Cosmic-Ray Transport, Energy Loss, and Influence in the Multiphase Interstellar Medium
Cosmic-Ray Transport, Energy Loss, and Influence in the Multiphase Interstellar Medium
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DOI：
10.3847/1538-4357/abf64c
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发表时间：
2020-12
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影响因子：
--
通讯作者：
Chad Bustard;E. Zweibel
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文献类型：
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作者：
Chad Bustard;E. Zweibel
GeV能量的宇宙射线的整体传播速度受到磁流体波频繁散射的限制。大多数解释这种限制的星系演化模拟都假设气体是完全电离的，宇宙射线与阿尔芬波很好地耦合;然而，在星系演化模拟中经常无法解决的多相密度不均匀性会导致宇宙射线碰撞和由宇宙射线去耦和部分中性气体中的流速度升高驱动的电离依赖的传输。宇宙射线如何导航和影响这样的介质，我们能用观测来约束这种传输吗？在本文中，我们模拟宇宙射线正面撞击理想化，部分中性云和对数正态分布的团块，有和没有电离依赖的运输。有了这些高分辨率的模拟，我们确定云接口的关键区域，宇宙射线的前锋可以开发一个阶梯压力梯度足以无碰撞地产生波，克服离子中性阻尼，并施加力的云。我们发现，冷云的加速是阻碍了只有一个因素的几个电离依赖的运输时，包括与磁场强度和云的维数的额外的依赖关系。我们还探讨了宇宙射线如何对背景气体进行采样并量化碰撞损失。当包含电离依赖的输运时，强子伽马射线发射图在性质上是不同的，但整体光度的变化只有一个很小的因素，因为宇宙射线在冷云中的短停留时间被宇宙射线采样的较高密度所抵消。
The bulk propagation speed of GeV-energy cosmic rays is limited by frequent scattering off hydromagnetic waves. Most galaxy evolution simulations that account for this confinement assume the gas is fully ionized and cosmic rays are well coupled to Alfvén waves; however, multiphase density inhomogeneities, frequently underresolved in galaxy evolution simulations, induce cosmic-ray collisions and ionization-dependent transport driven by cosmic-ray decoupling and elevated streaming speeds in partially neutral gas. How do cosmic rays navigate and influence such a medium, and can we constrain this transport with observations? In this paper, we simulate cosmic-ray fronts impinging upon idealized, partially neutral clouds and lognormally distributed clumps, with and without ionization-dependent transport. With these high-resolution simulations, we identify cloud interfaces as crucial regions where cosmic-ray fronts can develop a stairstep pressure gradient sufficient to collisionlessly generate waves, overcome ion–neutral damping, and exert a force on the cloud. We find that the acceleration of cold clouds is hindered by only a factor of a few when ionization-dependent transport is included, with additional dependencies on magnetic field strength and cloud dimensionality. We also probe how cosmic rays sample the background gas and quantify collisional losses. Hadronic gamma-ray emission maps are qualitatively different when ionization-dependent transport is included, but the overall luminosity varies by only a small factor, as the short cosmic-ray residence times in cold clouds are offset by the higher densities that cosmic rays sample.