Evaluating the ability of triaxial Schwarzschild modelling to estimate properties of galaxies from the Illustris simulation

Abstract

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 $\overline{R_{\mathrm{e}}}$ is recovered well with average deviations within $\pm 15$ percent. Stellar mass is underestimated by $\sim 24$ percent and dark matter overestimated by $\sim 38$ 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 $\sim 13$ percent for stellar mass and $\sim 18$ 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.

21 pages, 15 figures

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• Combining Stellar Populations with Orbit-Superposition Dynamical Modelling - the Formation History of the Lenticular Galaxy NGC 3115
• MaNGA DynPop -- I. Quality-assessed stellar dynamical modelling from integral-field spectroscopy of 10K nearby galaxies: a catalogue of masses, mass-to-light ratios, density profiles and dark matter
• The SAMI Galaxy Survey: The internal orbital structure and mass distribution of passive galaxies from triaxial orbit-superposition Schwarzschild models
• The Fornax3D project: Assembly histories of lenticular galaxies from a combined dynamical and population orbital analysis
• Dynamical model of the Milky Way using APOGEE and Gaia data
• Triaxial Orbit-based Dynamical Modeling of Galaxies with Supermassive Black Holes and an Application to Massive Elliptical Galaxy NGC 1453
• SDSS-IV MaNGA: Internal mass distributions and orbital structures of early-type galaxies and their dependence on environment
• SMART: A new implementation of Schwarzschild's Orbit Superposition technique for triaxial galaxies and its application to an N-body merger simulation
• The SAMI Galaxy Survey: Environmental analysis of the orbital structures of passive galaxies
• The Fornax3D project: Environmental effects on the assembly of dynamically cold disks in Fornax cluster galaxies
• Accuracy and precision of triaxial orbit models II: Viewing angles, shape and orbital structure
• Basis function expansions for galactic dynamics: Spherical versus cylindrical coordinates
• Comparing Lensing and Stellar Orbital Models of a Nearby Massive Strong-Lens Galaxy
• Supermassive black holes in a mass-limited galaxy sample