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Sorry to bother, I want to figure out the definition of 'id_group'. It's in the (i)'Molecular and atomic hydrogen (HI+H2) galaxy contents' of 'Supplementary Data Catalog'.

I'm so confused, since I have never found such field in the 'Groupcatalog' or 'Snapshot' or 'Merger Tree', and it's not the 'GroupFirstSub' or the index of group catalog arrays. Besides. the definition given by the table is not clear enough, so I have to ask you for help.

Could you please tell me the clear definition of this 'id_group', if possible? Or how can I correlate the data of HI of different halos with that of corresponding halo mass?

Thanks so much.

Hi Zhixing,

There is both id_subhalo and id_group, the latter is not really necessary but you can use it. It is the "ID" (also "index") of the parent FoF group of the subhalo.

So it should be exactly equal to SubhaloGrNr[id_subhalo] where SubhaloGrNr is the field loaded from the simulation group catalogs.

You're right then Group_M_Crit200[SubhaloGrNr[id_subhalo]] == Group_M_Crit200[id_group], which gives the halo mass (r200crit definition) in code units.