Polycyclic Aromatic Hydrocarbon and Emission Line Ratios in Active Galactic Nuclei and Starburst Galaxies

Abstract

We study the polycyclic aromatic hydrocarbons (PAH) bands, ionic emission lines, and midinfrared continuum properties, in a sample of 171 emission line galaxies taken from the literature plus 15 new active galactic nucleus (AGN) Spitzer spectra. We normalize the spectra at λ = 23µm and grouped them according to the type of nuclear activity. 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 µm) and the forbidden emission lines of [Si ii] 34.8 µm, [Ar ii] 6.9 µm, [S iii] 18.7 and 33.4 µm were detected in all the starbursts and in ∼80% of the Seyfert 2. Taking under consideration only the PAH bands at 7.7µm, 11.3µm, and 12.7µm we find that they are present in ∼80% of the Seyfert 1, while only half of this type of activity show the 6.2µm and 8.6 µm PAH bands. The observed intensity ratios for neutral and ionized PAHs (6.2µm/7.7µm × 11.3µm/7.7µm) were compared to theoretical intensity ratios, showing that AGNs have higher ionization fraction and larger PAH molecules (≥ 180 carbon atoms) than SB galaxies. The ratio between the ionized (7.7µm) and the neutral PAH bands (8.6µm and 11.3µm) 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 7.7µm and 11.3µm bands is nearly constant with the increase of [Ne iii]15.5µm/[Ne ii] 12.8µm, 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 (strongly decreasing) with [Ne iii]/[Ne ii], suggesting that the PAH molecules, emitting either ionized (7.7µm) or neutral (11.3µm) bands, may be destroyed with the increase of the hardness of the radiation field.

Destruction and multiple ionization of PAHs by X-rays in circumnuclear regions of AGNs

“…The absorption features of this very luminous Sy1 galaxy, which is located at 128.6 Mpc (Scott et al 2004), are consistent with the presence of a warm absorber, most likely a dusty torus (Ebrero et al 2010). Moreover, weak PAH emission features have been independently observed in Mrk 279 by different groups (Santos-Lleó et al 2001;Sales et al 2010).…”

“…Mrk 279 Gu & Huang (2002) 2001, 2007; Sales et al 2010Sales et al , 2013. The first two entries are Seyfert 1 (Sy1) AGNs, while the others are classified as Seyfert 2 (Sy2).…”

Hot graphite dust and the infrared spectral energy distribution of active galactic nuclei

“…This feature is strong and can be used as a reliable indicator for SF activity. Recent studies have shown that the ratio between the 11.3‐ and 7.7‐μm features in AGNs may differ significantly from those observed in SF galaxies (Diamond‐Stanic & Rieke 2010; Sales, Pastoriza & Riffel 2010). To further test this issue, we compared the luminosity ratio of the 11.3‐ and 7.7‐μm features in our sample to our SF templates.…”

Mid-infrared spectroscopic evidence for AGN heating warm molecular gas

“…Several models predict the impact of radiation on the ionization and grain sizes of PAHs (Li & Draine 2001;Draine & Li 2007). Although the relations between the PAH features and their environments are not completely understood (Sadjadi et al 2015;, empirically we measure low EQW[PAH 6.2 µm] in galaxies with AGN (Smith et al 2007;Sales et al 2010). This property is a powerful diagnostic of the AGN's contribution to the MIR emission.…”