Dear TNG Team,
I am looking to plot a density distribution, so I found the field named 'SubfindDensity', but this has led to some confusion: how do I link the density with the corresponding coordinates? This question has puzzled me for a while.
Today, I found a script in the API Getting Started Guide that might be what I need, but I am still confused about its content: I noticed that the 'masses' field is a one-dimensional list, so, If I set the 'weights' directly to 'masses', how do they correspond to the coordinates? The script in API Getting Started Guide I mentioned is:

>>> with h5py.File(cutout,'r') as f:
>>> x = f['PartType0']['Coordinates'][:,0] - sub_prog['pos_x']
>>> y = f['PartType0']['Coordinates'][:,1] - sub_prog['pos_y']
>>> dens = np.log10(f['PartType0']['Masses'][:])
>>>
>>> plt.hist2d(x,y,weights=dens,bins=[150,100])
>>> plt.xlabel('$\Delta x$ [ckpc/h]')
>>> plt.ylabel('$\Delta y$ [ckpc/h]');

Many thanks for your help! By the way, all of the discriptions of 'SubfindDensity' for different PartTypes is the same, i.e.: "The local total comoving mass density, estimated using the standard cubic-spline SPH kernel over all particles/cells within a radius of SubfindHsml." What does 'all' means? All cells in one PartType or all cells of all PartTypes?Thank you!

Dylan Nelson

28 Apr

You should generally not use SubfindDensity, this is a kernel density estimate of the total matter density (not just DM, or gas, or stars, but everything together, as the docs describe).

For gas you can use PartType0/Density. There is one entry per cell, just as with Masses. The positions of the same cells are in PartType0/Coordinates'.

Dear TNG Team,

I am looking to plot a density distribution, so I found the field named 'SubfindDensity', but this has led to some confusion: how do I link the density with the corresponding coordinates? This question has puzzled me for a while.

Today, I found a script in the

API Getting Started Guidethat might be what I need, but I am still confused about its content: I noticed that the 'masses' field is a one-dimensional list, so, If I set the 'weights' directly to 'masses', how do they correspond to the coordinates? The script inAPI Getting Started GuideI mentioned is:Many thanks for your help! By the way, all of the discriptions of 'SubfindDensity' for different PartTypes is the same, i.e.: "The local total comoving mass density, estimated using the standard cubic-spline SPH kernel over

particles/cells within a radius of SubfindHsml." What does 'all' means? All cells in one PartType or all cells of all PartTypes?Thank you!allYou should generally not use

`SubfindDensity`

, this is a kernel density estimate of the total matter density (not just DM, or gas, or stars, but everything together, as the docs describe).For gas you can use

`PartType0/Density`

. There is one entry per cell, just as with Masses. The positions of the same cells are in`PartType0/Coordinates`

'.thank you！