The Local Group was first recognised by Hubble in the early days of extragalactic research when distances to galaxies were first being measured. There was a distinct difference between those galaxies that resolved easily into stars and those that did not, implying that the Milky Way Galaxy is part of a small local cluster of galaxies that condensed out of the general expansion of the Universe. We now recognise aproximately 40 members of this condensation which we refer to as the Local Group. Most of the light (and mass) in the Local Group is contained in the two large spiral galaxies, the Milky Way and Andromeda, and most of the known smaller members of the Local Group are satellite galaxies orbiting these systems.
Membership of the Local Group is usually decided on either distance or velocity . By combining this information and using estimates for the total mass of the Local Group and the age of the Universe the following membership diagram can be obtained.
Objects lying to the left of the knotted boundary have enough time since the Big Bang to have moved to their current location. The upper and lower "tram lines" denote regions beyond which objects are unlikely to be bound to the Local Group.
Galaxy surveys since Messier have been biased towards finding galaxies with high surface brightness. Extreme dwarf and low surface brightness galaxies are difficult to find and are generally only found locally - most of the extreme examples come from within the Local Group. However, despite their unassuming appearance dwarf galaxies hold the key to many questions of galaxy formation, structure and evolution. Dwarf galaxies also provide important constraints on the distribution and nature of dark matter , and star formation in low density environments.
The most recent additions to the Local Group are: Tucana - an unusual isolated dwarf galaxy discovered in 1990 by Lavery on the outer fringes of the Local Group; Sextans - the 8th dwarf satellite galaxy found orbiting the Milky Way discovered in 1990 by Irwin; the Sagittarius dwarf the 9th dwarf satellite galaxy of the Milky Way - a galaxy caught in the act of being tidally destroyed and incorporated into the Milky Way was discovered by Ibata, Irwin and Gilmore in 1994; Antlia was recognised as a Local Group member by Whiting, Hau and Irwin in 1997; and over the last year 3 new companions to M31 have been discovered by Karachentsev & Karachentseva and by Armandroff and colleagues.
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