A quantum theory of the alignment and polarization of very small dust grains

TL;DR

This paper develops a quantum-mechanical model to quantify the polarization of spinning dust emission and related absorption, revealing how anisotropic illumination influences polarization levels in interstellar dust grains.

Contribution

It introduces a symmetric-top quantum model for small dust grains, linking polarization signatures to radiation anisotropy and grain properties, advancing understanding of AME polarization.

Findings

Polarization fractions near emission peaks can reach a few percent under optimal conditions.

UV/optical/IR absorption can be significantly polarized in strongly illuminated environments.

IR vibrational emission is predicted to be negligibly polarized.

Abstract

Context. Anomalous microwave emission (AME) is a component of interstellar medium emission peaking at 10-60 GHz. Its polarization is both a CMB foreground and a probe of the alignment physics of very small dust grains. Aims. We quantify when the purely rotational electric-dipole emission from very small interstellar grains (spinning dust/AME) can become measurably polarized, and we quantify related UV/optical/IR polarization diagnostics. Methods. We develop a quantum-mechanical symmetric-top model for an axisymmetric very small grain and express polarized emission and absorption coefficients in terms of irreducible density-matrix moments. Alignment is driven by anisotropic illumination; we solve a simplified two-manifold pumping model and compute (polarized) emission and absorption signatures for different dipole configurations and grain-size distributions. Results. Anisotropic illumination can generate polarized spinning-dust emission; in optimal geometries, polarization fractions near the emission peak reach the percent level, while more modest anisotropies reduce the polarization fraction. The same physics predicts polarized UV/optical/IR absorption that can be appreciable in strongly illuminated environments, whereas IR vibrational emission is predicted to be negligibly polarized. Conclusions. Spinning-dust polarization depends on radiation anisotropy, dipole geometry, and the competition between alignment pumping and depolarization by rotational emission. Joint constraints from AME polarization and UV/optical/IR absorption polarimetry provide a direct test of alignment by anisotropic radiation fields and help bound polarized microwave foregrounds.