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

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
>
> 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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