Charged polycyclic aromatic hydrocarbon clusters and the galactic extended red emission

Area of Science:

• Astrochemistry
• Interstellar Medium Physics
• Astrophysical Spectroscopy

Background:

• The origin of the broad extended red emission (ERE), observed since 1975, remains unidentified despite its widespread presence in the Galaxy.
• ERE is a photoluminescent process occurring in diverse interstellar environments, spanning 540-900 nm.
• Previous candidate carriers for ERE have been proposed but subsequently ruled out by observations.

Purpose of the Study:

• To identify the molecular species responsible for the extended red emission (ERE).
• To investigate the role of polycyclic aromatic hydrocarbon (PAH) related materials in ERE.
• To link ERE carriers to physical conditions and carbonaceous dust evolution in the interstellar medium.

Main Methods:

• Analysis of spectral properties and observational constraints of potential ERE carriers.
• Modeling of closed-shell cationic PAH dimers and their unique spectral characteristics.
• Investigating the emission from mixtures of charged PAH clusters.

Main Results:

• Closed-shell cationic PAH dimers exhibit peculiar spectra and unique properties that align with observational constraints for ERE.
• Emission from mixtures of charged PAH clusters is proposed as a significant contributor to the observed ERE.
• The study provides insights into the structure, stability, abundance, and ionization balance of PAH species in emission zones.

Conclusions:

• Charged PAH clusters, particularly dimers, are identified as the most plausible carriers of the extended red emission (ERE).
• This identification offers a fundamental understanding of nanoscale processes in carbon particle nucleation and growth.
• The findings shed light on the evolution of carbonaceous dust in the interstellar medium and reflect physical conditions within emission zones.