Polycyclic aromatic hydrocrbons

Polycyclic aromatic hydrocarbons (PAHs) are carbon-based molecules that typically consist of one or more carbon rings. These molecules absorb a substantial fraction of UV and optical photons, primarily from young stars, leading to their excitation. Once excited, PAH molecules emit characteristic infrared (IR) features—most prominently at 3.3, 6.2, 7.7, 8.6, 11.3, 12.7, and 17.0 µm—through vibrational relaxation. As such, PAH features are widely regarded as reliable tracers of star formation activity in star-forming (SF) galaxies, as well as in active galactic nuclei (AGN).

PAH emission features are commonly observed in the nuclear and circumnuclear regions of many active galactic nuclei (AGN), suggesting that complex organic molecules can persist even in the presence of intense radiation fields. However, their survival near AGN has been debated, as high-energy radiation—particularly UV and X-rays—can destroy or ionize PAH molecules. Several studies, including Ruschel-Dutra et al. (2014) and Voit (1992), have reported evidence for such destruction.

Recent observations with the James Webb Space Telescope (JWST) have supported this, revealing that the strengths of the 6.2, 7.7, and 8.6 micron PAH features are significantly suppressed within the central kiloparsec of Seyfert galaxies. This suggests that AGN activity can indeed reduce PAH emission, either by destroying the molecules or altering their ionization state (see García-Bernete et al. 2022; Armus et al. 2023).

However not all PAH features are equally affected. In local Seyfert galaxies, the 11.3 micron PAH emission has been found to correlate with the [Ne II] 12.8 micron line, a well-established tracer of star formation. This indicates that the 11.3 micron feature may be a more reliable indicator of star formation in AGN-hosting galaxies (Diamond-Stanic & Rieke 2010, 2012; Esquej et al. 2014). Furthermore, Alonso-Herrero et al. (2014) showed that this feature can survive in regions as close as 10 parsecs from the AGN, supporting the idea that it's more resilient to AGN radiation. Similar conclusions were drawn by Esquej et al. (2014) and Ramos Almeida et al. (2014).

These findings are consistent with more recent JWST studies (García-Bernete et al. 2024; Zhang et al. 2024), which suggest that they could be shield from dense material—such as the dusty torus or host galaxy structures— and then, they can protect PAH molecules from AGN radiation. This natural shielding allows the 11.3 micron feature to remain detectable even in close proximity to the AGN, making it a valuable tool for probing star formation in AGN-dominated environments.

Despite growing evidence that AGN radiation can affect PAH molecules, the overall influence of AGN on PAH emission is still not fully understood. While some studies report a clear suppression of certain PAH features in AGN-dominated regions, others find that specific features, such as the 11.3 micron band, can survive even in close proximity to the nucleus. This variability suggests that the impact of AGN on PAH emission may depend on several factors, including the geometry and density of surrounding material, the strength of the AGN, and the spatial scale being probed. As a result, the use of PAH features as reliable tracers of star formation in AGN hosts remains a matter of active investigation.

This uncertainty extends to the broader question of how AGN activity influences star formation—a key aspect of AGN feedback. Many efforts have been devoted to understanding the link between star formation rates (SFR) and AGN luminosity or accretion rate.