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From: Brad Krane <brad.krane_at_gmail.com>

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

directly to Gadget2 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?
*

*> >
*

*> >
*

*>
*

*>
*

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

directly to Gadget2 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:

-- Brad -- BradReceived on 2006-08-16 22:27:42

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