# Re: Unit Conversion

Date: Wed, 16 Aug 2006 16:27:41 -0400

The velocities that I am trying to convert are peculiar velocities in
units of proper km/s. The velocities are definitely not physical
because when I use the conversion v_gadget = v_pec/sqrt(a) = (v_phys -
v_comov)/sqrt(a) = (v_phys - x/a)/sqrt(a) where x is the particles
position in comoving coordinates. This gives me screwy results as the
velocities are supposed to peculiar velocities already.

However if I simply use v_gadget = v_pec/sqrt(a) I get structures that
form much too slowly. Even if I use v_gadget = v_pec/a things happen
too slowly but not nearly as slowly as the former method. I am
comparing these runs to the Santa Barbra (SB) IC of the same cosmology
(that I am aware of anyway) where I converted from the SB IC format

The only other thing that I can think of is that maybe there is a
difference between the peculiar velocities in units of proper km/s to
peculiar velocities in units of gadget2. Is there a real difference
here between km/s in proper units and km/s in gadget2?

On 8/15/06, Pedro Colin <p.colin_at_astrosmo.unam.mx> wrote:
> Did you add the term Hr, where H(z) is the Hubble constant a redshift z
> and r is the proper distance, to the peculiar velocity? v_phy = v_pec +
> Hr.
>
> Pedro
>
> > I am trying to convert an IC file that has the positions and
> > velocities in physical units to the Gadget2 format. I have converted
> > the the positions from physical to comoving units by multiplying by h.
> > Doing this yields obviously correct results. In converting the
> > velocities however this is not as obvious. In using the formula given
> > on page 32 of the Gadget2 documentation that v=u*sqrt(a) where v is
> > the physical velocity, u is the internal units and a=1/(1+z) gives
> > obviously incorrect results as the evolution of the system at z=0
> > looks more like an evolution of a system to z=3 give or take a good
> > bit. If I however use the formula that v=ua the results seem much more
> > reasonable though I am not entirely convinced that this is correct by
> > a simple visual inspection. Are the velocity units in Gadget2 comoving
> > (the documentation only specifies that velocities in Gadget2 are in
> > internal units) or is there an additional subtilty?
> >
> >
>
>

```--