The Canis Major Dwarf Galaxy located in the same part of the sky as the constellation of Canis Major. The galaxy contains a relatively high percentage of red giant stars, 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 neighbouring galaxy to the Milky Way, being located only 42,000 light-years from the Galactic Center and approximately 25,000 light-years away from our Solar System. 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 Milky Way.

The galaxy was first discovered in November 2003 by an international team of French, Italian, British and New Zealand astronomers. Although closer to the Earth than the centre of the galaxy itself, the Canis Major Dwarf galaxy was difficult to detect as it is located behind the plane of the Milky Way, where concentrations of stars, gas and dust are densest. This, along with its small size, explains why it was not discovered sooner.

The team of astronomers which discovered it were collaborating on analysis of data from the Two-Micron All Sky Survey, 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 over-density 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.

Astronomers believe that the Canis Major Dwarf Galaxy is in the process of being pulled apart by the gravitational field of the more massive Milky Way galaxy. The main body of the galaxy is extremely degraded. Tidal disruption causes a long 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 our galaxy three times. The stream of stars was first 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 Canis Major dwarf galaxy was discovered.

Globular clusters thought to be associated with the Canis Major Dwarf galaxy include NGC 1851 , NGC 1904, NGC 2298 and NGC 2808 , all of which are likely to be a remnant of the galaxy's globular cluster system before its accretion, or swallowing, into the Milky Way. NGC 1261 is another nearby cluster, but its velocity is different enough from that of the others to make its relation to the system unclear. The Canis Major Dwarf galaxy may also have associated open clusters, including Dol 25 and H18 , and possibly AM2. It is thought that the open clusters may have formed due to the dwarf galaxy's gravity perturbing material in the galactic disk and stimulating star formation.

The discovery of the Canis Major Dwarf Galaxy 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 neighbours. 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.

See also

• Sagittarius (dwarf galaxy)
• Messier 79
• Omega Centauri
• Galaxy formation and evolution

References

• N. F. Martin, R. A. Ibata, M. Bellazzini, M. J. Irwin, G. F. Lewis, W. Dehnen, (February 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 (1) 12.
• Official press release announcing discovery (University of Strasbough)
• SEDS page on the Canis Major dwarf galaxy