Evaluating the ability of triaxial Schwarzschild modelling to estimate
  properties of galaxies from the Illustris simulation
  We evaluate the capabilities of Schwarzschild's orbit-superposition method by
applying it to galaxies from the large scale, high resolution Illustris
simulation. Nine early-type galaxies with a range of triaxiality are selected,
and we create mock integral field unit data for five line-of-sight projections
of each galaxy. Each of the 45 mock data sets is taken as an independent
observed galaxy. Using van den Bosch's 2008 triaxial Schwarzschild
implementation, we assess model estimates of various galaxy properties,
covering mass profiles, intrinsic shapes, stellar orbit distributions and
velocity anisotropies. Total mass within $\overline{R_{\rm e}}$ is recovered
well with average deviations within $\pm15$ percent. Stellar mass is
underestimated by $\sim24$ percent and dark matter overestimated by $\sim38$
percent (assuming an NFW dark matter profile and allowing for degeneracy
between stellar mass and dark matter mass). Using a gNFW profile, these values
improve to $\sim13$ percent for stellar mass and $\sim18$ percent for dark
matter. Axis ratio estimates show a moderate bias of $\Delta (b/a)=0.07$ and
$\Delta (c/a)=0.14$ ($a\ge b\ge c$). Distributions of the orbit circularities
$\lambda_z$ and $\lambda_x$, representing rotation about the minor and major
axes, are well reconstructed. Separating orbits into thermal categories, our
models match the average fractions of these categories to within $10$ percent.
Velocity anisotropy is well estimated with values matching in the inner regions
but becoming slightly radially biased in the outer regions. Overall, the galaxy
property estimates we obtained using Schwarzschild modelling are not
implausible and are representative of the simulated galaxies we modelled.
