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Galaxies 2018, 6(4), 129; https://doi.org/10.3390/galaxies6040129

Is There a Polarization Horizon?

1,2,3
1
Department of Physics and Astronomy, University of British Columbia, Vancouver, BC V6T 1Z1, Canada
2
Space Science Institute, Boulder, CO 80301, USA
3
Dominion Radio Astrophysical Observatory, National Research Council, Penticton, BC V2A 6J9, Canada
Received: 29 October 2018 / Revised: 23 November 2018 / Accepted: 25 November 2018 / Published: 30 November 2018
(This article belongs to the Special Issue The Power of Faraday Tomography)
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Abstract

Modern radio spectrometers make measurement of polarized intensity as a function of Faraday depth possible. I investigate the effect of depolarization along a model line of sight. I model sightlines with two components informed by observations: a warm ionized medium with a lognormal electron density distribution and a narrow, denser component simulating a spiral arm or Hii region, all with synchrotron-emitting gas mixed in. I then calculate the polarized intensity from 300–1800 MHz and calculate the resulting Faraday depth spectrum. The idealized synthetic observations show far more Faraday complexity than is observed in Global Magneto-Ionic Medium Survey observations. In a model with a very nearby Hii region observed at low frequencies, most of the effects of a “depolarization wall” are evident: the Hii region depolarizes background emission, and less (but not zero) information from beyond the Hii region reaches the observer. In other cases, the effects are not so clear, as significant amounts of information reach the observer even through significant depolarization, and it is not clear that low-frequency observations sample largely different volumes of the interstellar medium than high-frequency observations. The observed Faraday depth can be randomized such that it does not always have any correlation with the true Faraday depth. View Full-Text
Keywords: techniques: radio polarization; ISM: turbulence; ISM: magnetic fields techniques: radio polarization; ISM: turbulence; ISM: magnetic fields
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Hill, A.S. Is There a Polarization Horizon? Galaxies 2018, 6, 129.

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