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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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