[yt-dev] "Overdensity" in yt 2.x and 3 and the Enzo Dark_Matter_Density field

[Josh Moloney]

For the past couple of days I've been trying to figure out halo profiles
and virial radius calculations in yt, which so far has mostly consisted of
trying to understand the differences in the "Overdensity" field between yt
versions.

The biggest difference is due to yt-2.x using an incorrect redshift scaling
for the overdensity calculation. The field is defined as

Matter_Density / (rho_crit_now * data.pf.hubble_constant**2 * (1 +
data.pf.current_redshift)**3)

which scales the critical density by (1+z)^3 instead of only the matter
component.

This error is fixed in yt 3, but I'm not sure if it's corrected for or
known by people still using legacy versions of yt.

After accounting for this, there is still a ~1.5% difference in the mean
overdensity in my simulation between yt 2.x and 3 (the yt 3 value is
lower). This difference comes from the particle_density component of the
matter_density. The yt 2.x version uses the particle masses from Enzo and
yt's CICDeposit_3. The yt 3 version uses the Dark_Matter_Density field from
Enzo, which uses Enzo's build in cic method. As far as I understand,
differences in cic methods could lead to different values over subdomains
of a simulation, but shouldn't lead to different total masses over the
entire simulation box.

I went through each grid of my simulation individually, and in every one of
them the sum of the Dark_Matter_Density values is less than the sum of the
particle_mass values. Differences range from 0.4% to 3.5% across individual
grids. My guess is that this means the Dark_Matter_Density field is losing
mass that is deposited in the ghost zones outside of the grid boundaries.

I'm not familiar enough with the internal workings of Enzo to know if this
has any impact on the actual simulations, or if it is only an issue with
the data outputs. Either way, it might be a good idea to have yt return to
calculating the particle_density field from particle masses and do the cic
deposition in yt, at least for Enzo data.
    - Josh
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