Extended Red Emission and the evolution of carbonaceaous nanograins in NGC 7023

TL;DR

This study investigates the origin of Extended Red Emission in NGC 7023, linking it to transitional species formed during dust grain destruction, and suggests singly ionized PAH dimers as likely carriers.

Contribution

It provides new insights into the carriers of ERE, using multi-wavelength observations and blind signal separation to connect ERE with dust grain evolution.

Findings

ERE is prominent in regions where VSGs are photo-evaporated.

ERE is absent in regions dominated by PAH$^+$.

Singly ionized PAH dimers are likely ERE carriers.

Abstract

Extended Red Emission (ERE) was recently attributed to the photo-luminescence of either doubly ionized Polycyclic Aromatic Hydrocarbons (PAH$^{++}$), or charged PAH dimers. We analysed the visible and mid-infrared (mid-IR) dust emission in the North-West and South photo-dissociation regions of the reflection nebula NGC 7023.Using a blind signal separation method, we extracted the map of ERE from images obtained with the Hubble Space Telescope, and at the Canada France Hawaii Telescope. We compared the extracted ERE image to the distribution maps of the mid-IR emission of Very Small Grains (VSGs), neutral and ionized PAHs (PAH$^0$ and PAH$^+$) obtained with the Spitzer Space Telescope and the Infrared Space Observatory. ERE is dominant in transition regions where VSGs are being photo-evaporated to form free PAH molecules, and is not observed in regions dominated by PAH$^+$. Its carrier makes a minor contribution to the mid-IR emission spectrum. These results suggest that the ERE carrier is a transition species formed during the destruction of VSGs. Singly ionized PAH dimers appear as good candidates but PAH$^{++}$ molecules seem to be excluded.