Gravitational potential in halos without the contribution from particles external to the halo

Justo Antonio Gonzalez Villalba
• 10 May '22

Dear Illustris-TNG team,

I would like to calculate the total potential energy of gas particles in a Illustris-TNG300-1 halo, but without the contribution from particles external to the halo.

Is there a way to correct the Potential value in the Halo cuts to remove the contribution of particles external to the halo?

Thank you!

Dylan Nelson
• 10 May '22

As you say, the `Potential` field in the snapshots is a global quantity, and there isn't any way to separate it into different contributions.

If you need a local measurement of the potential, it would make sense to simply compute this directly (e.g. using only halo particles, one halo at a time - this is not too expensive).

Justo Antonio Gonzalez Villalba
• 2
• 11 May '22

I see, then in order to calculate the potential I have a couple of questions regarding softening lengths:

• In Nelson et al. 2018 the DM/Stars softening for TNG-300-1 at z=0 is 1.48kpc, but in the rest API (https://www.tng-project.org/api/TNG300-1/) we have softening_stars_comoving=2.0. If I multiply this by hubble 0.6774 then I get 1.3548kpc. Where is the difference coming from?

• In Pillepich+ (2018) there is a comment that the gravitational forces become Newtonian at 2.8 x epsilon. So I assume for distances greater then 2.8 x epsilon I should calculate the potential as plain Newtonian and not Plummer-Equivalent?

• Also, in the rest API we have softening_stars_max_phys=1.0 but both values mentioned above are greater than this threshold. Is the threshold really enforced?

Thanks!

Dylan Nelson
• 1
• 11 May '22

I would probably suggest to ignore softenings for the potential calculation, as a first step.

For Illustris and TNG the softenings are comoving until z=1, then fixed. So this value `2.0 ckpc/h` is the prior. Note that you divide by h, rather than multiply, to remove the hubble parameter from code lengths. So the box size of TNG100 is `75 Mpc/h = 110 Mpc`. Then `1.0 / 0.6774 = 1.48`.

Justo Antonio Gonzalez Villalba
• 1
• 18 May '22

Hi Dylan,

So I have tried some quick tests with small halos, for example halo 1010 from TNG-300-1 snapshot 99 (z = 0 / a = 1)

I calculate the halo-only potential in the position of the less bounded dark matter particle (with id 7805765839). The halo-only potential is -159054.84 (km/s)^2 and the tabulated potential is -1944362.75 (km/s)^2. Therefore there is an offset of 1785307.88 (km/s)^2 which I assume to be due to the overall matter distribution in the box.

Then I calculate the sum of the potential energy off all DM particles in the halo, after removing the background offset, and I compare this total potential energy to the kinetic energy, which I calculate subtracting GroupVel from the tabulated particle velocities. This gives me a ratio of (Kinetic Energy)/(Potential Energy) = 0.07, that is quite far from what I would expect from a virialized halo (T= -V/2).

It seems all halos are in this state, where the kinetic energy is much lower than half of the potential energy from the hosting halo. What do you think about this?

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