Dynamical models for the counter-rotating galaxy NGC 1366

Early-type galaxies hosting counter-rotating (CR) stellar and/or gaseous components offer key insights into galaxy assembly. About 30% of S0 galaxies display such CR structures. The S0 NGC 1366 was found via spectroscopic studies as hosting CR stellar components. However, spectroscopy alone provide only a partial understanding of the system’s dynamics. In this context, dynamical modelling serves as a powerful and complementary tool to investigate the galaxy’s internal structure and dynamics. The aim of this work is to compare models with the kinematic data, to constrain the number of stellar components, their mass and mass-to-light ratio, and then the relative contribution of the CR component. The dynamical modelling is performed solving the Jeans equations for axisymmetric, stationary, and collisionless multi-component systems with the JASMINE2 code. A set of two-component models is explored by varying the Satoh parameters governing the decomposition of azimuthal motions, the mass fractions regulating the relative weight of each component or the mass-to-light ratios accounting for the stellar population properties. The projected photometric and kinematic fields are generated through a dedicated post-processing pipeline, extended with modules designed to complete the model, implement the weighting scheme, convert dimensionless quantities into physical units, and produce the final observables. The modelling reveals a lower limit on the bulge mass fraction (Rb>0.336), imposed by constraints within the Satoh decomposition framework. The explored two component models successfully reproduce the observed line-of-sight velocity profile, particularly in the inner (out to R≈Re,b) and the outer (1÷2kpc) observed regions. None of the models though is able to reproduce the velocity dispersion profile, failing to replicate the observed off-centre peaks. The outcome suggests that a more complex dynamical configuration is required to fully account for the galaxy’s kinematics.