Black hole mass

Daeun Jeong
  • 5 Oct '23

Hi! I have a question regarding TNG snapshot data. I'm attempting to calculate values, such as 'SubhaloMassInHalfRad' (Sum of masses of all particles/cells within the stellar half-mass radius), directly from particle data. In the case of snapshot PartType5 black holes, there are two provided mass values, 'BH_Mass' and 'Masses.' I'm unsure which one to use and would appreciate some guidance. While considering using 'Masses' as with other particle types, the description for 'Masses' reads, 'Total mass of the black hole particle. Includes the gas reservoir from which accretion is tracked onto the actual BH mass (see BH_Mass).' This raises concerns about potential duplicate inclusion of gas cell mass when I calculate the total mass using particle data. Please let me know if my explanation lacks clarity. Thank you.

Dylan Nelson
  • 5 Oct '23

I'd suggest to plot the SMBH mass vs galaxy mass relations using both fields, to get a feel for what they mean. See this thread.

The PartType5/Masses are the particle masses used in the simulation, e.g. for the gravity calculation. These will also be the masses summed into fields such as SubhaloMassType[5].

However, the PartType5/BH_Mass field is more physical, I would call this the "actual" SMBH mass. It is summed into the SubhaloBHMass field.

The only difference is for low-mass SMBHs, or in low resolution simulations, as you can see in the thread linked.

Daeun Jeong
  • 7 Oct '23

Thank you for your clear answer!

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