Spitzer IRS observations of Seyfert 1.8 and 1.9 galaxies:: A comparison with Seyfert 1 and Seyfert 2

We present Spitzer mid-infrared spectra of 12 Seyfert 1.8 and 1.9 galaxies over the 5-38 mu 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 mu m feature and the emission-line fluxes have enabled us to characterize the mid-IR properties of Seyfert 1.8/1.9s. We find that the EWs of the 10 mu 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 mu m silicate absorption features are highly inclined and/or merging galaxies. It is likely that these absorption features originate primarily in the dusty interstellar medium of the host galaxy rather than in a dusty torus on parsec scales close to the central engine. We find that the EW of the PAH band at 6.2 mu m correlates strongly with the 20-30 mu 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 to most Seyfert 2s. They show strong PAH bands and a strong red continuum toward 30 mu m. The strengths of the high-ionization forbidden narrow emission lines [O IV] 25.89 mu m, [Ne III] 15.56 mu m, and [Ne V] 14.32 mu m relative to [Ne II] 12.81 mu m are weaker in Seyfert 1.8/1.9s and Seyfert 2s than in Seyfert 1s. The weakness of high-ionization lines in Seyfert 1.8-1.9s 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.