Stellar Bars Form Dark Matter Counterparts in TNG50

Abstract

Dark matter (DM) bars that shadow stellar bars have been previously shown to form in idealized simulations of isolated disk galaxies, but have yet to be studied in a fully cosmological context. In this work, we analyze a population of disk galaxies within the TNG50 simulation to determine the characteristics of their dark bars. We estimate bar strength and orientation using both the in-plane Fourier A2 density moments and the quadrupolar coefficients of the spherical harmonic basis function expansions of the density. We additionally present two novel methods for measuring the bar pattern speed Ωp and rotation axis orientation using these coefficients, and apply them to one sample galaxy located in a TNG50 subbox. Consistent with isolated simulations, DM bars are shorter than their stellar counterparts and are 75% weaker in A2. DM bars dominate the shape of the inner halo potential and are apparent in the time series of quadrupolar coefficients. In our selected subbox galaxy, the stellar and dark bars remain co-aligned throughout the last 8 Gyr and have identical Ωp. Pattern speed Ωp evolves considerably over the last 8 Gyr, consistent with torques on the bars due to dynamical friction and gas accretion, and is seen to increase following a merger at tlb = 1.5 Gyr. Rather than remaining static in time, the bar rotation axis displays both precession and nutation possibly caused by torques outside the plane of rotation. We find that the shape of the stellar and DM mass distributions are tightly correlated with Ωp.