The formation time of the halo is defined by the relation M200c (a=a_form)=M200c (a=1)/2 (here a is the scale factor). This quantity is available only for catalogs of the present epoch, z=0. The halo history is obtained by following the SUBLINK merger tree of the central/primary subhalo from a given halo.
The sublink data file tree_extended.N.hdf5 of the merger tree does not have halo form redshift or scale factor a. May I ask how to get the time (a or z) when halo forms half of the total mass（z=0）?
I'll be grateful for your help.
This is available in the (q) Halo Structure supplementary catalog, or you can just calculate it yourself (load the main progenitor branch of the tree, then find the snapshot where the total mass e.g. Group_M_Crit200 is half its current value).
Thank you so much for your help!