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Polarized emission from polycyclic aromatic hydrocarbons (PAHs) potentiallyprovides 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 intoaccount the effect of PAH alignment with the magnetic field. We first generatea large sample of the grain angular momentum $\bf J$ by simulating thealignment of PAHs due to resonance paramagnetic relaxation that accounts forvarious interaction processes. We then calculate the polarization level of PAHemission features, for the different phases of the ISM, including the coldneutral medium (CNM), reflection nebulae (RN), and photodissociation regions(PDRs). We find that a moderate degree of PAH alignment can significantlyenhance the polarization degree of PAH emission compared to the previousresults obtained with randomly oriented $\bf J$. In particular, we find thatsmallest, negatively charged PAHs in RN can be excited to slightly suprathermalrotation due to enhanced ion collisional excitation, resulting in an increaseof the polarization with the ionization fraction. Our results suggest that RNis the most favorable environment to observe polarized PAH emission and to testalignment physics of nanoparticles. Finally, we present an explicitrelationship between the polarization level of PAH emission and the degree ofexternal alignment for the CNM and RN. The obtained relationship will beparticularly useful for testing alignment physics of PAHs by futureobservations.

Effect of alignment on polarized infrared emission from polycyclic aromatic hydrocarbons