2D vs 3D plots

[Samuel Grima]

Hi, 

Can anyone explain what the difference between a 2D and 3D profile is when using the class pynbody.analysis.profile.Profile(), please? My intuition says that the latter would plot out a 3D figure of whatever I am plotting (for example the stellar radial density), but the output is still a 2D figure (only 2 axes). However, choosing either 2D or 3D, the actual plots (lines) are different.

I am quite new to this package and I am only using it to carry out a practical for one of my Physics unit. I barely have any knowledge on cosmology :(

Any help is much appreciated.

[Jeremy Bailin]

Hi Samuel,

A 3D profile divides the volume up into spherical shells binned by spherical radius r.

A 2D profile that the system that you’re looking at is more or less a flat disk in the xy plane, and divides the volume up into hollow cylinders by cylindrical radius R.

Aside from the way the volume is divided up, a few quantities are actually different… most importantly, ‘density’ for a 3D profile means the volume density (e.g. in units of Msun/pc^3), i.e. the mass in the shell divided by the volume of the shell, but for a 2D profile it means surface density (e.g. in units of Msun/pc^2), i.e. the mass in the cylinder divided by the area of the annulus that is the cross-section of the cylinder (the volume of the the cylinder is technically infinite, so a volume density doesn’t make sense in that case).

Cheers,
-Jeremy.

--
Dr. Jeremy Bailin
Associate Professor, Dept of Physics and Astronomy
he / him

The University of Alabama
GL 311B, Tuscaloosa, AL 35487
office 205-348-4613
jba...@ua.edu | http://physics.ua.edu/

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