Public Data Access Overview / Discussion Forum

Can I find the BCG(galaxy) in the GroupFirstSubhalo?

Using SubhaloParent and SubhaloFlag, I can distinguish the subhalos which are considered as galaxy and included in second massive (or third...and so on) subhalo.
[please see the attached figure]

But, I didn't know whether GroupFirstSubhalo has including subhalos(A.K.A sub-sub halo or satellite galaxy) or not.

How can I know that from Group catalog?
(I used the Illustris-TNG 300-1 groupcat at the snapshot 99(z=0))

Hi Hyowon,

Each massive halo ("group") will in general have one central ("BCG") and many satellites.

Both the central and all the satellites are all "subhalos" in the catalog, and for one halo, they will all share a common SubhaloGrnr.

Thus I would suggest, for a given satellite index i, the value GroupFirstSubhalo[ SubhaloGrNr[i] ] gives the index of its central. Does that make sense?

Thank you for your reply!

Umm...But, I think that what I really want to ask thing is not exactly expressed, because of the broken figure.

Each subhalo are not included sub-subhalos? I mean the "more" lower hierarchy structure is not in there?
For example, If we assume the hierarchy index of the Halo is 0 and subhalo is 1, then Isn't there an object the hierarchy index 2?

If there are existing, How can I see that?
Especially, I want to see the sub-subhalo of central("BCG")!

Hi Hyowon,

Yes you're right, Subfind does search for heirarchical (sub)substructure, and this information is present in the SubhaloParent field.

It is typically quite rare to have more than 1 level (i.e. satellite subhalos inside a halo) at the resolution of the TNG simulations. That is, it would be rare (but possible) for a satellite galaxy to itself have substructures identified by Subfind.

Thank you, Dylan.
Can I ask one more thing to check it?

I wonder the case of the BCG which is doing interaction like galaxy-galaxy merging.
Does merging galaxy considered as "one subhalo" or distinguishes it as "two different subhalos"?

The reason why I wondered about the above thing is that central subhalo of Illustris-TNG is so bigger than other subhalos of itself, and has larger values than observed BCGs. Even It has about 10 to 14 solar mass for total mass in the biggest halo of snap 99. In the observation, until now, the biggest mass of the BCG is 10 to 13 solar mass. Thus I feel suspicious whether "merging subhalos" is considered as "one subhalo" or not.

Hi Hyowon,

(1) A merger will go through three stages: (i) when the systems are still far apart (>>rvir separation), they will belong to separate halos, and each will be the central subhalo of their parent halo. (ii) once the two halos coalesce into a single group (~rvir separation), both galaxies will become subhalos of that group. One will be the central, while one will be a "satellite". (iii) at some point, when Subfind can no longer tell them apart (<<rvir separation), there will be only one central subhalo left. The point when this happens would be called the merger event.

Any particular system could be in any one of these stages - you can get a good sense by looking at the merger tree (i.e. the mass history).

(2) I am not sure if you refer to stellar mass or total mass? The largest halo in TNG100 has a total mass of ~2e14 msun, while the largest halo in TNG300 has a total mass of ~1e15 msun. The stellar mass of the central galaxies of those objects will be around ~1e12 msun. The exact value will depend sensitively on the definition (i.e. aperture) and separation between "BCG mass" and "ICL mass". The total stellar mass of the entire central subhalo (i.e. halo) could be quite large of course, while the galaxy stellar mass itself will be much less.