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From: Brad Krane <brad.krane_at_gmail.com>

Date: Mon, 14 Aug 2006 21:34:18 -0400

Hi,

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: Mon, 14 Aug 2006 21:34:18 -0400

Hi,

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?

-- BradReceived on 2006-08-15 03:34:21

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