### how to get the Mass stellar Volume Density data?

Di Yihuan
• 12 Jan '20

Hi TNG teams and other friends,
I have a question about "Visualize Galaxies and Halos".
This online tool can help me to draw a "Mass stellar Volume Density" picture, but how can I see the data?

I tried to download the hdf5 data of this galaxy(like TNG100-1 @ Snapshot 33: Subhalo ID: 58771 (Parent Halo ID: 153),cutout_58771.hdf5 ). But I find there is no stellar density in the file, how can I calculate it? By the way, I also need to calculate the "Mass DM Volume Density".

And My goal is to get the distribution of density with radius.

thanks
Yihuan

Dylan Nelson
• 13 Jan '20

Hello Yihuan,

The tool generates projections, i.e. the results are surface density (e.g. units of `Msun / kpc^2`).

If you want volume density (e.g. units of `Msun / kpc^3`), then this cannot be represented on a two-dimensional grid/image.

At the particle level, perhaps you can think to use `PartType4/Masses` and `PartType4/StellarHsml` to define a volume and thus obtain density.

Di Yihuan
• 13 Jan '20

ok,I see. thanks!

Di Yihuan
• 30 Jun '20

Hi Dylan，
Sorry to bother you again.
I calculate the stellar volume density with (M*32) / (4/3 pi r^3) , where 'M' is PartType4/Masses and 'r' is PartType4/StellarHsml. Is it right?

And I have a question about galactic center: if the coordinates of the black hole(PartType5/Coordinates) can be thought to the center of the galaxy at snapshot 33(z=2)? I haven't seen anything like "stellar mass center".

Dylan Nelson
• 30 Jun '20

Hello,

`PartType4/StellarHsml` is the radius enclosing the nearest 32 star particles. So the mass `M` should not be `PartType4/Masses` of that star particle, but rather `32 * mean(PartType4/Masses)` for example. Note that because of stellar mass loss, you could have up to a factor of two error in this estimate, because the masses of the 32 nearest stars could be different from the average across the simulation. Ideally, you would want to sum up the masses of these 32 nearest stars, but to do this you would have to locate them again e.g. by calculating distances.

For the position of a galaxy we would normally use `SubhaloPos` from the group catalog, not any value from the particles. You could calculate (yourself) a `SubhaloCMType` in analogy to `SubhaloCM`, if you want a center of mass position for DM/gas/stars separately.

Di Yihuan
• 7 Jul '20

Thinks!
I think the SubhaloPos is enough to me.

Here is another basic question: I found the star particles can be tens of pc away form the center, and DM particles can be hundreds of pc away from the center(TNG-100). Is that means the resolutoin of TNG-100 canbe tens of pc because of the star particles? I confused because I heard the resolution of TNG-100 is hundreds of pc.

Dylan Nelson
• 7 Jul '20

Hello,

Resolution is hard to describe with a single number. You can see many relevant numbers in the simulation table, we prefer to talk about resolution in terms of mass (the mass of particles), because there is no ambiguity.

But, it is also useful to think of the spatial resolution. Then you could quote the gravitational softening length, or the size of gas cells, or the distance between star particles as you say. For TNG100-1 the gravitational softening for dark matter and stars is 700 parcsecs (z=0), and for gas it is smaller, down to ~300 parsecs in the centers of galaxies. But you're right, there does exist structure below this scale, because cells or particles can be closer. But, you would not want to analyze the statistics of structure below this scale, because e.g. the gravitational force is softened.

Di Yihuan
• 7 Jul '20

Got it! Thank you so much.

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