2024MNRAS.530..129Y

Fuzzy dark matter (FDM), an attractive dark matter candidate comprising ultralight bosons (axions) with a particle mass m_a ∼ 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 m_a = 0.2-1.2 × 10^–22 eV and halo masses M_h = 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 m_a, reaching dh/dt ≃ 0.04 (0.4) kpc Gyr^–1 and ${\rm d}\sigma _z^2/{\rm d}t \simeq 4$ (150) km² s^–2 Gyr^–1 for m_a = 1.2 (0.2) × 10^–22 eV and $M_{\rm h}=7\times 10^{10} \, \rm {M}_{\odot }$, where h is the disc scale height and σ_z 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 ${\rm d}\sigma ^2_z/{\rm d}t \propto m_a^{-3}$. Finally, we examine the applicability of the Fokker-Planck approximation in FDM granulation modelling and the robustness of the m_a exclusion bound derived from the Galactic disc kinematics.