Dwarf Spheroidal Galaxy]] into a Stellar Stream. It was during the investigation of the Monoceros Ring, and a closely spaced group of [[globular clusters]] similar to those associated with the Sagittarius Dwarf Elliptical Galaxy, that the CMa Overdensity was discovered.

Globular clusters thought to be associated with the CMa include [[NGC 1851]], [[Messier 79|NGC 1904]], [[NGC 2298]] and [[NGC 2808]], all of which may have been part of the galaxy's globular cluster system before [[accretion (astrophysics)|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 [[NGC 2467|H18]] are [[open cluster]]s that may have formed when the dwarf galaxy perturbed material in the [[Disc (galaxy)|galactic disk]], stimulating [[star formation]].<ref name="Martin2003" />

The discovery of the CMa and subsequent analysis of its associated stars has supported the current theory that galaxies may grow in size by swallowing their smaller neighbors. Martin et al.<ref name="Martin2003" /> 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.<ref name="Lopez-Corredoira20012">{{Cite arXiv |eprint=1207.2749 |class=astro-ph.GA |first1=M. |last1=Lopez-Corredoira |first2=A. |last2=Moitinho |title=Comments on the "Monoceros" affair |date=Jul 2012 |last3=Zaggia |first3=S. |last4=Momany |first4=Y. |last5=Carraro |first5=G. |last6=Hammersley |first6=P. L. |last7=Cabrera-Lavers |first7=A. |last8=Vazquez |first8=R. A.}}</ref> Some research suggests that the trail of stars is 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.<ref name="Momany2004">{{Cite journal |last1=Momany |first1=Y. |last2=Zaggia |first2=S. R. |last3=Bonifacio |first3=P. |last4=Piotto |first4=G. |last5=De Angeli |first5=F. |last6=Bedin |first6=L. R. |last7=Carraro |first7=G. |date=July 2004 |title=Probing the Canis Major stellar over-density as due to the Galactic warp |journal=Astronomy and Astrophysics |volume=421 |issue=2 |pages=L29 |arxiv=astro-ph/0405526 |bibcode=2004A&A...421L..29M |doi=10.1051/0004-6361:20040183|s2cid=6371010 }}</ref> Investigation of the area in 2009 yielded only ten [[RR Lyrae variable]] stars, consistent with the Milky Way's [[Galactic halo|halo]] and thick disk populations rather than a separate dwarf spheroidal galaxy.<ref>{{Cite journal |last1=Mateu, Cecilia |last2=Vivas, A. Katherina |last3=Zinn, Robert |last4=Miller, Lissa R. |last5=Abad, Carlos |year=2009 |title=No Excess of RR Lyrae Stars in the Canis Major Overdensity |journal=The Astronomical Journal |volume=37 |issue=5 |pages=4412–23 |arxiv=0903.0376 |bibcode=2009AJ....137.4412M |doi=10.1088/0004-6256/137/5/4412|s2cid=18967866 }}</ref>

==In fiction==
British science fiction writer [[Stephen Baxter (author)|Stephen Baxter]] included the Canis Major Overdensity in his 2004 novella ''Mayflower II'' as the destination of the eponymous starship.

==See also==
* [[Messier 79]]
* [[Galaxy formation and evolution]]
* [[Stellar kinematics]]
* [[Milky Way]]
* [[Sagittarius Dwarf Spheroidal Galaxy]]
* [[Monoceros Ring]]
* [[Virgo Stellar Stream]]

==References==
{{Reflist}}

==External links==
* {{Cite web |title=Canis Major Dwarf |url=http://messier.seds.org/more/cma_dw.html |website=SEDS Messier Database}}
* {{cite simbad|title=NAME CMa Dwarf Galaxy}}

{{Milky Way}}
{{Canis Major}}
{{Portal bar|Physics|Stars|Outer space}}
{{Sky|07|12|35.0|-|27|40|00|25000}}

[[Category:Canis Major Overdensity| ]]
[[Category:Dwarf irregular galaxies]]
[[Category:Local Group]]
[[Category:Milky Way Subgroup]]
[[Category:Canis Major]]
[[Category:Astronomical objects discovered in 2003|20031101]]
[[Category:Principal Galaxies Catalogue objects|5065047]]