[M79]

Canis Major Dwarf

Irregular Galaxy Canis Major Dwarf, centered in Canis Major

[CMa Dw in IR]
Right Ascension 07 : 15 (h : m)
Declination -28: (deg : m)
Distance 25.0 (kly)
Apparent Dimension 720 (arc min)

Discovered in November 2003 by Martin, Ibata, Bellazzini, Irwin, Lewis, and Dehnen.

In November 2003, a team of astronomers from France, Italy, the UK, and Australia announcede the discovery of a small nearby galaxy, or galaxy remnant centered in Canis Major, of about 12 degrees (720 arc min) in angular size in Galactic longitude. It was found from the analysis of data on asymmetries in the population of Galactic red giant stars (of spectral type M) in the 2MASS All Sky catalog. These stars show several large-scale asymmetries, the most significant of which is a strong elliptical-shaped over-density close to the Galactic plane, around the position l=240deg, b=-8deg in Galactic coordinates, or about RA=07:20, Dec=-30. The distance of this concentration was estimated at 25,000 light-years from us and 42,000 light-years from the center of our Milky Way Galaxy.

The authors estimated that this concentration might be the nucleus of a dwarf galaxy, which is in a progressive state of disruption, as it orbits our Milky Way Galaxy. This galaxy would thus be our nearest known intergalactic neighbor, and a new dwarf member of the Local Group. The major part of its matter was thought to have been distributed along its orbit, and form arc structures around the Milky Way. It was considered to have been a considerable though small galaxy at one time, having of the order of one billion stars, and may have contributed up to about one percent of the matter of our Milky Way.

The image on the right shows the Canis Major Dwarf's nucleus just below the stellar band indicating the equatorial plane of our Milky Way. This is an infrared view, composed from the Two-Micron All Sky Survey (2MASS).

In many respects such as size, orbit and the process of dissolution, the Canis Major Dwarf would be similar to SagDEG, which had been discovered in 1994 and taken the place as the nearest known neighbor for nine years.

Some Milky Way globular clusters are loosely grouped around the nucleus of the presumed Canis Major dwarf, and at least some of them may have their origin in the halo of this small galaxy, and may represent the remnant of its former globular cluster system: M79, NGC 1851, NGC 2298, and NGC 2808. Open clusters AM-2 and Tombaugh 2 were considered strong candidates for being physically associated with the Canis Major Dwarf (Bellazzini et.al. 2004).

More recent research has revealed that this object is probably not a dwarf galaxy or galaxy remnant, but just a chance overdensity of stars and matter caused by a density wave in the Milky Way's disk. The clusters thought to be associated with it, in particular the globulars, have been found to have their origin in another progenitor satellite galaxy. This progenitor satellite was found in 2018 from the analysis of astrometrical data acquired by the Gaia satellite, and was named "Gaia Sausage" or "Gaia Enceladus". For M79, newer analysis (2022) has shown that its origin may be from Gaia Enceladus only with a probability of about 40%, and it may come from another progenitor called "Helmi" instead, with a probability of about 59% (Callingham et.al. 2022).

The overdensity is also related to another structure discovered in 2005, the Monoceros Ring, a tidal stellar stream (Penarubia et.al. 2005, De Boer et.al. 2018).

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References: More links:

Hartmut Frommert
Christine Kronberg
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Last Modification: February 18, 2026
Former Definitive Version: July 31, 2022
Former Definitive Version: November 22, 2003