Re: Comoving Density Conversion Question

From: Jared Coughlin <Jared.W.Coughlin.29_at_nd.edu>
Date: Thu, 6 Oct 2016 21:42:48 -0400

Hi Antonio,
Thanks for the response! I agree that what you wrote will convert the
density to cgs, but won't it still be comoving?
-Jared

On Oct 6, 2016 9:19 PM, "Antonio Bibiano" <antbbn_at_gmail.com> wrote:

> In the user guide it says:
>
>> Units are again in internal code units, i.e. for the above system of
>> units,
>
> rho is given in 10^10 h^-1 M_sun / ( h^-1 kpc)^3.
>>
> It's kind of confusing, but I think it might the same thing as every
> other quantity in the gadget snapshots.
>
> To convert to cgs you should just grab the number from the snapshot,
> let's call it rho, and do
> rho * UnitMass_in_g / UnitLength_in_cm^3 * HubbleParam^2
>
> With the last multiplication only if you want to get rid of little h.
>
> Let me know if the result makes sense.
>
> Antonio
>
> 2016-10-06 14:42 GMT+11:00 Jared Coughlin <Jared.W.Coughlin.29_at_nd.edu>:
>
>> Hello! I know this is a pretty stupid question, but I'm drawing a blank
>> on it so I figured I'd ask. I want to know how to convert density from
>> comoving gadget units to proper cgs units. I've run a cosmological
>> simulation (so comoving integration is on), which means that Gadget is
>> writing the densities in comoving internal code units. For ease of talking
>> about them, I've been calling a gadget unit of mass a GUM and a gadget unit
>> of length a GUL.
>>
>> (NOTE: I'm using the tex for gmail plugin on chrome, if that helps make
>> this more readable)
>>
>> That is,
>>
>> 1GUL = 3.085678e21 cm/h = X cm/h
>> 1GUM = 1.989e43 g/h = Y g/h (the use of X and Y is just for ease of
>> writing)
>>
>> in the default system, which is what I'm using. For one of my analysis
>> codes I need to convert the densities from comoving gadget units to proper
>> cgs units. Let a subscript c is for comoving, and a subscript p is for
>> proper, and the cg subscript is for comoving gadget units.
>>
>> [image: \rho_{cg} =
>> \left(\frac{|\rho_{cg}|\text{GUM}}{\text{GUL}_c^3}\right)\left(\frac{Y\text{g}}{h\text{GUM}}\right)\left(\frac{h\text{GUL}_c}{X\text{cm}_c}\right)^3\left(\frac{1\text{cm}_c}{\frac{1\text{cm}_p}{a}}\right)^3]
>>
>>
>> Where the || just means the magnitude of the density (no units attached).
>> This is the number contained in the snapshot. The above simplifies to:
>> [image: \rho_{cg} =
>> |\rho_{cg}|\left(\frac{Y}{X^3}\right)a^3h^2\text{gcm}^{-3}_p]
>>
>>
>>
>> Therefore, it seems to me, from the above, that the magnitude of the
>> density in proper cgs units is the value given in the snapshot ([image:
>> |\rho_{cg}|]) multiplied by the scale factor cubed and then the other
>> constants. That is:
>>
>> [image: |\rho_{p\text{cgs}}|=|\rho_{cg}|\left(\frac{Y}{X^3}\right)a^3h^2]
>>
>>
>> However, this seems wrong to me, as I know that:
>>
>> [image: \rho_c=a^3\rho_p]
>>
>>
>> So it seems like I could do:
>>
>> [image: \rho_{pg}=\frac{\rho_{cg}}{a^3}]
>>
>>
>> That is, the density in proper gadget units is just the value given in
>> the snapshot divided by the scale factor cubed. This quantity could then
>> undergo the conversion to cgs units as above, with the only difference
>> being that this method has me dividing by the scale factor cubed, which is,
>> of course, different than what happened the first time.
>>
>> I've thought myself into a corner on this, if that makes any sense, and
>> so my question is this: Can anyone tell me the right way to convert the
>> densities from comoving gadget units to proper cgs units? I would greatly
>> appreciate it, and I apologize for such a stupid question. Thank you very
>> much!
>>
>> Sincerely,
>> -Jared
>>
>>
>>
>>
>>
>>
>>
>>
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Received on 2016-10-07 03:42:52

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