PAHs Under Pressure: Tracing Star Formation in the Harsh Environments of AGN

Mid-infrared polycyclic aromatic hydrocarbon (PAH) features are among the most widely used tracers of star formation in galaxies, as they originate from small dust grains excited by ultraviolet photons from young stars. In active galactic nuclei (AGN), however, the strong radiation field from the central engine can modify or destroy PAH molecules, potentially biasing star-formation rate (SFR) estimates based on these features. Understanding how PAHs behave in AGN environments is therefore crucial to disentangling the nuclear and host contributions to the mid-infrared emission.
In this talk, I will present results from an analysis of low-resolution Spitzer/IRS spectra of 148 nearby AGN (⟨z⟩ = 0.03). We measured the fluxes of the 6.2, 7.7, 8.6, and 11.3 μm PAH bands, derived PAH-based SFRs, and compared them with X-ray properties such as luminosity, obscuration, and Eddington ratio. We find that the 11.3 μm feature is detected in over 90% of our sample and, Its luminosity shows a clear positive correlation with the AGN hard X-ray luminosity, suggesting that these parameters are connected on galactic scales. Interestingly, this correlation becomes stronger in unobscured systems, consistent with the idea that PAH emission (and therefore star formation) can be affected by the AGN radiation field. No significant trends are found with the Eddington ratio or black hole mass, implying that global star formation is not directly regulated by the instantaneous accretion rate. Altogether, our results show that PAH emission cannot be used to trace star formation in galaxies hosting AGN, until the effects of AGN luminosity and obscuration are properly taken into account. This work helps us understand how black hole growth and star formation coexist and/or possibly interact.