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Canals in Milky Way radio polarization maps

Lookup NU author(s): Dr Andrew Fletcher, Professor Anvar Shukurov

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Abstract

Narrow depolarized canals are common in maps of the polarized synchrotron emission of the Milky Way. Two physical effects that can produce these canals have been identified: the presence of Faraday rotation measure (RM) gradients in a foreground screen, and the cumulative cancellation of polarization known as differential Faraday rotation. We show that the behaviour of the Stokes parameters Q and U in the vicinity of a canal can be used to identify its origin. In the case of canals produced by a Faraday screen we demonstrate that, if the polarization angle changes by 90 degrees. across the canal, as is observed in all fields to-date, the gradients in RM must be discontinuous. Shocks are an obvious source of such discontinuities and we derive a relation of the expected mean separation of canals to the abundance and Mach number of supernova-driven shocks, and compare this with recent observations by Haverkorn, Katgert & de Bruyn. We also predict the existence of less common canals with polarization angle changes other than 90 degrees. Differential Faraday rotation can produce canals in a uniform magneto-ionic medium, but as the emitting layer becomes less uniform the canals will disappear. We show that for moderate differences in emissivity in a two-layer medium, of up to 1/2, and for Faraday depth fluctuations of standard deviation <= 1 rad, canals produced by differential rotation will still be visible.


Publication metadata

Author(s): Fletcher A, Shukurov A

Publication type: Article

Publication status: Published

Journal: Monthly Notices of the Royal Astronomical Society

Year: 2006

Volume: 371

Issue: 1

Pages: L21-L25

ISSN (print): 0035-8711

ISSN (electronic): 1365-2966

Publisher: Wiley-Blackwell Publishing Ltd.

URL: http://dx.doi.org/10.1111/j.1745-3933.2006.00200.x

DOI: 10.1111/j.1745-3933.2006.00200.x


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