D8 Astronomical Polarimetry

Topic

Light polarization reveals information that photometry and spectroscopy alone cannot show.

When interstellar dust is aligned by a large-scale magnetic field, it polarizes the light from background stars passing through it; this polarization thereby reveals the orientation of the galactic magnetic field itself—the very field that caused the alignment.

Synchrotron emission produced by jets from active galactic nuclei and pulsars is, in turn, intrinsically polarized, as are certain patterns in the cosmic microwave background which, if clearly detected, would signal the presence of primordial gravitational waves.

The degree and angle of polarization of the received light are derived from the complete set of parameters describing that light's polarization state.

Faraday rotation—the twisting of a radio wave's polarization plane as it passes through a magnetized plasma—increases with both electron density and magnetic field strength along the line of sight, as well as with the square of the observed wavelength; consequently, measuring this rotation at various distinct wavelengths allows for the reconstruction of the magnetic field that produced it. Thus, from the additional information revealed by polarization—with which this document began—to the magnetic field reconstruction with which it concludes, and encompassing aligned interstellar dust and intrinsically polarized synchrotron emission, light polarization makes visible an entire dimension of astronomical information that photometry and spectroscopy, on their own, leave completely invisible.

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