Resumen:
One of the most challenging aspects of studying the interstellar medium (ISM) of active galaxies is the intense obscuration by dust. This phenomenon raises a series of debates about the most notable power engine of active galaxies: star formation activity, nuclear activity (AGN), or a combination of both. One way to investigate this issue is by inspecting the dust, particularly the Polycyclic Aromatic Hydrocarbons (PAHs). This class of complex molecules, detected by their broad features at mid-infrared, is a tracer of ionizing radiation by young stars, ubiquitous in the universe, and, more recently, has been found even in circumnuclear regions of AGN. In this work, we use low-resolution spectroscopy by IRS/Spitzer to probe the ISM astrochemistry in 600 dust-rich galaxies to explore the relationship between PAH molecular diversity (size, charge, and composition) and the AGN strength. We use a sample of dusty galaxies (0$\textless$z$\textless$3.7) encompassing different nuclear activity degrees, such as pure starbursts, U/LIRGs, Seyfert galaxies, and quasars. We proceed with a spectral decomposition and a molecular breakdown, using a suite of theoretical spectra from PAH vibrational transitions. We notice PAH infrared emission is well represented by neutral, small ($\textless$50 atoms of Carbon), and pure molecules (with no metal, except Carbon). We see stronger AGNs seem to host more neutral PAHs (77\%$\pm$15\%) than star-forming galaxies (56\%$\pm$10\%), which can be explained by shielding of molecular gas and photochemical processes, such as electronic recombination and PAH dehydrogenation, where the latter may be favored in stronger AGN. These results help us to understand how the dust is affected by the coexisting activities of supermassive black hole growth and star formation bursts that occur in the evolution process of gas-rich galaxies. With the new generation of IFU instruments, the technique developed in this work can be used to explore the ISM of dusty galaxies.
