In a recent study, a team of researchers led by Dr. N. Thatte from the University of Oxford, along with colleagues from various institutions including the University of Sussex, the University of Cambridge, and the Instituto de Astrofísica de Andalucía, have made significant strides in understanding the behavior of Polycyclic Aromatic Hydrocarbons (PAHs) in the interstellar medium. Their work, published in the journal Nature Astronomy, provides new insights into the conditions that shield these complex organic molecules from destructive ultraviolet (UV) radiation.
The researchers utilized archival data from the James Webb Space Telescope (JWST) to conduct a detailed analysis of the 3.4-micron spectral feature associated with PAHs. For the first time in an external galaxy, NGC 6240, they identified two distinct spectral components of the PAH 3.4-micron feature. The shorter wavelength component, at 3.395 microns, is attributed to short aliphatic chains tightly attached to the aromatic rings of the PAH molecules. The longer wavelength feature, at 3.405 microns, arises from longer, more fragile aliphatic chains that are weakly attached to the parent PAH molecule.
The study revealed that these longer aliphatic chains are more easily destroyed by far-ultraviolet photons (>5eV). PAH thermal emission, therefore, only occurs where PAH molecules are shielded from more energetic photons by dense molecular gas. The researchers observed a strong correlation between the morphology of the PAH 3.395-micron feature and the PAH 3.3-micron emission, which arises from robust aromatic PAH molecules. Similarly, they found a strong correlation between the PAH 3.405-micron morphology and the warm molecular gas, as traced by H2 vibrational lines.
The team demonstrated that the flux ratio PAH_3.395/PAH_3.405 < 0.3 strongly corresponds to regions where PAH molecules are shielded by dense molecular gas. In these regions, only modestly energetic UV photons penetrate to excite the PAHs. This finding provides a robust diagnostic tool for understanding the physical conditions of the interstellar medium in external galaxies and quantifying the energies of the photon field penetrating molecular clouds. For the energy sector, this research offers valuable insights into the behavior of complex organic molecules in extreme environments. Understanding how PAHs behave under different conditions can inform the development of materials and technologies that are resilient to harsh radiation and thermal conditions. This knowledge could be particularly relevant for the development of advanced materials for energy storage, conversion, and transmission, as well as for the design of space-based solar power systems and other energy technologies that operate in challenging environments. In summary, the study by Thatte and colleagues provides a significant advancement in our understanding of PAHs and their role in the interstellar medium. Their findings offer practical applications for the energy sector, particularly in the development of materials and technologies that can withstand extreme conditions. The research was published in the journal Nature Astronomy, providing a valuable resource for scientists and engineers working in the field of energy and materials science. This article is based on research available at arXiv.