There was some earlier discussion of how early in the universe
galaxies could form. I happened to run across a simulation that
gives one answer:
http://adsabs.harvard.edu/abs/2016MNRAS.461L..51W

I haven't read this in detail, and I'm sure it is far from the final
answer we will have, but it seems to make credible use of known
physics to give predictions. Upcoming observations, particularly
with JWST, will show whether the predictions are accurate or not.

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