Drivers of disc tilting I: Correlations and possible drivers for Milky Way analogues

The direction of the spin vectors of disc galaxies change over time. We present the tilting rate of a sample of galaxies in the NIHAO suite of cosmological hydrodynamical simulations. Galaxies in our sample have been selected to be isolated and to have well-determined spins. We compare the tilting rates to the predicted observing limit of Gaia, finding that our entire sample lies above the limit, in agreement with previous work. To test the role of dark matter and of gas, we compare the weighted Pearson's correlation coefficients between the tilting rates and various properties. We find no correlation between the dark halo's tilting rate, shape, or misalignment with respect to the disc, and the tilting rate of the stellar disc. Therefore, we argue that, in the presence of gas, the dark halo plays a negligible role in the tilting of the stellar disc. On the other hand, we find a strong correlation between the tilting rate of the stellar disc and the misalignment of the cold gas warp. Adding the stellar mass fraction improves the correlation, while none of the dark matter's properties together with the cold gas misalignment improves the correlation to any significant extent. This implies that the gas cooling on to the disc is the principal driver of disc tilting.