Quote:
Originally Posted by Dave2042
This reminds me of my Honours thesis long ago, which was on cluster galaxies. I ran into essentially this problem only much worse.
The definition of a cluster member is that it is gravitationally bound to the cluster - simple enough in theory. However, looking at a particular galaxy 'near' a cluster and deciding whether it is a member is very difficult. There was an agreed measure, but it was obviously a wild approximation. I don't know if anyone's improved on this since.
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That does sound like a real head scratcher, if two clusters are in alignment in their relative postion to our vantage point, and we are all heading in the same direction, it might take hundreds of thousands, I guess even millions of years before the expansion of space make the motion apparent enough to distinguish the two. Thanks for the brain food! I am going to look a bit deeper into this. Redshift of the individual members would be the way to determine this would be my first guess?