How much z variation within a cluster of galaxies?
The SDSS image associated with this one, is packed full of galaxies, whose z seems to vary quite a lot, is there a rule of thumb of some kind for using z to tell if galaxies are actually close to each other rather than just randomly lining up as seen from Earth? I googled for an answer and couldn't find anything.
Happy New Year, 2015, Ptd! 😃
Here's an SDSS image, centered on (158.247, 30.761) (the same center are ARG0001ok1), and approximately the same angular size (so it covers about the same piece of sky):
The sole object with a red box is SDSS J103256.04+304459.5. It has a red box to indicate that it has an SDSS spectrum; its spectroscopic redshift (z_sp) is 0.420.
Every galaxy in this field has a photometric redshift (z_ph); i.e. a redshift estimated from each galaxy's 'colors'. In fact, there are two, and each comes with an estimated uncertainty (or 'error'). For example, SDSS J103256.04+304459.5 has a z_ph of 0.378 ± 0.0255 and 0.388 ± 0.0601. However, we know* its redshift is 0.420, so the first z_ph estimate is quite off ("0.378 ± 0.0255" = '~67% chance the redshift is between 0.353 and 0.393'), but the second is OK.
So, what about the z_ph estimates of the 'arc' of galaxies at/near the host of the #wat? Here are some: (0.460 ± 0.0171/0.458 ± 0.0673), (0.412 ± 0.0201/0.412 ± 0.0615), (0.327 ± 0.0956/0.320 ± 0.1012), (0.239 ± 0.0267/0.261 ± 0.0792), and (0.388 ± 0.0487/0.407 ± 0.0716). With one exception, these all have 'the same' redshift as SDSS J103256.04+304459.5.
Although the above does not answer your actual question, I think it addresses the underlying issue.
Does this help?
*sometimes z_sp's are unreliable; however, in this case it's pretty good