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?
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.
Thank you very much for the clarification!