I wonder if for PartType4, each particle literally is a single 'star' it represents a 'galaxy'?
They are in the middle - i.e. simple stellar populations.
You mean there are just several stars in a particle? Thanks.
And I have an additional question.
I know that snapshot data is ordered according to FoF Groups and SubFind Groups. For Illustris-3, 'subhalo len' field says that First Subfind group has 515481 number of gas particle, so I drew a plot, with indices for x axis and x coordinates for y axis. The result was below:
I don't understand why it is like this, because I thought the whole 515481 particles would have gathered around. Why are there particles far far from the center of this group?
All Coordinates are "Spatial position within the periodic box of size 75000 ckpc/h. Comoving coordinate.", the important part here is the 'periodic box'.
To better understand this, you can plot the distance dx between each particle x and the center of the halo/subhalo (e.g. SubhaloPos). To do this, you need to account for the periodic boundary conditions, e.g. as the pseudo code under "Distance and vector between objects should obey the minimum image criterion." describes.
Thank you so much. Really helped a lot!