Hello, I have a question regarding the particle snapshot data. Would it be possible to download only the particles, which are within a subbox of the actual Illustris-1 simulation box. For example only the particles in the regions with: 0<x<1000 kpc, 0<y<1000 kpc, 0<z<1000 kpc without downloading the particle data of the whole simulation box. Thank you very much in advance!
No unfortunately this is not possible - you'll have to download the entire box.
What you can do is only download exactly the particle types and fields you need (see Task 11 of the API cookbook).