Orbit properties of massive prolate galaxies in the Illustris simulation
  We explore orbit properties of 35 prolate-triaxial galaxies selected from the
Illustris cosmological hydrodynamic simulation. We present a detailed study of
their orbit families, and also analyse relations between the relative abundance
of the orbit families and the spin parameter, triaxiality, the ratio of the
angular momentum and the baryon fraction. We find that box orbits dominate the
orbit structure for most prolate-triaxial galaxies, especially in the central
region. The fraction of irregular orbits in the prolate-triaxial galaxies is
small, and it increases with galaxy radius. Both the x-tube and z-tube orbits
are important in prolate-triaxial galaxies, especially in the outer regions.
The fraction of box orbits for prolate-triaxial galaxies ($0.7<T<1$) decreases
with the triaxiality of the stars, while the fraction of x-tube orbits
increases with the triaxiality for a given axis ratio. The fraction of box
orbits increases and the fractions of x-tube and z-tube orbits weakly decrease
with increasing baryon fraction. These results help to understand the structure
of prolate-triaxial galaxies and provide cross-checks for constructing
dynamical models of prolate-triaxial galaxies by using the Schwarzschild or the
Made-to-Measure methods.
