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arXiv 1502.03456astro-ph.COastro-ph.GAhep-phhep-th
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轴子暗物质、孤立子和尖核问题

Axion dark matter, solitons, and the cusp-core problem

David J. E. Marsh, Ana-Roxana Pop

备注：
17 pages, 5 figures. v2: corrected omission in section 3.3. Discussions improved. References added. v3 Discussion added, sections reordered for brevity. Published in MNRAS
发表信息：
MNRAS, 451, 2479 (2015)
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摘要：

自引力玻色子场可以支持稳定和局部的场配置。对于实场，这些解随时间振荡，称为振荡子。密度分布是静态的，是孤立子。这样的孤子在轴子暗物质模型中应该是普遍存在的，孤子的特征质量和大小取决于轴子质量的某个逆幂。用薛定谔-泊松系统数值研究了非相对论性轴子的稳定组态。这种方法，以及由此产生的孤子密度分布，进行审查。利用标度对称性和测不确定原理，孤子的核心尺寸可以以一种普遍的方式与中心密度和轴子质量$m_a$相关联。孤子具有恒定的中心密度，由于压力支持，不同于冷暗物质（CDM）的尖形轮廓。这一事实的一个结果是，由超轻轴子（ULA）组成的孤子可以解决CDM的“尖点核”问题。在DM晕中，热力学将导致在大半径处具有类似CDM的Navarro-Frenk-White轮廓，在小半径处具有中心孤子核心。利用Monte-Carlo技术，探索这种形式的可能的密度分布，拟合矮球状星系的恒星运动学数据。为了使ULAs解决尖点核问题（不依靠重子反馈或其他天体物理效应），轴子质量必须满足$m_a<1.1\times 10^{-22}\text{ eV}$在95\%C. L。另一方面，具有$m_a\lesssim 1\times 10^{-22}\text{ eV}$的ULA与宇宙学结构的形成存在某种张力。一个轴子解决方案的尖核问题，从而使新的预测，未来的测量时代的再电离。另一方面，这可以被看作是结构形成可以很快在轴子/标量场DM上施加\emph{Catch 22}的证据，类似于热DM的情况。

英文摘要：

Self-gravitating bosonic fields can support stable and localised field configurations. For real fields, these solutions oscillate in time and are known as oscillatons. The density profile is static, and is soliton. Such solitons should be ubiquitous in models of axion dark matter, with the soliton characteristic mass and size depending on some inverse power of the axion mass. Stable configurations of non-relativistic axions are studied numerically using the Schrödinger-Poisson system. This method, and the resulting soliton density profiles, are reviewed. Using a scaling symmetry and the uncertainty principle, the core size of the soliton can be related to the central density and axion mass, $m_a$, in a universal way. Solitons have a constant central density due to pressure-support, unlike the cuspy profile of cold dark matter (CDM). One consequence of this fact is that solitons composed of ultra-light axions (ULAs) may resolve the `cusp-core' problem of CDM. In DM halos, thermodynamics will lead to a CDM-like Navarro-Frenk-White profile at large radii, with a central soliton core at small radii. Using Monte-Carlo techniques to explore the possible density profiles of this form, a fit to stellar-kinematical data of dwarf spheroidal galaxies is performed. In order for ULAs to resolve the cusp-core problem (without recourse to baryon feedback or other astrophysical effects) the axion mass must satisfy $m_a<1.1\times 10^{-22}\text{ eV}$ at 95\% C.L. On the other hand, ULAs with $m_a\lesssim 1\times 10^{-22}\text{ eV}$ are in some tension with cosmological structure formation. An axion solution to the cusp-core problem thus makes novel predictions for future measurements of the epoch of reionisation. On the other hand, this can be seen as evidence that structure formation could soon impose a \emph{Catch 22} on axion/scalar field DM, similar to the case of warm DM.

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