Polycyclic Aromatic Hydrocarbon and Emission Line Ratios in Active
  Galactic Nuclei and Starburst Galaxies
  We study the Polycyclic Aromatic Hydrocarbons (PAH) bands, ionic emission
lines, and Mid-infrared continuum properties, in a sample of 171 emission line
galaxies taken from literature plus 15 new active galactic nuclei (AGN) Spitzer
spectra. The continuum shape steeply rises for longer wavelengths and can be
fitted with a warm blackbody distribution of T=150-300K. The brightest PAH
spectral bands (6.2, 7.7, 8.6, 11.3, and 12.7$\mu$m) and the forbidden emission
lines of [Si II] 34.8$\mu$m, [Ar II] 6.9, [S III] 18.7 and 33.4 were detected
in all the Starbursts and in ~80% of the Seyfert~2. Taking under consideration
only the PAH bands at 7.7$\mu$m, 11.3$\mu$m, and 12.7$\mu$m we find they are
present in ~80% of the Seyfert 1, while only half of this type of activity show
the 6.2$\mu$m and 8.6 PAH bands. The observed intensities ratios for neutral
and ionized PAHs (6.2/7.7 x 11.3/7.7) were compared to theoretical intensity
ratios, showing that AGNs have higher ionization fraction and larger PAH (> 180
carbon atoms) than SB galaxies. The ratio between the ionized (7.7) and the
neutral PAH bands (8.6 and 11.3) are distributed over different ranges for AGNs
and SB galaxies, suggesting that these ratios could depend on the ionization
fraction, as well as on the hardness of the radiation field. The ratio between
the 7.7 and 11.3 bands is nearly constant with the increase of [Ne III]15.5/[Ne
II], indicating that the fraction of ionized to neutral PAH bands does not
depend on the hardness of the radiation field. The equivalent width of both PAH
features show the same dependence with [Ne III]/[Ne II], suggesting that the
PAH, emitting either ionized (7.7) or neutral (11.3) bands, may be destroyed
with the increase of the hardness of the radiation field.
