Polarization of Infrared Emission from Polycyclic Aromatic Hydrocarbons

TL;DR

This paper presents a new model for polarized infrared emission from PAHs, incorporating magnetic field alignment effects, and supports it with observational data, enhancing the potential of PAHs as magnetic field tracers.

Contribution

The paper introduces a novel model of polarized PAH emission that accounts for grain alignment with magnetic fields, supported by observational polarization measurements.

Findings

Negatively charged small PAHs can be suprathermally rotated, increasing polarization.

The model predicts a polarization level of about 2% at 11.3 μm, consistent with observations.

PAHs can be aligned with magnetic fields, enabling polarization-based magnetic field tracing.

Abstract

Polarized infrared emission from polycyclic aromatic hydrocarbons (PAHs) is important for testing the basic physics of alignment of ultrasmall grains and potentially offers a new way to trace magnetic fields. In this paper, a new model of polarized PAH emission is presented, taking into account the effect of PAH alignment with the magnetic field. The polarization level of PAH emission features, for the different phases of the diffuse interstellar medium (ISM) is discussed. We find that negatively charged smallest PAHs in the reflection nebula can be excited to slightly suprathermal rotation due to enhanced ion collisional excitation, which enhances the degree of PAH alignment and the polarization level of PAH emission. The polarization level and polarization angle predicted by our model including PAH alignment are supported by the first detection of the polarization of $p\sim 2\%$ at 11.3 $\mu$m PAH feature from MWC 1080 nebula by Zhang et al. The theoretical and observational progress reveals that PAHs can be aligned with the magnetic field, resulting in a moderate polarization level of spinning dust emission. Polarized infrared PAH polarization would be useful for tracing the magnetic fields.