reliable minimum radius given gravitational softening
Melika Sarrami
24 Jun
Dear Dylan,
I am wondering at which softening radii the DM density profile and its inner slope are considered reliable in the TNG100 simulation. I frequently encounter references to a threshold of approximately twice the softening length. Could you tell me the reliable radii, or how I should determine them?
Thanks
Dylan Nelson
25 Jun
It depends what you mean by "reliable".
I assume you mean that the resulting value i.e. density is converged (to some tolerance) if one were to change the softening length. The only quantitative answer would be to perform a resolution study using the different resolution boxes. You can find an example of such a study in Pillepich+ (2019) regarding stellar sizes.
Mathematically, gravity becomes unsoftened at 2.8 times the gravitational softening length. You can find the softening length of dark matter in the documentation, for a given TNG simulation. However, even below this value the magnitude of softening is very small, and results could certainly be "reliable". You can find a more detailed discussion of this in Springel+ (2024) the Gadget4 paper, for example.
As a rule of thumb, I would suggest to plot radial profiles to r=0 and draw a line indicating the softening length. Caution should be used in interpreting or using values at or near this radius.
Melika Sarrami
9h
Thank you so much.
Why does the gravitational force become effectively unsoftened at 2.8 softening radii ?
Is there a paper you can refer me to that supports this specific number?
Dylan Nelson
2h
This is the definition of "Plummer equivalent gravitational softening", you can find related discussion in many papers including e.g. the Gadget-2 and Gadget-4 code papers.
Dear Dylan,
I am wondering at which softening radii the DM density profile and its inner slope are considered reliable in the TNG100 simulation. I frequently encounter references to a threshold of approximately twice the softening length. Could you tell me the reliable radii, or how I should determine them?
Thanks
It depends what you mean by "reliable".
I assume you mean that the resulting value i.e. density is converged (to some tolerance) if one were to change the softening length. The only quantitative answer would be to perform a resolution study using the different resolution boxes. You can find an example of such a study in Pillepich+ (2019) regarding stellar sizes.
Mathematically, gravity becomes unsoftened at 2.8 times the gravitational softening length. You can find the softening length of dark matter in the documentation, for a given TNG simulation. However, even below this value the magnitude of softening is very small, and results could certainly be "reliable". You can find a more detailed discussion of this in Springel+ (2024) the Gadget4 paper, for example.
As a rule of thumb, I would suggest to plot radial profiles to r=0 and draw a line indicating the softening length. Caution should be used in interpreting or using values at or near this radius.
Thank you so much.
Why does the gravitational force become effectively unsoftened at 2.8 softening radii ?
Is there a paper you can refer me to that supports this specific number?
This is the definition of "Plummer equivalent gravitational softening", you can find related discussion in many papers including e.g. the Gadget-2 and Gadget-4 code papers.