Polarization of Infrared Emission from Polycyclic Aromatic Hydrocarbons

Thiem Hoang

Published 2017-12-13Version 1

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.