Re: Comoving Density Conversion Question

From: Antonio Bibiano <antbbn_at_gmail.com>
Date: Fri, 7 Oct 2016 12:16:25 +1100

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 03:16:29

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