Galactic disc heating by density granulation in fuzzy dark matter simulations

Abstract

Fuzzy dark matter (FDM), an attractive dark matter candidate comprising ultralight bosons (axions) with a particle mass $m_a\sim 10^{-22}$ 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 $m_a=0.2-1.2\times 10^{-22}$ eV and halo masses $M_{\text{h}}=0.7-2.8\times 10^{11}$ M${}_{\odot }$. 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 $dh/dt\simeq 0.04$ ($0.4$) kpc Gyr${}^{-1}$ and $d{\sigma }_z^2/dt\simeq 4$ ($150$) km${}^2$s${}^{-2}$Gyr${}^{-1}$ for $m_a=1.2$ ($0.2$) $\times 10^{-22}$ eV and $M_{\text{h}}=7\times 10^{10}{\text{M}}_{\odot }$, where $h$ is the disc scale height and ${\sigma }_z$ 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 $d{\sigma }_z^2/dt\propto m_a^{-3}$. 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.

20 pages, 26 figures; Accepted for publication in MNRAS

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