No evidence of polarization in the $11.3\,\mu$m PAH emission line by independent analyses

TL;DR

This study re-analyzed previous claims of polarized PAH emission at 11.3 micrometers and found no statistically significant polarization, challenging earlier interpretations and supporting current dust models.

Contribution

The paper demonstrates that earlier reported polarization signals are consistent with instrumental effects, providing a more accurate assessment of PAH polarization in the ISM.

Findings

No statistically significant polarization detected in the 11.3 μm PAH emission line.

Previous polarization measurements can be explained by instrumental polarization.

Supports current dust models that predict unpolarized PAH emission.

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are commonly used as proxies for star formation, molecular gas content, and other interstellar medium (ISM) properties in our Galaxy and other galaxies. Given their abundance and brightness, polarization measurements of PAH features could, in principle, provide a probe of the ISM magnetic field and intrinsic PAH properties; however, the diagnostic power of PAH polarization remains to be established. Previous studies reported that the $11.3\,\mu$m PAH emission line in the northwestern nebula of the Herbig Be star MWC 1080 was polarized at $1.9\pm0.2$%. This level of polarization was explained via the paramagnetic relaxation process, which may allow the characterization of magnetic fields in the ISM. Using the same observations, here, we re-analyzed the $8-13\,\mu$m spectro-polarimetric observations taken with CanariCam on the 10.4-m Gran Telescopio CANARIAS (GTC), and we measure a polarization of $0.5\pm0.6$% within $11.3\pm0.2\,\mu$m, consistent with an unpolarized source, $0.6\pm0.2$% (instrumental polarization). We reproduce the previously reported polarized PAH emission line if the polarization fraction spectrum is oversubtracted by a constant instrumental polarization and the polarization uncertainties, which is inconsistent with the fundamentals of polarimetric data analysis. Thus, the published $8-13\,\mu$m spectro-polarimetric data taken with CanariCam/GTC provide no statistical evidence for a polarized $11.3\,\mu$m PAH emission line, in agreement with current dust models.