[yt-users] Issues with code_length versus physical lengths for positions in x, y, z space

[John Wise]

Hi Devin,

1.4 is awfully close to 1/h.  I bet there's a missing factor of 1/h in 
the Rockstar interface since Rockstar calculates almost everything 
(mass, lengths, positions) with inverse-h.

John

On 04/20/2015 01:12 PM, Devin Silvia wrote:
> Hi all,
>
> I've been trying to do some data analysis of an Enzo dataset to
> extract information about volumes contained with the virial radius of
> rockstar identified halos.
>
> As a test, I've identified the most massive halo and created sphere at
> that halo position using a bit of code that looks like this:
>
> sp = data_ds.sphere([most_massive.quantities['particle_position_x'],
>                       most_massive.quantities['particle_position_y'],
>                       most_massive.quantities['particle_position_z']],
>                      most_massive.quantities['virial_radius'])
>
> Then, when I actually try to use the sphere to analyze data or make
> plots, it seems like there is something screwy going on with the
> position information, which you can see if you look at the
> calculations done here:
>
> http://paste.yt-project.org/show/5522/
>
> When you look at the "center" based on a calculation of the positions
> with respect to the domain, you get a different center than the one
> that just comes of sp.center in "code_length".  Specifically, the
> "code_length" center values all seem to be off from the actual
> position of the halo by a factor of 1.4:
>
> In [98]: sp.center.in_units("kpc")/data_ds.domain_width.in_units("kpc")
> / sp.center.d
> Out[98]: YTArray([ 1.40845077,  1.40845077,  1.40845077]) (dimensionless)
>
> I'm not sure where this factor is coming from, but it seems to be
> buried in some sort of unit conversion step somewhere.
>
> Thanks,
> Devin
>

-- 
John Wise
Assistant Professor of Physics
Center for Relativistic Astrophysics, Georgia Tech
http://cosmo.gatech.edu
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