Polycyclic aromatic hydrocarbons and the ionized gas

Abstract

We present a study for a sample of galaxies with active nuclei to characterize the main type of PAH molecules present in these objects and the local physical conditions of their irradiating sources, as well as the characteristics of the residing ionized gas, by combining optical and infrared data. Photoionization models were built with the {\sc CLOUDY} code to reproduce optical emission line ratios in combination with PAH intensity ratios. We find that the species containing 10 $-$ 82 carbon atoms are the most abundant in the sample. We suggest that family of species with only two or three fused rings of and a nitrogen hanging, such as small aromatic amides are important targets worthy of consideration in future experimental/theoretical as well as observational studies. We find that the AGN photoionization models reproduce most of the observational data in the log (6.2/11.3) versus log ([\ion{N}{ii}]$λ$6584/H$α$) diagram with the optical to X-ray spectral index of $α_{\rm ox}=-1.4$. The flux of small PAH, as well as the flux of ionized PAHs and PANH, decrease as the logarithm of the ionization parameter (log $U$) increases. The 6.2/11.3 PAH intensity ratio presents anti correlation between the oxygen abundance and log $U$, in the sense that the 6.2/11.3 ratio decreases as the oxygen abundance and log $U$ increases. Finally, we found that the ionization degree of PAH species increases with the decreasing of the 11.3/7.7 ratio and the log U, in agreement with the models proposed by Draine \& Li.