The Canis Major Overdensity (''CMa Overdensity'') or Canis Major Dwarf Galaxy (''CMa Dwarf'') is a disputed

dwarf irregular galaxy An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, ...

in the Local Group, located in the same part of the sky as the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Canis Major Canis Major is a constellation in the Southern Celestial Hemisphere, southern celestial hemisphere. In the second century, it was included in Ptolemy's 48 constellations, and is counted among the 88 modern constellations. Its name is Latin fo ...

. The supposed small

galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

contains a relatively high percentage of

red giant A red giant is a luminous giant star of low or intermediate mass (roughly 0.3–8 solar masses ()) in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius large and the surface temperature around or ...

s and is thought to contain an estimated one billion stars in all. At the time of its announcment, 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 the Milky Way, being located about away from the Solar System and 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 Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph), also known as the Sagittarius Dwarf Elliptical Galaxy (Sgr dE or Sag DEG), is an elliptical loop-shaped satellite galaxy of the Milky Way. It contains four globular clusters ...

, the previous contender for closest galaxy to the Earth's location in the Milky Way, 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 Australian astronomers, who claimed their study pointed to a newly discovered dwarf galaxy: the Canis Major Dwarf Galaxy. This structure is located closer to the Sun than the center of this galaxy, at approximately from the Sun. The team of astronomers that discovered it was collaborating on analysis of data from the

Two-Micron All Sky Survey The Two Micron All-Sky Survey, or 2MASS, was an astronomical survey of the whole sky in infrared light. It took place between 1997 and 2001, in two different locations: at the U.S. Fred Lawrence Whipple Observatory on Mount Hopkins, Arizona, and ...

(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 In physics, a gravitational field is a model used to explain the influences that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational phenome ...

of the more massive Milky Way galaxy. The main body of the CMa is extremely degraded.

Tidal disruption The tidal force is a gravitational effect that stretches a body along the line towards the center of mass of another body due to a gradient (difference in strength) in gravitational field from the other body; it is responsible for diverse phenome ...

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. See section "The Canis Major Dwarf". The stream of stars was discovered in the early 21st century by astronomers conducting the

Sloan Digital Sky Survey The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project began in 2000 a ...

. 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 1851 (also known as Caldwell 73) is a relatively massive globular cluster located in the southern constellation of Columba. Astronomer John Dreyer described it as not very bright but very large, round, well resolved, and clearly consisting of ...

, NGC 1904,

NGC 2298 NGC 2298 is a globular cluster in the southern constellation of Puppis. Discovered by James Dunlop on May 30, 1826, it is probably a former member of the disputed Canis Major Dwarf galaxy. The cluster is being disrupted by the galactic tide, tra ...

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 NGC 1261 (also known as Caldwell 87) is a globular cluster of stars in the southern constellation of Horologium, first discovered by Scottish astronomer James Dunlop in 1826. The cluster is located at a distance of from the Sun, and from the G ...

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 Star formation is the process by which dense regions within molecular clouds in The "medium" is present further soon.-->interstellar space

. 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. 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. 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. Investigation of the area in 2009 yielded only ten

RR Lyrae variable RR Lyrae variables are periodic variable stars, commonly found in globular clusters. They are used as standard candles to measure (extra) galactic distances, assisting with the cosmic distance ladder. This class is named after the prototype and ...

stars which is consistent with the Milky Way's halo and thick disk populations rather than a separate dwarf spheroidal galaxy.

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.

References

External links