Galactic disc heating by density granulation in fuzzy dark matter simulations

Fuzzy dark matter (FDM), an attractive dark matter candidate comprising ultralight bosons (axions) with a particle mass mₐ~10⁻²² eV, is motivated by the small-scale challenges of cold dark matter and features a kpc-size de Broglie wavelength. Quantum wave interference inside an FDM halo gives rise to stochastically fluctuating density granulation; the resulting gravitational perturbations could drive significant disc thickening, providing a natural explanation for galactic thick discs. Here we present the first self-consistent simulations of FDM haloes and stellar discs, exploring mₐ=0.2-1.2×10⁻²² eV and halo masses Mₕ = 0.7-2.8×10¹¹ M_. Disc thickening is observed in all simulated systems. The disc heating rates are approximately constant in time and increase substantially with decreasing mₐ, reaching dh/dt 0.04 ($0.4$) kpc Gyr⁻¹ and dσz²/dt 4 ($150$) km²s⁻²Gyr⁻¹ for mₐ=1.2 ($0.2$) ×10⁻²² eV and Mₕ =7×10¹⁰ M_, where h is the disc scale height and σz is the vertical velocity dispersion. These simulated heating rates agree within a factor of two with the theoretical estimates of Chiang et al., confirming that the rough estimate of Church et al. overpredicts the granulation-driven disc heating rate by two orders of magnitude. However, the simulation-inferred heating rates scale less steeply than the theoretically predicted relation dσ²z/dt ∝ mₐ⁻³. Finally, we examine the applicability of the Fokker-Planck approximation in FDM granulation modelling and the robustness of the mₐ exclusion bound derived from the Galactic disc kinematics.