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The Question

(Submitted November 24, 2000)

How are the "Local Group" of galaxies selected? Are they just galaxies that are close to each other or are they connected by other means (By gravity, similarities, etc)? What other galaxies, apart from Andromeda and The Milky Way, are included in the "Local Group?"

The Answer

Galaxies are not uniformly distributed in space, but tend to be concentrated in groups and clusters. In some clusters, the density of galaxies is so much higher and the combined gravitational pull so deep that membership is easy to determine. Groups are looser associations, and so are harder to define cleanly.

To quote http://www-hpcc.astro.washington.edu/papers/localgroup/lg.html (a page which appears to have disappeared):

"What is the criterion for inclusion in the Local Group? Proximity is the cleanest and often used to the exclusion of any other. If we grant membership to all galaxies within 4 million light-years, we have a club with 30 members, three of which barely got in. We can also use velocities to find out if the last three are on their way in or out. That is, we can accept all the fellow travelers. If we do this, the three squeakers become full members and we are pressed to include a few more distant objects such as Leo A and Pegasus, both small irregular galaxies."

The motions of all the Local Group galaxies are strongly influenced by the combined gravity of M31 and our Milky Way Galaxy, the two dominant members.

For a full list of members, see http://www.seds.org/messier/more/local.html

Hope this helps,

Koji & Bish
for "Ask an Astrophysicist"

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