SubhaloVelDisp calculation

Blaise Veres

Hello,

I am trying to do calculations to find the subhalo velocity dispersion and check it with the SubhaloVelDisp field, however, the values I get do not match the SubhaloVelDisp values. Specifically, how is the SubhaloVelDisp field being calculated?

Thanks!

Dylan Nelson
  • 6 Jul

Hi Blaise,

There are some subtleties of the units, etc, it is computed as (where the num loop is over the member particles):

  disp = 0;
  mass = 0;

  for(i = 0; i < num; i++)
    {
      for(j = 0; j < 3; j++)
        {
          ddxx = GRAVITY_NEAREST_X(P[p].Pos[j] - SubhaloCM[j]); // periodic distance
          dx[j] = current_scalefactor * ddxx;
          dv[j] = vel_to_phys * (P[p].Vel[j] - SubhaloCMVel[j]); // units to km/s
          dv[j] += H_of_a * dx[j]; // hubble correction

          disp += P[p].Mass * dv[j] * dv[j];
       }
       mass += P[p].Mass;
   }
veldisp = sqrt(disp / (3 * mass));    /* convert to 1d velocity dispersion */
Blaise Veres
  • 9 Jul

Hi Dylan,

Thanks for your response. In the code you sent above, do you know how the GRAVITY_NEAREST_X, vel_to_phys, and H_of_a are defined?

Dylan Nelson

Hello,

GRAVITY_NEAREST_X is the usual periodic distance function, e.g.

    if x > BoxHalf:
        return x-BoxSize
    else:
        if x < -BoxHalf:
            return x+BoxSize
        else:
            return x

vel_to_phys is whatever you need to get that quantity into km/s, e.g. likely 1/a.

H_of_a is the usual H(z) = H0 * sqrt(Ωm(1+z)^3 + ΩΛ + Ωk(1+z)^2).

Blaise Veres
  • 13 Jul

Also, for the P[p].Mass above, I figure that the Mass is a list of the masses of each particle in num. I am trying to recreate that whole code in Python, but I am not sure what mass value I would use in replace of P[p].Mass. Do you know what value/field I should use for trying to recreate that code? Thanks.

Dylan Nelson
  • 13 Jul

It should just be the Masses particle field of the stars.

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