Canals in Milky Way radio polarization maps

Details Canals in Milky Way radio polarization maps Canals in Milky Way radio polarization maps

2006-07-03

arxiv.org/abs/astro-ph/0607027v1

Mon.Not.Roy.Astron.Soc.Lett. 371 (2006) L21-L25

Astronomy and Astrophysics

Astrophysics

Accepted for publication in MNRAS Letters. 5 pages, 3 figures

Scientific paper

10.1111/j.1745-3933.2006.00200.x

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 $90\degr$ 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 \citet{Haverkorn03}. We also predict the existence of less common canals with polarization angle changes other than $90\degr$. 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 $\lesssim 1 \mathrm{rad}$, canals produced by differential rotation will still be visible.

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