Probing Star-Formation and AGN activity with mid-infrared spectroscopy

Strong emission features dominate the spectra of galaxies in the local Universe and out to redshift of 4. These features are attributed to IR fluorescence of Polycyclic Aromatic Hydrocarbons (PAH) molecules upon absorption of UV photons. PAHs are ubiquitous in the Universe and have been used to trace Star Formation activity in star-forming, normal and Active Galactic Nuclei. However, the use of PAHs to trace Star Formation in galaxies is primarily based on empirical relations without much insight about the properties of PAH molecules or, if and how galaxy properties (especially those of the ISM) influence PAH emission. For instance, it is not known how PAHs respond to intense UV/X-ray radiation (as found in the surroundings of an AGN) or how they are affected by UV photo-processing and shocks. As a result, the exact influence of the AGN on PAH emission or how this influence might be mediated by the presence of molecular hydrogen (H2) in the circumnuclear regions of AGN is not yet known. In addition, lack of theoretical models suitable for the harsh environments of galaxies are not widely available.

Previous investigations of PAH emission in galaxies utilised spectra from ISO and Spitzer and were limited by poor angular and spectral resolution. With its superb
spectral and spatial resolution, wavelength coverage and unprecedented sensitivity especially for low brightness regions, the James Webb Space Telescope is expected to revolutionize PAH research. These new cutting edge observations combined with new theoretical models developed by our group here in Oxford means that we are able to tackle key questions on galaxy evolution. The focus of this DPhil will be on the following questions:
1) what is the role of the AGN on PAH emission in nuclear and circumnuclear regions?
2) what is the role of PAHs in regulating a galaxy's star formation efficiency though
photoelectric heating and does it change with redshift?
3) can we use PAH emission from a distant galaxy to glean information about the conditions of the ISM at those redshifts?

To answer these questions the DPhil student will use new observations acquired with the JWST. These observations will be fully analysed with in-house developed software and modelled using our own PAH theoretical models. These are unique computations that we have been able to produce for the very first time. In addition, the DPhil student will investigate a new way to model PAH emission from galaxies by developing `synthetic' MIR spectra based on the combination of theoretical PAH models and real galaxy continua. If successful this technique has the potential to revolutionise the way MIR spectra are interpreted in the future. The outcome of the investigation is bound to establish PAHs as robust Star-Formation Rate indicators applicable not only in local galaxies but all the way to distant galaxies at redshift~7 which will become accessible with the JWST.