A high angular resolution view of the PAH emission in Seyfert galaxies using the James Webb Space Telescope

Ismael G. Bernete

Nowadays, there is evidence that most galaxies can host supermassive black holes. It also is widely accepted that most galaxies undergo an active phase in their evolution. The impact of the energy released by active galactic nuclei (AGN) in the interstellar medium (ISM) of the host galaxy has been proposed as a key mechanism responsible for regulating star formation (SF). The mid-infrared (IR) is the ideal spectral range to investigate the nuclear/circumnuclear regions of AGN since dust extinction is significantly lower compared to the visible range. In addition, it provides unique tracers to study the AGN-SF connection such as H2 rotational lines, fine structure lines and Polycyclic Aromatic Hydrocarbons (PAHs).

PAH molecules are made up of multiple aromatic rings, mainly containing carbon and hydrogen. They are well known on Earth as a by-product of incomplete combustions (e.g. roasted coffee). PAHs are ubiquitous and abundant in space and are amongst the most widespread organic compounds in the Universe. PAH features are also a powerful tool to characterize the interstellar medium (ISM) in different astrophysical objects and environments. However, our understanding of the effect of the hardness of the radiation field on these molecules is limited.

In this talk, I will summarise our first results on the PAH properties of AGN using new James Webb Space Telescope data. In this work, we used JWST/MIRI MRS spectroscopy of 3 Seyferts to compare their nuclear PAH emission with that of star-forming regions. Our results showed that a suite of PAH features is present in the innermost parts of luminous Seyfert galaxies. We found that the nuclear regions of AGN lie at different positions of the PAH diagnostic diagrams, whereas the SF regions are concentrated around the average values of SF galaxies. The nuclear PAH emission mainly originates in neutral PAHs. In contrast, PAH emission originating in the SF regions favours small ionised PAH grains. Therefore, our results provide evidence that the AGN have a significant impact on the ionization state and size of the PAH grains on scales of ~142 – 245 pc.

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