For example, there is a galaxy which Subfind ID = 13333 at snap = 7. We can find its descendent in the tree, which Subfind ID= 5007 at snap = 9. Obviously, there is no record for this galaxy at snap=8 in the tree. I guess it is because there is no merger for this galaxy at snap=8, right? But this galaxy must be alive. Can I know its Subfind ID and other information like mass at snap=8?
For some reason it was missed at that particular snapshot.
If you want any properties at that snapshot, e.g. mass, or position, I would suggest to simply interpolate between the two snapshots on each side.
That mean TNG can't provide the accurate information at that snapshot? I am wondering is it reliable to get the properties by interpolation. But it seems like there is no other ways to get properties. And what about the Subfind ID? Does it mean that I can't get SubfindID at that snapshot?
I'd suggest to read the SubLink paper for some additional information on why a subhalo progenitor/descendant link may "skip" a snapshot.
For example, there may be no discrete subhalo (as identified by Subfind) at the intermediate snapshot, so a "Subfind ID" is not well defined.
If you need a guess at the values at that time, you can interpolate. But if you need precise values for some reason, you would want to be more careful.
Thank you! That helps me a lot