[yt-users] HOP overdensity parameter

[Stephen Skory]

Shankar,


> 1. Does anyone know where to change the default overdensity of 160.0 ?  

Run it like this for a threshold of 100:

>>> haloes = HaloFinder(pf,threshold=100.0)

By the way, you can see the available inputs and defaults to any function in Python by using help(), so

>>> help(HaloFinder)

would get you something like this (there is more output than this, but I'm not quoting it for brevity).

class HOPHaloFinder(GenericHaloFinder, HOPHaloList)
 |  Method resolution order:
 |      HOPHaloFinder
 |      GenericHaloFinder
 |      yt.lagos.ParallelTools.ParallelAnalysisInterface
 |      HOPHaloList
 |      HaloList
 |      __builtin__.object
 |  
 |  Methods defined here:
 |  
 |  __init__(self, pf, threshold=160, dm_only=True, padding=0.02)

> 2. Is it (160.0) the Delta-peak, as defined in the 1998 HOP paper ?

No. Threshold corresponds to delta outer. Following the recommendations of the paper, delta merge = 2.5 * threshold, and delta peak = 3 * threshold. If you wish to change this, you'll have to edit lines 114 and 115 in hop_regroup.c and re-build yt by running 'python setup.py install' when in the yt-trunk-svn directory.

> 3. I have done 512^3 grid, 512^3 dark-matter (dm) particle Enzo Cosmology 
> Simulations in a 200 Mpc/h Box using wmap5
>    What should I choose in place of 160.0 ?

There is no 'correct' answer to this question. You need to see if the haloes provided are what you want. For example, if you want low-mass haloes, you should see if there are enough found by HOP. If not, you could try lowering the threshold. Halo finders are never definitive, you must interpret the results and decide if they give you what you need.

Good luck!


 _______________________________________________________
sskory at physics.ucsd.edu           o__  Stephen Skory
http://physics.ucsd.edu/~sskory/ _.>/ _Graduate Student
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