| Canis Major Overdensity | |
|---|---|
  Location of the Canis Major Overdensity (red circle) | |
| Observation data (J2000epoch) | |
| Constellation | Canis Major |
| Right ascension | 07h 12m 35.0s[1] |
| Declination | −27° 40′ 00″[1] |
| Distance | 25,000ly |
| Characteristics | |
| Type | Irr |
| Number of stars | 1 billion |
| Apparent size (V) | 12degrees × 12 degrees |
| Other designations | |
| CMa Dwarf,[1] PGC 5065047 | |
TheCanis Major Overdensity (CMa Overdensity) orCanis Major Dwarf Galaxy (CMa Dwarf) is a disputeddwarf irregular galaxy in theLocal Group, located in the same part of the sky as theconstellationCanis Major.
The supposed smallgalaxy contains a relatively high percentage ofred giants 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 theMilky Way, located about 25,000 light-years (7.7 kiloparsecs) away from theSolar System[2] and 42,000 ly (13 kpc) 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.
An international team of French, Italian, British, and Australianastronomers 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.[3] This structure is located closer to theSun than the center of the milky way, at approximately 7.7 kpc (25,000 ly) from the Sun.
The team of astronomers that discovered it was collaborating on the analysis of data from theTwo-Micron All Sky Survey (2MASS), a comprehensive survey of the sky ininfrared 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 ofclass 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.
Some astronomers believe that the CMa overdensity is a dwarf galaxy in the process of being pulled apart by thegravitational 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 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 Stream, which is thought to be the result of theMilky Way tearing stars and gases from theSagittarius Dwarf Spheroidal Galaxy into a Stellar Stream. It was during the investigation of the Monoceros Ring, and a closely spaced group ofglobular 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 includeNGC 1851,NGC 1904,NGC 2298 andNGC 2808, all of which may have been part of the galaxy's globular cluster system beforeaccreting 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 andH18 areopen clusters that may have formed when the dwarf galaxy perturbed material in thegalactic disk, stimulatingstar formation.[3]
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.[3] believe that the preponderance of evidence points to the accretion of a smallsatellite galaxy of the Milky Way, which was orbiting roughly in the plane of the galactic disk.
Several studies cast doubts on the true nature of this overdensity.[5] Some research suggests that the trail of stars is part of the warped galacticthin disk andthick disk population and not a result of the collision of theMilky Way with a dwarf spheroidal galaxy.[6] Investigation of the area in 2009 yielded only tenRR Lyrae variable stars, consistent with the Milky Way'shalo and thick disk populations rather than a separate dwarf spheroidal galaxy.[7]
British science fiction writerStephen Baxter included the Canis Major Overdensity in his 2004 novellaMayflower II as the destination of the eponymous starship.[citation needed]
