Testing physical models for cosmic ray transport coefficients on galactic scales: self-confinement and extrinsic turbulence at ∼GeV energies
Testing physical models for cosmic ray transport coefficients on galactic scales: self-confinement and extrinsic turbulence at ∼GeV energies
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DOI：
10.1093/mnras/staa3691
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发表时间：
2020-02
影响因子：
4.8
通讯作者：
P. Hopkins;J. Squire;T. K. Chan;E. Quataert;Suoqing Ji;D. Keres̆;C. Faucher-Giguère
中科院分区：
文献类型：
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作者：
P. Hopkins;J. Squire;T. K. Chan;E. Quataert;Suoqing Ji;D. Keres̆;C. Faucher-Giguère
星系尺度上的~ GeV宇宙射线（CR）输移的微观物理学仍然非常不确定，几乎所有的研究都采用简单的公式（例如恒定扩散率）。我们在矮星系和~ L*星系的高分辨率宇宙反馈现实环境（FIRE）模拟中探索了不同的物理驱动、各向异性、动态CR输移尺度，其中散射率随局域等离子体特性的变化而变化，这些特性是由外在湍流（ET）或自约束（SC）场景引起的，并对未分辨尺度上的湍流功率谱、alfv<s:1> -波阻尼等进行了不同的假设。我们自我一致地预测观测值，包括γ射线（Lγ）、克重、停留时间和CR能量密度，以约束模型。我们证明了许多非线性动力学效应（在更简单的模型中未被捕获）倾向于增强约束。例如，在多相介质中，即使允许在中性气体中任意快速输运，也不会实质性地减少CR停留时间（或Lγ），因为输运受到电离的WIM和“内部CGM”气体晕（104-106 K的气体，小于10\!-\!）的速率限制。30\,$ kpc)，和Lγ可以通过捕获小的“补丁”来控制。大多数物理ET模型对~ 1-10 GeV cr的散射贡献可以忽略不计，但考虑各向异性和阻尼（特别是快模）是至关重要的，否则散射率将违反观测结果。我们发现，在湍流和朗道阻尼占主导地位的热电离介质（WIM）和内部CGM中，SC模型中最广泛假设的标度会产生超过约100因子的过度约束。这表明，要么是用于推导SC中CR输运参数的准线性理论的崩溃，要么是其他新的阻尼机制在中密度电离气体中占主导地位。
The microphysics of ∼ GeV cosmic ray (CR) transport on galactic scales remain deeply uncertain, with almost all studies adopting simple prescriptions (e.g. constant diffusivity). We explore different physically motivated, anisotropic, dynamical CR transport scalings in high-resolution cosmological Feedback In Realistic Environment (FIRE) simulations of dwarf and ∼L* galaxies where scattering rates vary with local plasma properties motivated by extrinsic turbulence (ET) or self-confinement (SC) scenarios, with varying assumptions about e.g. turbulent power spectra on un-resolved scales, Alfvén-wave damping, etc. We self-consistently predict observables including γ-rays (Lγ), grammage, residence times, and CR energy densities to constrain the models. We demonstrate many non-linear dynamical effects (not captured in simpler models) tend to enhance confinement. For example, in multiphase media, even allowing arbitrary fast transport in neutral gas does not substantially reduce CR residence times (or Lγ), as transport is rate-limited by the ionized WIM and ‘inner CGM’ gaseous halo (104–106 K gas within $\lesssim 10\!-\!30\,$ kpc), and Lγ can be dominated by trapping in small ‘patches’. Most physical ET models contribute negligible scattering of ∼1–10 GeV CRs, but it is crucial to account for anisotropy and damping (especially of fast modes) or else scattering rates would violate observations. We show that the most widely assumed scalings for SC models produce excessive confinement by factors ≳100 in the warm ionized medium (WIM) and inner CGM, where turbulent and Landau damping dominate. This suggests either a breakdown of quasi-linear theory used to derive the CR transport parameters in SC, or that other novel damping mechanisms dominate in intermediate-density ionized gas.