On Predicting the Polarization of Low Frequency Emission by Diffuse Interstellar Dust

Abstract

Abstract. Several of the current and next-generation cosmic microwave background (CMB) experiments have polarimetric capability, promising to add to the finesse of precision cosmology. One of the contaminating Galactic foregrounds is thermal emission by dust. Since optical interstellar polarization is commonly seen, from differential extinction by aligned aspherical dust particles, it is expected that thermal emission from these grains will be polarized. Indeed, in the Galactic plane and in dark (molecular) clouds, dust emission in the infrared and submillimetre has been measured to be polarized. It seems likely that the faint diffuse cirrus emission, of more relevance to CMB experiments, will be polarized too. We discuss how well the amount of polarization of this component can be predicted, making use of what is known about optical (and infrared and ultraviolet) interstellar polarization and extinction. Some constraints on the alignment of the carrier of the dust-correlated anomalous microwave emission can be made as well.

Planck2015 results

“…Through correlations involving many lines of sight two ratios are determined, R S /V = (P/I)/(p V /τ V ) and R P/p = P/p V , the latter focusing directly on the polarization properties of the grains contributing to polarization. The first ratio, R S /V , is compatible with predictions based on a range of dust models that have been developed for the diffuse interstellar medium (e.g., Martin 2007;Draine & Fraisse 2009). This estimate provides new empirical validation of many of the common underlying assumptions of the models, but is not very discriminating among them.…”

“…The relatively large grains that are in thermal equilibrium and emit the radiation seen by Planck in the submillimetre also extinguish and polarize starlight in the visible (e.g., Martin 2007). Comparison of polarized emission and starlight polarization on lines of sight probed by stars provides insight into the properties of polarizing grains.…”

Planckintermediate results. XXI. Comparison of polarized thermal emission from Galactic dust at 353 GHz with interstellar polarization in the visible

“…Because many of the factors driving interstellar polarization, like grain shape, alignment efficiency, and magnetic field orientation, affect p V and P S in similar ways (Martin 2007), we are motivated to examine two polarization ratios,…”

“…It is a non-dimensional quantity where both numerator and denominator are themselves non-dimensional ratios that apply to the same geometry (common beam and common portion of the line of sight). Furthermore, by virtue of the normalization both P S /I S and p V /τ V , the submillimetre and visible polarization fractions, do not depend on the column density and become less sensitive to various factors like the size distribution, grain heating, and opacity that affect the numerator and denominator in similar ways (Martin 2007). R S/V is therefore a robust tool for the data analysis.…”

“…The most recent are further constrained by fitting the (pre-Planck) spectral energy distribution of dust emission in the infrared and submillimetre (Draine & Fraisse 2009;Siebenmorgen et al 2014). Efforts have also been made to predict the polarized thermal emission quantitatively (Martin 2007;Draine & Fraisse 2009;Draine & Hensley 2013). This paper is organized as follows.…”