I'm looking at the cutout data from the TNG50 Milky Way/Andromeda (MW/M31) sample and have noticed something peculiar about the SubfindID numbers assigned to individual galaxies. Specifically, while I expected each galaxy's cutout dataset to have a uniform SubfindID, reflecting all particles belonging to a single galaxy (for instance, all particles in the galaxy with ID 479938 having the SubfindID 479938), this doesn't seem to be the case. Instead, I observe that a significant portion of particles have a SubfindID of 0, and there are even instances where a couple of particles have a SubfindID, for example, of 512345. Could you clarify why there are discrepancies in the SubfindID numbers, particularly why many are set to 0, and what does SubfindID = 0 mean? Thank you in advance!
Dylan Nelson
27 Feb
In general this isn't true, because these cutouts are "spatial", i.e. they include all particles/cells in the region of space which encompasses the halo.
So only the particles/cells belonging to the central subhalo will have SubfindID equal to the cutout ID. Particles/cells in satellites, for example, will have a different SubfindID. It is also possible a nearby (central) halo is in the cutout, and these particles/cells would have a different SubfindID.
A value of 0 is a bit surprising. It is unlikely it actually means subhalo ID zero. Perhaps it means "IGM", i.e. not belonging to any subhalo. I will ask about this and get back to you.
Dylan Nelson
6 Mar
The SubfindID field for all cutouts has been updated. (It seems that the previous values for PartType1 and PartType4 were somehow wrong).
Can you try to download the cutout again, and check?
Fish Yu
6 Mar
Thank you for your help! I just checked, the zero IDs are -1 now, which means it doesn't belong to any subhalo?
Hello,
I'm looking at the cutout data from the TNG50 Milky Way/Andromeda (MW/M31) sample and have noticed something peculiar about the SubfindID numbers assigned to individual galaxies. Specifically, while I expected each galaxy's cutout dataset to have a uniform SubfindID, reflecting all particles belonging to a single galaxy (for instance, all particles in the galaxy with ID 479938 having the SubfindID 479938), this doesn't seem to be the case. Instead, I observe that a significant portion of particles have a SubfindID of 0, and there are even instances where a couple of particles have a SubfindID, for example, of 512345. Could you clarify why there are discrepancies in the SubfindID numbers, particularly why many are set to 0, and what does SubfindID = 0 mean? Thank you in advance!
In general this isn't true, because these cutouts are "spatial", i.e. they include all particles/cells in the region of space which encompasses the halo.
So only the particles/cells belonging to the central subhalo will have SubfindID equal to the cutout ID. Particles/cells in satellites, for example, will have a different SubfindID. It is also possible a nearby (central) halo is in the cutout, and these particles/cells would have a different SubfindID.
A value of 0 is a bit surprising. It is unlikely it actually means subhalo ID zero. Perhaps it means "IGM", i.e. not belonging to any subhalo. I will ask about this and get back to you.
The
SubfindIDfield for all cutouts has been updated. (It seems that the previous values for PartType1 and PartType4 were somehow wrong).Can you try to download the cutout again, and check?
Thank you for your help! I just checked, the zero IDs are -1 now, which means it doesn't belong to any subhalo?