Thanks for your reply and your explanation. So, I agree, in general you need gas to cool (and potentially recombine) in order to collapse.
However, as you may know, galaxies seem to be able to accrete IGM gas which is already cool enough to infall without being heated by shocks when penetrating...
Hi,
Once the Universe has completed reionization (at redshift >~6 ), the intergalactic medium (IGM) is completely ionized (even if some neutral clumps can persist). On the other hand, it seems that galaxies were accreting a lot of gas from the IGM after reionization so that they could form...