A quantum theory of the alignment and polarization of very small dust grains
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arXiv
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| Format: | Preprint |
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2026
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| author | Lankhaar, Boy |
| author_facet | Lankhaar, Boy |
| contents | 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. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2604_26001 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | A quantum theory of the alignment and polarization of very small dust grains Lankhaar, Boy Astrophysics of Galaxies 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. |
| title | A quantum theory of the alignment and polarization of very small dust grains |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2604.26001 |