模擬波暗物質中密度顆粒驅動的星系盤加熱

Abstract

波暗物質（Fuzzy dark matter，FDM）是一種暗物質候選，由超輕玻色子（軸子）m_a∼10^{-22} eV 組成，其動機出自於冷暗物質的小尺度問題，並且具有千秒差距尺度的德布羅意波長。波暗物質暈内的量子波干涉引起的密度顆粒會在宇宙時間尺度上產生隨機起伏且不會衰減。由此引起的引力擾動可能導致星系盤顯著增厚，為銀河系和其他外部星系中觀測到的厚星系盤提供了自然的解釋。根據Chiang等人（2023）的詳細分析，由FDM顆粒引起的星系盤垂直方向的加熱速率與粒子質量呈 m_a^{-3} 的關係，並從測量的星系盤動力學中得出了一個下限 m_a≳0.4×10^{-22} eV，且需要進行仔細的數值驗證。我們提供了首個自洽的波暗物質暈和星系盤共同演化的模擬，探索了粒子質量 m_a=0.2-1.2×10^{-22} eV 和暗物質暈的維里質量 M_h = 0.7–2.8 ×10^{11} M⊙。在所有模擬情况下都觀察到星系盤增厚。此外，對於粒子質量 m_a=0.4×10^{-22} eV，理論估計的來自波暗物質的星系盤加熱速率與模擬結果吻合。對於具有其他粒子質量的暗物質暈，測量的加熱速率顯示出一些偏差，且比預期的關係 m_a^{-3} 要緩和。我們討論了利用福克-普朗克方法得出的顆粒造成的加熱之有效性以及上述 m_a 排除界線的相關含義。

Fuzzy dark matter (FDM), an attractive dark matter candidate comprising ultralight bosons (axions) 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 undamped on cosmological timescales. The resulting gravitational perturbations could drive significant disc thickening, providing a natural explanation for the presence of stellar thick disc components observed in the Milky Way and other external galaxies. The detailed analytical estimate by Chiang et al. (2023) indicates that the FDM-granulation-driven disc vertical heating rate scales sharply with the ultralight axion mass as m_a^{-3} and places an exclusion bound m_a≳0.4×10^{-22} eV from the measured Galactic disc kinematics, which requires careful numerical corroboration. We present the first self-consistent simulations of FDM halo and N-body stellar disc co-evolution, exploring m_a=0.2–1.2×10^{-22} eV and halo virial masses M_h = 0.7–2.8×10^{11} M⊙. Disc thickening is observed in all simulated cases. Furthermore, analytical estimates agree well with disc heating rate measurements from FDM haloes fixing m_a=0.4×10^{-22} eV. For haloes with other values of m_a, the measured heating rates show some deviation and scale less steeply than the predicted relation m_a^{-3}. We discuss relevant implications on the validity of the Fokker-Planck approach to granulation-induced stellar heating estimate and the aforementioned m_a exclusion bound.