Galactic Loops as Sources of Polarized Emission

Since I seem to have established myself as an arch-sceptic concerning the cosmological interpretation of the the BICEP2 measurement of the polarization of the cosmic microwave background (CMB), I couldn’t resist posting a link to an interesting paper by Liu et al. that has just appeared on the arXiv.

The abstract is:

We investigate possible imprints of galactic foreground structures such as the `radio loops’ in the derived maps of the cosmic microwave background. Surprisingly there is evidence for these not only at radio frequencies through their synchrotron radiation, but also at microwave frequencies where emission by dust dominates. This suggests the mechanism is magnetic dipole radiation from dust grains enriched by metallic iron, or ferrimagnetic molecules. This new foreground we have identified is present at high galactic latitudes, and potentially dominates over the expected B-mode polarisation signal due to primordial gravitational waves from inflation.

The authors argue that foreground emission from our own Galaxy has not been fully subtracted from maps of the cosmic microwave background. This emission could result in significant contamination of the CMB polarization if it is associated with dust grains aligned with the Galaxy’s magnetic field.

I’m grateful to one of the authors of the paper, Philip Mertsch, for sending me this map of the Galactic Loops with the BICEP2 region superimposed on it, demonstrating that there is potential for a contribution…

 

 

This paper is likely to provoke quite a discussion, so I thought I’d suggest one possible way of testing it, namely by updating the analysis presented by myself and Patrick Dineen in 2003 with new data. Here’s the abstract of our old paper:

We present a diagnostic test of possible Galactic contamination of cosmic microwave background sky maps designed to provide an independent check on the methods used to compile these maps. The method involves a non-parametric measurement of cross-correlation between the Faraday rotation measure (RM) of extragalactic sources and the measured microwave signal at the same angular position. We argue that statistical properties of the observed distribution of rotation measures are consistent with a Galactic origin, an argument reinforced by a direct measurement of cross-correlation between dust, free-free and synchrotron foreground maps and RM values with the strongest correlation being for dust and free-free. We do not find any statistically compelling evidence for correlations between the RM values and the COBE DMR maps at any frequency, so there is no evidence of residual contamination in these CMB maps. On the other hand, there is a statistically significant correlation of RM with the preliminary WMAP individual frequency maps which remains significant in the Tegmark et al. Wiener-filtered map but not in the Internal Linear Combination map produced by the WMAP team.

The idea is that cross-correlating the CMB pattern with Faraday rotation measures should provide an independent diagnostic of the effect of magnetic fields. Our analysis was based on old CMB data, so there’s an interesting project to be done updating it with, e.g., Planck CMB data and a larger set of rotation measures. See the comment below for a reference to more recent work along these lines, but still not including Planck.

Anyway, this all goes to show that there’s one question you can always ask about an astrophysics result: have you considered the possible role of magnetic fields?

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