Scale Factor

Anthony Young
  • 1 Nov '16

I am interested in comparing data across multiple redshifts. Obviously I need to account for the scale factor when doing so. In the table for the data specifications, for dm particles for example, velocity is given in units of (km/s)sqrt(a) and in the notes it says that to get peculiar velocities, one should multiply by sqrt(a). I also see in the group (FoF) table, a weighted velocity is given as km/s/a and in the notes it says to multiply the values by 1/a to get peculiar velocities. Likewise, the dm Potential is given in (km/s)^2 x 1/a. I am a little confused between the difference in those velocities as well as what I need to do to the potential. Any help would be much appreciated.

Dylan Nelson
  • 3 Nov '16

Hi Anthony,

As with "little h" factors, the notation for "a" factors is ambiguous (everywhere). It is never clear if, in the units of a field, or on the axis of a plot, if km/s/a means that to get to just km/s you should multiply or divide by a. Thus the hints. I believe, though, the convention -should- be consistent for everything in Illustris. (For example, "75 Mpc/h" ~= 105 Mpc).

In the case of Potential, you indeed want to divide the snapshot value by a to recover physical units.

In the case of Velocities, you are right that there are three different unit conventions for velocities (!). Particles, halos, and subhalos each have different factors of a.

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