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From: Antonio Bibiano <antbbn_at_gmail.com>

Date: Fri, 7 Oct 2016 13:12:06 +1100

Hi Jared,

You are right, my answer didn't actually respond.

I went back and reread your mail and I think you have it right in the last

part,

rho_phys = rho_comoving / a^3

Don't know excatly what's the error in the first formula, I suspect the

last term should be elevated to the -3 .

But this does not change the dimension conversion,when changing form Mpc/h

to cm and

M_sun/h to grams no factor of "a" should appear. They only appear when you

convert from comoving to physical.

So to recap:

Gadget Sanpshot Units -> cgs:

rho_cgs_comoving = rho * UnitMass_in_g / UnitLength_in_cm^3 * HubbleParam^2

Comoving cgs -> physical cgs:

rho_cgs_physical = rho_cgs_comoving / a^3

Does this make more sense?

Antonio

2016-10-07 12:42 GMT+11:00 Jared Coughlin <Jared.W.Coughlin.29_at_nd.edu>:

*> 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
*

*>>>
*

*>>>
*

*>>>
*

*>>>
*

*>>>
*

*>>>
*

*>>>
*

*>>>
*

*>>> -----------------------------------------------------------
*

*>>>
*

*>>> If you wish to unsubscribe from this mailing, send mail to
*

*>>> minimalist_at_MPA-Garching.MPG.de with a subject of: unsubscribe
*

*>>> gadget-list
*

*>>> A web-archive of this mailing list is available here:
*

*>>> http://www.mpa-garching.mpg.de/gadget/gadget-list
*

*>>>
*

*>>>
*

*>>
*

*>>
*

*>> -----------------------------------------------------------
*

*>>
*

*>> If you wish to unsubscribe from this mailing, send mail to
*

*>> minimalist_at_MPA-Garching.MPG.de with a subject of: unsubscribe
*

*>> gadget-list
*

*>> A web-archive of this mailing list is available here:
*

*>> http://www.mpa-garching.mpg.de/gadget/gadget-list
*

*>>
*

*>>
*

*>
*

*> -----------------------------------------------------------
*

*>
*

*> If you wish to unsubscribe from this mailing, send mail to
*

*> minimalist_at_MPA-Garching.MPG.de with a subject of: unsubscribe gadget-list
*

*> A web-archive of this mailing list is available here:
*

*> http://www.mpa-garching.mpg.de/gadget/gadget-list
*

*>
*

*>
*

Received on 2016-10-07 04:12:10

Date: Fri, 7 Oct 2016 13:12:06 +1100

Hi Jared,

You are right, my answer didn't actually respond.

I went back and reread your mail and I think you have it right in the last

part,

rho_phys = rho_comoving / a^3

Don't know excatly what's the error in the first formula, I suspect the

last term should be elevated to the -3 .

But this does not change the dimension conversion,when changing form Mpc/h

to cm and

M_sun/h to grams no factor of "a" should appear. They only appear when you

convert from comoving to physical.

So to recap:

Gadget Sanpshot Units -> cgs:

rho_cgs_comoving = rho * UnitMass_in_g / UnitLength_in_cm^3 * HubbleParam^2

Comoving cgs -> physical cgs:

rho_cgs_physical = rho_cgs_comoving / a^3

Does this make more sense?

Antonio

2016-10-07 12:42 GMT+11:00 Jared Coughlin <Jared.W.Coughlin.29_at_nd.edu>:

Received on 2016-10-07 04:12:10

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