[yt-users] possible bugs in analytical mass function code and halo profiler code

[Matthew Turk]

Hi Mike,

I'll leave the rest for Britton, Stephen, etc, but I can address your
final point.

> 3) Lastly, one question about the radial profiles. I've noticed that
>   the 'CellVolume' field is rarely equal to 4.0/3.0 * pi *
>   ('RadiusMpc' * 3.09e24)**3. Typically 'CellVolume' is about 35 -
>   40% larger. Maybe this is because the grid cells partly "overshoot"
>   the edge of the sphere? But I'm surprised that this is a 35%
>   effect... This potentially affects the determination of the virial
>   radius.

CellVolume is always calculated by summing up the values of the cells
that are included, which are determined by the AMRSphere object.
There are a couple potential weaknesses here, but they will be
consistent with the cells that have been included.  This is done in
two routines:

yt/data_objects/object_finding_mixin.py line 162:
ObjectFindingMixin.find_sphere_grids
yt/data_objects/data_containers.py line 2936:
AMRSphereBase._get_cut_mask (also see _is_fully_enclosed just above)

This uses the RadiusCode field, which is defined on line 746 of
yt/data_objects/universal_fields.py.

The is_fully_contained (which you can verify is not causing the
problem inserting "return False" at the top of that routine and
checking again) routing determines whether or not the grid requires
additional masking.  The masking is done by comparing RadiusCode to
the radius of the sphere.  RadiusCode comes from the center of the
cells.  If you catch too many root grid cells, for instance, this may
result in an overestimate.  It's possible that this could be 30-40%,
if it's integrated over the entire sphere, catching cells here and
there on the exterior that contribute far too much.  (You could, at
best, be adding on an entire ~3/4 of a cell, and effectively extending
the radius by sqrt(3) dx.)  It's worth investigating if this is indeed
where the issue is coming from.

That aside, the selection method is identical everywhere, so I do
believe that taking the sum of CellVolume is the correct method,
rather than using 4/3 pi r^3.

-Matt

>
>
> Thanks,
>
> Mike
>
>
> --
> *********************************************************************
> *                                                                   *
> *  Dr. Michael Kuhlen              Theoretical Astrophysics Center  *
> *  email: mqk at astro.berkeley.edu   UC Berkeley                      *
> *  cell phone: (831) 588-1468      601 Campbell Hall                *
> *  skype username: mikekuhlen      Berkeley, CA 94720               *
> *                                                                   *
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