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arXiv 1608.08138astro-ph.COastro-ph.GA
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尘埃极化与ISM turbulence

Dust polarization and ISM turbulence

Robert R. Caldwell, Chris Hirata, Marc Kamionkowski

摘要:

也许普朗克尘埃偏振测量的最有趣的结果是观察到E模式偏振的功率是B模式的两倍,而不是普朗克之前预期的E和B模式大致相等的尘埃功率。在这里,我们展示了如何E-和B-模式的权力取决于详细的磁化星际介质中的波动属性。这些波动被分为慢、快和阿尔文磁流体动力学(MHD)波,一旦气体与磁场压力的比值被指定,这些波动就被确定了。我们还参数化模型的功率振幅和功率各向异性的三种类型的波。我们发现,所观察到的EE/BB比(及其标度不变性)和TE的正相关性不能很容易地解释的MHD湍流的青睐模型。所观察到的温度/极化波动的幂律指数也不利于MHD湍流。因此,我们推测,这些尘埃偏振测量探测的0.1- 30 pc的长度尺度不是由MHD湍流描述的,而是探测驱动ISM湍流的大尺度物理。我们开发了一个简单的唯象模型,基于随机位移的磁化流体,产生EE/BB $\simeq2$和一个积极的TE互相关。根据这个模型,EE/BB和TE信号是由于纵向,而不是横向,模式在随机位移场,提供,也许,一些线索的机制,搅拌ISM。未来的调查涉及的EE/BB比的空间依赖性,TE的相关性,并从统计各向同性的尘埃偏振图,以及进一步测试的一些假设,在此分析中,局部偏离,概述。这项工作也可能有助于改善前景分离技术的研究CMB偏振。

英文摘要:

Perhaps the most intriguing result of Planck's dust-polarization measurements is the observation that the power in the E-mode polarization is twice that in the B mode, as opposed to pre-Planck expectations of roughly equal dust powers in E and B modes. Here we show how the E- and B-mode powers depend on the detailed properties of the fluctuations in the magnetized interstellar medium. These fluctuations are classified into the slow, fast, and Alfvén magnetohydrodynamic (MHD) waves, which are determined once the ratio of gas to magnetic-field pressures is specified. We also parametrize models in terms of the power amplitudes and power anisotropies for the three types of waves. We find that the observed EE/BB ratio (and its scale invariance) and positive TE correlation cannot be easily explained in terms of favored models for MHD turbulence. The observed power-law index for temperature/polarization fluctuations also disfavors MHD turbulence. We thus speculate that the 0.1--30 pc length scales probed by these dust-polarization measurements are not described by MHD turbulence but, rather, probe the large-scale physics that drives ISM turbulence. We develop a simple phenomenological model, based on random displacements of the magnetized fluid, that produces EE/BB $\simeq2$ and a positive TE cross-correlation. According to this model, the EE/BB and TE signals are due to longitudinal, rather than transverse, modes in the random-displacement field, providing, perhaps, some clue to the mechanism that stirs the ISM. Future investigations involving the spatial dependence of the EE/BB ratio, TE correlation, and local departures from statistical isotropy in dust-polarization maps, as well as further tests of some of the assumptions in this analysis, are outlined. This work may also aid in the improvement of foreground-separation techniques for studies of CMB polarization.

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