I am converting the IllustrisTNG outputs in the Gadget output format. The old Illustris files used to have smoothing length for each particle. The new one does not have such information.
What should be the smoothing length?
I am currently following a suggestion from yt guys (link: https://github.com/yt-project/yt/pull/1807)
I am assuming the cell is spherical and calculate the radius from density and mass. Then I multiply that by factor 2.5.
Let me know if I am doing something wrong.
Yes this is a fine, and is an approach we often take. Note that this is of course an approximation, and that the number 2.5 is arbitrary.
Also note that the SmoothingLength in the original Illustris files is not this value, it is something else.