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.
Highlights
Polarized radio continuum emission in the Milky Way typically originates as synchrotron emission caused by relativistic particles accelerating due to magnetic fields
There is always a limit in which a spectral window is narrow enough so that the data are consistent with a straight line; in such a narrow window, a single rotation measure (RM) would describe the data fully
In the models presented here, depolarization acts on the single-frequency polarized intensity largely in accordance with the models by Burn [1], Tribble [13], and Sokoloff et al [2]
Summary
Polarized radio continuum emission in the Milky Way typically originates as synchrotron emission caused by relativistic particles accelerating due to magnetic fields. As the polarized emission propagates through the magnetized, ionized component of the interstellar medium (ISM), it undergoes. A number of physical processes can cause polarization vectors to add destructively, leading to depolarization [1,2,3]. Uyaniker et al [4] introduced the “polarization horizon”, a distance beyond which all emission is depolarized (typically due to a combination of depth and beam depolarization). The line of sight beyond the polarization horizon does not contribute to the observed polarized emission. The polarization horizon is wavelength, angular resolution, and direction dependent and, true to the metaphor of a horizon, is not a solid wall beyond which we cannot see
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