[yt-dev] Why are particle masses returned in code units?

[Nathan Goldbaum]

Hi all,

Does anyone know why particle fields are turned in code units?  For example:

>>> import yt
>>> ds = yt.load('IsolatedGalaxy/galaxy0030/galaxy0030')
>>> ad = ds.all_data()
>>> print ad['all', 'particle_mass']
[  5.24322377e-06   5.24322377e-06   5.24423058e-06 ...,   2.51688903e-06
   2.52040900e-06   2.52345330e-06] code_mass
>>> print ad['all', 'particle_position_x']
[ 0.50070677  0.50118382  0.50118984 ...,  0.49867166  0.49857796
  0.49865383] code_length

For fluid fields, we have aliases for on-disk fields that get returned in
code units, but the 'standard' yt fields are all in CGS by default:

>>> print ad['gas', 'density']
[  4.92775113e-31   4.94005233e-31   4.93824694e-31 ...,   1.12879234e-25
   1.59561490e-25   1.09824903e-24] g/cm**3

>>> print ad['enzo', 'Density']
[  1.78469285e-01   1.78914800e-01   1.78849414e-01 ...,   4.08816836e+04
   5.77886836e+04   3.97754906e+05] code_mass/code_length**3

I guess we don't really have a distinction like that on the particle side
of things.

I tend to think that it should be easier to access particle fields in CGS
units and that we should default to having 'standard' particle fields in
CGS, just like the fluid fields.

That said, looking at YTEP-0003, it seems we focused mostly on fluid fields
and didn't think very hard about how particle fields should work.  For
example, 'particle_mass' is never mentioned.

I realize that completely refactoring the particle fields will be a fair
bit of work so we may not be able to change things before yt 3.0 is
released, but I think this should be part of the discussion as we develop a
more consistent set of rules for how yt fields should work, including
multifluid, multiparticle, and multispecies datasets.

I'd love to hear other's thoughts on this.

-Nathan
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