### 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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