# Effect of alignment on polarized infrared emission from polycyclic aromatic hydrocarbons

## TL;DR

This paper develops a new model for polarized emission from PAHs considering their alignment with magnetic fields, predicting observable polarization levels in different interstellar environments to test nanoparticle alignment physics.

## Contribution

It introduces a comprehensive model of polarized PAH emission incorporating alignment effects and provides explicit relationships useful for future observational tests.

## Key findings

- Moderate PAH alignment significantly increases polarization levels.
- Small negatively charged PAHs in reflection nebulae show enhanced polarization due to suprathermal rotation.
- Reflection nebulae are optimal for observing polarized PAH emission to test alignment physics.

## Abstract

Polarized emission from polycyclic aromatic hydrocarbons (PAHs) potentially provides a new way to test basic physics of the alignment of ultrasmall grains. In this paper, we present a new model of polarized PAH emission that takes into account the effect of PAH alignment with the magnetic field. We first generate a large sample of the grain angular momentum $\bf J$ by simulating the alignment of PAHs due to resonance paramagnetic relaxation that accounts for various interaction processes. We then calculate the polarization level of PAH emission features, for the different phases of the ISM, including the cold neutral medium (CNM), reflection nebulae (RN), and photodissociation regions (PDRs). We find that a moderate degree of PAH alignment can significantly enhance the polarization degree of PAH emission compared to the previous results obtained with randomly oriented $\bf J$. In particular, we find that smallest, negatively charged PAHs in RN can be excited to slightly suprathermal rotation due to enhanced ion collisional excitation, resulting in an increase of the polarization with the ionization fraction. Our results suggest that RN is the most favorable environment to observe polarized PAH emission and to test alignment physics of nanoparticles. Finally, we present an explicit relationship between the polarization level of PAH emission and the degree of external alignment for the CNM and RN. The obtained relationship will be particularly useful for testing alignment physics of PAHs by future observations.