Polycyclic aromatic hydrocarbons and the ionized gas
  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}]$\lambda$6584/H$\alpha$) diagram with the optical to X-ray
spectral index of $\alpha_{\rm ox}=-1.4$. 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.
