### Neutral Hydrogen Abundance in Mini Snaps Ryan Flynn
• 30 Jul '21

I've being working with one of the 'mini' snapshots for TNG100, and found that NeutralHydrogenAbundance was not a data field for the mini snaps. I just wanted to ask if there was any method or work-around to compute neutral hydrogen abundance with the data supplied in the mini snaps? Dylan Nelson
• 30 Jul '21

Yes, this field was removed from mini snapshots since it can be, in theory, reconstructed from the other available fields, following Rahmati+13. I post some code which may be a useful starting point:

``````def neutral_fraction(nH, sP, temp=1e4, redshift=None):
""" The neutral fraction from Rahmati+ (2012) Eqn. A8. """
# recombination rate from Rahmati+ (2012) Eqn. A3, also Hui & Gnedin (1997). [cm^3 / s] """
lamb    = 315614.0/temp
alpha_A = 1.269e-13*lamb**1.503 / (1+(lamb/0.522)**0.47)**1.923

# photoionization rate
if redshift is None:
redshift = sP.redshift

_, gamma_UVB_z = uvbPhotoionAtten(np.log10(nH), np.log10(temp), redshift)

# A6 from Theuns 98
LambdaT = 1.17e-10*temp**0.5*np.exp(-157809.0/temp)/(1+np.sqrt(temp/1e5))

A = alpha_A + LambdaT
B = 2*alpha_A + gamma_UVB_z/nH + LambdaT

return (B - np.sqrt(B**2-4*A*alpha_A))/(2*A)
`````` Ryan Flynn
• 2 Aug '21

Thank you! One question about the code you sent - what is the function uvbPhotoionAtten? Is that also from the Rahmati paper? Dylan Nelson
• 2 Aug '21

Yes `gamma_UVB_z` is the value of the hydrogen photoionization rate, at the redshift of interest. Table 2 of Rahmati+ (should use FG09 to be most consistent with TNG).

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