Unsure what BH_Density, BH_Mass, Masses fields are

Mikkel Theiss Kristensen
  • 1
  • 10 Aug '21

Hi TNG team,

I have a couple of questions regarding the specification of certain BH fields:

BH_Density - Local comoving gas density averaged over the nearest neighbors of the BH.
Is this the same density used to calculate the Bondi accretion rate (and is specified in Weinberger2017+)? Or what does 'nearest neighbours' cover?

Masses - Total mass of the black hole particle. Includes the gas reservoir from which accretion is tracked onto the actual BH mass (see BH_Mass).
Is the gas reservoir the sphere of which accretion is tracked (and also as mentioned in Weinberger2017+)?
Is the only way to obtain the gas reservoir to do BH_Mass - Masses?


Dylan Nelson
  • 11 Aug '21

Dear Mikkel,

Indeed BH_Density is the neighboring gas value, estimated within the local kernel, as directly used by the accretion (as well as feedback) routines.

I think the descriptions of BH_Mass versus PartType5/Masses are a bit confusing, and suggest to look at these values. While the former is what I would call the "true" or "physical" BH mass, the latter is the mass used in the gravitational force calculation, i.e. the dynamical mass. If you plot the M_BH-M_star relation in TNG300-1, for example, you can see that the physical (seed) BH masses go lower than the particle (dynamical) masses, due to the low resolution of the simulation. Similarly, I wouldn't consider the 'gas reservoir' for any analysis, this is just a sub-component within the model.

Mikkel Theiss Kristensen
  • 13 Aug '21

Thank you very much for the clarification!

Yossi Cohen
  • 13 Feb

Hi everyone,

I encountered a possibly relevant problem.
I wanted to calculate the energy injected by AGN feedback using Eq. 13 in Weinberger+2017.

In order to do so, I need to estimate the "gas mass in the feedback region".
Since the feedback region is described as being n_gbl particles away from the BH, I assumed that the relevant gas mass can be obtained by taking the 'Masses' field minus the 'BH_Mass' field.
When testing that on a sample of BH particles, it appears that in most cases the difference between these two values is 0.
There doesn't seem to be any preference to a specific redshift.

  1. Is my understanding wrong?
  2. If the difference I am looking at is indeed correct, does that mean that AGN feedback events are truly this rare in TNG (on average every 3-5 snapshots)?


Dylan Nelson
  • 13 Feb

The total energy injected by a SMBH (in its past history) is stored in the BH_CumEgyInjection_QM and BH_CumEgyInjection_RM fields. I would suggest you use these.

Yossi Cohen
  • 14 Feb

Thank you for the quick response.

I was interested in estimating the energy injected by the SMBH in specific times.
Doesn't the cumulative energy field also account for mergers? Would I need to account for those as well?

Dylan Nelson
  • 14 Feb

Yes that's correct, when two SMBHs merger, these fields are summed. You may want to try to take this out, if needed.

You could calculate the difference of these fields, between each snapshot, to obtain the total energy injected between each snapshot.

Otherwise, the only other value would be the "instantaneous rate of energy injection" - you can calculate this from Mdot (from the snapshot) and Eqn. 7 or Eqn. 8, depending on mode, from Weinberger+17.

Yossi Cohen
  • 14 Feb

Thank you!

So you wouldn't suggest trying to estimate m_enc? Is the data simply not recorded?

Dylan Nelson
  • 14 Feb

You can certainly sum the "gas mass in the feedback/accretion region". Of course, only instantaneously.

To do so, you would sum(PartType0/Masses[inds]) where inds correspond to the nearest N gas cells. You would not use any fields from PartType5.

Yossi Cohen
  • 14 Feb

I understand.

Again, thank you very much for the help!

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