Oxford University researchers have used JWST observations to analyse polycyclic aromatic hydrocarbons in the nuclear region of active galaxies.
In a first-of-its-kind study led by Oxford University researchers, tiny dust molecules known as polycyclic aromatic hydrocarbons (PAHs) were analysed in the nuclear region of active galaxies. The study utilised early observations from the James Webb Space Telescope (JWST) and is the first UK-led paper to use spectroscopic data from the JWST’s Mid-Infrared Instrument (MIRI). Through this study, the researchers aim to address one of the biggest challenges in modern physics – understanding how galaxies form and evolve.
The study is published in Astronomy and Astrophysics.
Utilising polycyclic aromatic hydrocarbons
Polycyclic aromatic hydrocarbons are tiny dust molecules that are among the most widespread organic molecules in the Universe, and act as important astronomical tools. These molecules are regarded as the fundamental building blocks of prebiotic compounds, which could have had a vital role in the origin of life.
When PAH molecules are illuminated by stars, they produce extremely bright emission bands in the infrared region. This allows astronomers to trace star-formation activity and use the molecules as sensitive barometers of the local physical conditions.
The importance of the JWST to research
The new analysis, led by Dr Ismael García-Bernete from Oxford University’s Department of Physics, used JWST instruments to characterise the PAH properties in the nuclear region of three luminous active galaxies. Dr García-Bernete based the study on spectroscopic data from the JWST’s MIRI, which measures light in the five to 28 micron wavelength range. These observations were then compared to theoretical predictions for these molecules.
Dr García-Bernete stated: “The JWST MIRI provides us with a fantastic opportunity to observe galaxies in a way that just hasn’t been possible up until now. We were excited to find that these organic molecules can actually survive in extremely harsh conditions.”
The results were surprising, overturning results from previous studies that predicted that polycyclic aromatic hydrocarbons would be destroyed in the vicinity of the black hole at the centre of an active galaxy.
Instead, the new research revealed that PAH molecules can survive in this region, even where very energetic photons could potentially rip them apart. This could be due to a large amount of molecular gas protecting the molecules in the nuclear region.
Limitations to using the molecules
The results showed, however, that even where polycyclic aromatic hydrocarbons survived, the supermassive black holes, located at the centre of the galaxies, significantly impacted the properties of the molecules. More fragile, small, and charged PAH molecules may have been destroyed here, as the proportion of larger and neutral molecules increased. This is a major limitation of using the PAH molecules to explore how rapidly an active galaxy creates new stars.
Dr García-Bernete explained: “This research is of great interest to the wider astronomy community, particularly those focused on the formation of planets and stars in the most distant and faint galaxies.
“It is incredible to think that we can observe PAH molecules in the nuclear region of a galaxy and the next step is to analyse a larger sample of active galaxies with different properties. This will enable us to better understand how PAH molecules survive and which are their specific properties in the nuclear region. Such knowledge is key to using PAHs as an accurate tool for characterising the amount of star formation in galaxies, and thus, how galaxies evolve over time.”
The PAH models were developed by Professor Dimitra Rigopoulou’s research group, in collaboration with the Physical Chemistry group at the University of Oxford. The research was funded by the University of Oxford’s Fell Fund.
