[Yt-dev] pHOP in YT question

[Geoffrey So]

 I would humbly suggest that you rethink what you are
doing if you are trying to find the ellipsoidal extent of a four
particle "halo."
The temporary fix was to not crash the script (and identify what was causing
the crash), so I can get some result when testing, it was not meant as a
permanent solution.  After going through Brian's 2005 paper as Matt
suggested, it seems that the cutoff they've chosen is to discard any "halo"
with less than 32 particles, and after talking to Prof. Norman, he recalled
that Brian mentioned that to be conservative, should really use 100
particles as the cutoff for unigrid simulations.

Stephen, I think I recall now that  the cutoff you mentioned is done in the
filtering step, where if you specify the mass to be 10x the DM particle
mass, then your halo will be at least 10 particles (if dm_only=True) etc,
pretty vague in my memory.  So I guess what I can do is in my ellipsoidal
analysis to discard/filter haloes that are less than 100 particles to be
safe.

In addition to the particle count cutoff, currently I am also discarding all
haloes that have C (the smallest axis) less than a cell width similar to
what is done with spheres in YT.  I think this is a pretty problem dependent
number that can vary depending on what the user want, but I was wondering if
there's a general consensus on how many cells constitute a good enough
ellipsoid?  I can throw in a switch for user to define this number as well.

As an alternative to cell count, I was thinking maybe if the volume of the
"perfect" ellipsoid is a "close enough" fraction of the cell volume, then to
keep the ellipsoid, else discard it.  This would be useful for keeping
poorly resolve objects that exist in a couple cells and you'd think that it
is big enough to consider all cells part of the object. In this case, the
user would input the fraction instead of the cell count, what do you guys
think?

Any comments are appreciated.

From
G.S.


On Fri, Oct 28, 2011 at 12:22 PM, Stephen Skory <s at skory.us> wrote:

> Matt,
>
> > Identifying halos with only a single particle by itself makes a pretty
> > good argument that pHOP should apply a "minimum number of particles"
> > setting.
>
> I see no reason that this can't be done. I'll add that to my todo list.
>
> --
> Stephen Skory
> s at skory.us
> http://stephenskory.com/
> 510.621.3687 (google voice)
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