[yt-users] HOP threshold

[Stephen Skory]

Shankar,


> Hi, I am generating Halos using HOP (via yt). I see that the default overdensity 
> is 160. I guess it means 160 particles per cell. 

No, it is not 160 particles per cell. See below.

> But isn't cell size a function 
> of the simulation box volume and the grid ?
> 
> For example, if Box=200Mpc and Ngrid=512^3, then cell=(0.390625 Mpc)^3
> Isn't 0.390625 Mpc too big for a galaxy ?

Yes, but, and no, this is not actually what matters. Yes, galaxies are not 390 Kpc on a side, typically, but you're not looking for galaxies with HOP. You're looking for dark matter haloes (which is where galaxies live), which are much larger than galaxies. They may not be as large as 390 Kpc across, but luckily, HOP doesn't actually care about the cell size, so having a dark matter halo smaller than the cell is not a problem.

> What is the calculation behind 160 ?

The over-density threshold in HOP refers to the cutoff density for the least-dense members of any halo. That number is the multiple of the overall average density *in the simulational box*, not the universe, which is an important point to remember. All HOP cares about is the positions and masses of dark matter particles, it knows nothing about the gridding, cosmology, or day of the week. I suggest you read the HOP method paper, especially section 2.2, which explains what the over-density threshold parameter means.

http://adsabs.harvard.edu/abs/1998ApJ...498..137E

Good luck!

 _______________________________________________________
sskory at physics.ucsd.edu           o__  Stephen Skory
http://physics.ucsd.edu/~sskory/ _.>/ _Graduate Student
________________________________(_)_\(_)_______________