Dust-polarization Maps and Interstellar Turbulence

Journal Article · Wed Apr 19 00:00:00 EDT 2017 · The Astrophysical Journal (Online)

DOI:https://doi.org/10.3847/1538-4357/aa679c· OSTI ID:1537182

Caldwell, Robert R.  ^[1]; Hirata, Chris ^[2]; Kamionkowski, Marc  ^[3]

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1. Dartmouth College, Hanover, NH (United States). Dept. of Physics and Astronomy, Wilder Lab.
2. The Ohio State Univ., Columbus, OH (United States). Center for Cosmology and Astroparticle Physics
3. Johns Hopkins Univ., Baltimore, MD (United States). Dept. of Physics and Astronomy

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 the 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 (ISM). These fluctuations can be decomposed into slow, fast, and Alfvén magnetohydrodynamic (MHD) waves, which comprise a complete basis that can be used to describe linear fluctuations of a magnetized fluid. They can alternatively be decomposed in terms of one longitudinal and two transverse components of a fluid-displacement field. The intensity (T) and E- and B-mode amplitudes induced by each of the three types of waves, in both decompositions, are then calculated. To illustrate how these tools can be applied, we consider a toy model of the ISM in which dust traces a single component of plasma, and obtain the EE/BB ratio and TE correlation for several ansatzes for the power in slow/fast/Alfvén waves and in longitudinal/transverse waves. Although our model may be too simplistic to properly describe the nonlinear structure of interstellar magnetic fields, we find that the observed EE/BB ratio (and its scale invariance) and positive TE correlation—as well as the observed power-law index for the angular spectrum of these fluctuations—are not easily accommodated in terms of simple MHD turbulence prescriptions for the expected powers in slow, fast, and Alfvén waves. We 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 find that a slightly anisotropic spectrum of random fluid displacements produces $${EE}/{BB}\simeq 2$$ and a positive TE cross-correlation. Furthermore, we find that large EE/BB and positive TE are due primarily 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 help in the improvement of foreground-separation techniques for studies of cosmic microwave background polarization.

• Research Organization: Dartmouth College, Hanover, NH (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: SC0010386
• OSTI ID: 1537182
• Journal Information: The Astrophysical Journal (Online), Vol. 839, Issue 2; ISSN 1538-4357
• Publisher: Institute of Physics (IOP)
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
Astronomy & Astrophysics
ISM: general
cosmic background radiation