Spitzer IRS Observations of Seyfert 1.8 and 1.9 Galaxies: A Comparison with Seyfert 1 and Seyfert 2

We present Spitzermid-infrared spectra of 12 Seyfert 1.8 and 1.9 galaxies over the 5Y38 m region. We compare the spectral characteristics of this sample to those of 58 Seyfert 1 and 2 galaxies from the Spitzer archives. An analysis of the spectral shapes, the silicate 10 m feature and the emission-line fluxes have enabled us to characterize the midIR properties of Seyfert 1.8/1.9s. We find that the EWs of the 10 m silicate feature are generally weak in all Seyfert galaxies, as previously reported by several studies. The few Seyfert galaxies in this sample that show deep 10 m silicate absorption features are highly inclined and/or merging galaxies. It is likely that these absorption features originate primarily in the dusty interstellarmediumof the host galaxy rather than in a dusty torus on parsec scales close to the central engine.We find that the EWof the PAH band at 6.2 mcorrelates stronglywith the 20Y30 m spectral index. Either of these quantities is a good indicator of the amount of starburst contribution to the mid-IR spectra. The spectra of Seyfert 1.8s and 1.9s are dominated by these starburst features, similar tomost Seyfert 2s. They show strong PAHbands and a strong red continuum toward 30 m. The strengths of the high-ionization forbidden narrow emission lines [O iv] 25.89 m, [Ne iii] 15.56 m, and [Ne v] 14.32 m relative to [Ne ii] 12.81 m are weaker in Seyfert 1.8/1.9s and Seyfert 2s than in Seyfert 1s. The weakness of high-ionization lines in Seyfert 1.8Y1.9s is suggestive of intrinsically weak AGN continua and/or stronger star formation activity leading to enhanced [Ne ii]. We discuss the implications of these observational results in the context of the unified model of AGNs. Subject headinggs: dust, extinction — galaxies: active — galaxies: Seyfert