[yt-users] units
gso at physics.ucsd.edu
gso at physics.ucsd.edu
Wed Mar 10 22:33:35 PST 2010
Yeah I was just confused by the comment
> """
> Returns the mass-weighted RMS velocity for the halo
> particles in code units.
> """
because I got values and thought it was in code units so multiplying by
the velocity conversion factor gave speeds exceeding c by orders of
magnitude. So it is indeed already in CGS then.
From
G.S.
> Hi Geoffrey,
>
> Here's the code for calculating RMS:
>
> def rms_velocity(self):
> """
> Returns the mass-weighted RMS velocity for the halo
> particles in code units.
> """
> bv = self.bulk_velocity()
> pm = self["ParticleMassMsun"]
> sm = pm.sum()
> vx = (self["particle_velocity_x"] - bv[0]) * pm/sm
> vy = (self["particle_velocity_y"] - bv[1]) * pm/sm
> vz = (self["particle_velocity_z"] - bv[2]) * pm/sm
> s = vx**2. + vy**2. + vz**2.
> ms = na.mean(s)
> return na.sqrt(ms) * pm.size
>
> particle_velocity_[xyz] is returned in cgs (I just checked.) Bulk
> velocity is also cgs, and ParticleMassMsun is the weighting factor, so
> it doesn't need to be in any units. Looking at this code, it could be
> substantially more memory efficient, but as-is it should be correct
> and returning in cgs. Why do you think it might be wrong? Did you
> get bizarre answers?
>
> -Matt
>
> On Wed, Mar 10, 2010 at 6:32 PM, <gso at physics.ucsd.edu> wrote:
>> When the line
>> self["particle_velocity_x"]
>> is called in HaloFinding.py, does it automatically give the number in
>> CGS
>> units already or is it in code units? It's used in calculating the rms
>> velocity for each of the dark matter haloes, but it's subtracting
>> centers
>> and multiplying other values in code units I believe, just want to make
>> sure the calculations are done in the same units.
>>
>> From
>> G.S.
>>
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