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In this case, as you can see from the M51 images, they are physically connected by a trail of stars.  These two galaxies are so close they are messing each other up, gravitationally.

I'm not sure how large M51 is, but our Milk Way is 100 000 light years across - so a companion might be a few times further than that distance.

In fact, galaxies are often found in groups - clusters.  The Milky Way and the Andromeda Galaxy are the heavyweights in the Local Group cluster.  These groups are typically, say 5 million light years across (50 times bigger than the Milky Way).

Jonathan Ransom 2 on 15 May 2006 at 22:51 +0000 wrote:
How far or how close does an object need to be to be considered a compaion of another object, as in the case of M51?

Jon