I’ve been trying to confirm the threshold density for star formation for use in a post-processed dust model. In Pillepich et al. (2018) they say that the threshold density is n_H~0.1 cm^-3 and in Vogelsberger et al. (2013) they give it as rho_SFR = 0.13 cm^-3. I assume the latter is also a hydrogen number density, but I would like to avoid making assumptions where possible. If I understand correctly, the threshold density for star formation is not a value that is directly fed into the model, but rather is computed internally as a mass density based on the input supernovae fraction and temperature, star formation timescale, and the cooling function at some reference temperature and density. The cooling function makes it non-trivial to verify the TNG threshold density with independent calculations.
So instead, I looked at the densities of all gas cells with non-zero SFRs in the z=0 snapshot. (I started looking in only a few chunks, but after not finding any near the stated threshold, I verified by looking through all the chunks.) In both TNG100 and 300, the lowest density of any star-forming cell is about 0.00755 in code units, or 5.11e-25 g/cm^3. Assuming a total hydrogen composition gives a number density of 0.305cm^-3, and taking a hydrogen number density (rho*X/m_H) gives 0.232 cm^-3.
Is there any reason why none of the gas cells in the entire z=0 snapshot would be close to the threshold density? Am I misunderstanding something? And is there another way to verify the threshold density used in the simulations? Thanks!
Dylan Nelson
13 Jun
Some debug output of use:
USE_SFR: A0=573 PhysDensThresh=0.000754654 (int units) 0.232138 h^2 cm^-3 expected fraction of cold gas at threshold=0.904362 tcool=4.82656e-10 dens=277.475 egyhot=2104.87
USE_SFR: run-away sets in for dens=0.172583 dynamic range for quiescent star formation=228.692 USE_SFR: isotherm sheet central density=0.0481551 z0=0.0519156
USE_SFR: SNII energy=272312 [internal units] = 5.41628e+48 [erg/M_sun] = 0.00541628 [1e51 erg/Msun]
where you can find the parameter meanings, and full model description, back in Springel & Hernquist (2003), although I am not sure why the exact value is useful?
Hello Dylan and all,
I’ve been trying to confirm the threshold density for star formation for use in a post-processed dust model. In Pillepich et al. (2018) they say that the threshold density is n_H~0.1 cm^-3 and in Vogelsberger et al. (2013) they give it as rho_SFR = 0.13 cm^-3. I assume the latter is also a hydrogen number density, but I would like to avoid making assumptions where possible. If I understand correctly, the threshold density for star formation is not a value that is directly fed into the model, but rather is computed internally as a mass density based on the input supernovae fraction and temperature, star formation timescale, and the cooling function at some reference temperature and density. The cooling function makes it non-trivial to verify the TNG threshold density with independent calculations.
So instead, I looked at the densities of all gas cells with non-zero SFRs in the z=0 snapshot. (I started looking in only a few chunks, but after not finding any near the stated threshold, I verified by looking through all the chunks.) In both TNG100 and 300, the lowest density of any star-forming cell is about 0.00755 in code units, or 5.11e-25 g/cm^3. Assuming a total hydrogen composition gives a number density of 0.305cm^-3, and taking a hydrogen number density (rho*X/m_H) gives 0.232 cm^-3.
Is there any reason why none of the gas cells in the entire z=0 snapshot would be close to the threshold density? Am I misunderstanding something? And is there another way to verify the threshold density used in the simulations? Thanks!
Some debug output of use:
where you can find the parameter meanings, and full model description, back in Springel & Hernquist (2003), although I am not sure why the exact value is useful?