An artist’s impression of the Monoceros Ring, which wraps three times around the Milky Way.
Credit: Nicolas Martin & Rodrigo Ibata, Observatoire de Strasbourg, 2003.

A small, irregular galaxy with a mass of about a billion solar masses, the Canis Major dwarf galaxy is one of our closest neighbours, lying approximately 25,000 light years from the Sun and 42,000 light years from the centre of the Milky Way. Until recently, however, this dwarf galaxy lay unobserved behind the dust and gas in the disk of the Milky Way. It was only discovered during the Two-Micron All Sky Survey (2MASS) infrared survey, which allowed astronomers to see beyond the dust, in many regions for the first time.

Gravitational interactions with the Milky Way have stripped long tidal tails from the Canis Major dwarf galaxy, which have wrapped themselves around the Milky Way three times in a structure known as the ‘Monoceros Ring’. Several of the globular clusters found in the Milky Way are also thought to have come from the Canis Major dwarf, whose ultimate fate is to merge with the Milky Way over the next billion years or so. With a mass only 1% of that of the Milky Way, this is one of two minor mergers known to be currently occurring in the Milky Way. The other involves the Sagittarius dwarf galaxy.