Accuracy and precision of triaxial orbit models – II. Viewing angles, shape, and orbital structure

Abstract

ABSTRACT We explore the potential of our novel triaxial modelling machinery in recovering the viewing angles, the shape, and the orbit distribution of galaxies by using a high-resolution N-body merger simulation. Our modelling technique includes several recent advancements. (i) Our new triaxial deprojection algorithm shape3d is able to significantly shrink the range of possible orientations of a triaxial galaxy and therefore to constrain its shape relying only on photometric information. It also allows to probe degeneracies, i.e. to recover different deprojections at the same assumed orientation. With this method we can constrain the intrinsic shape of the N-body simulation, i.e. the axis ratios p = b/a and q = c/a, with Δp and Δq ≲ 0.1 using only photometric information. The typical accuracy of the viewing angles reconstruction is 15°–20°. (ii) Our new triaxial Schwarzschild code smart exploits the full kinematic information contained in the entire non-parametric line-of-sight velocity distributions along with a 5D orbital sampling in phase space. (iii) We use a new generalized Akaike information criterion AICp to optimize the smoothing and to select the best-fitting model, avoiding potential biases in purely χ2-based approaches. With our deprojected densities, we recover the correct orbital structure and anisotropy parameter β with Δβ ≲ 0.1. These results are valid regardless of the tested orientation of the simulation and suggest that even despite the known intrinsic photometric and kinematic degeneracies the above described advanced methods make it possible to recover the shape and the orbital structure of triaxial bodies with unprecedented accuracy.