2005ApJ...633..706W

Full low-resolution (65<R<130) and high-resolution (R∼600) spectra between 5 and 37 µm obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope are presented for eight classical active galactic nuclei (AGNs) that have been extensively studied previously. Spectra of these AGNs are presented as comparison standards for the many objects, including sources at high redshift, that are being observed spectroscopically in the mid-infrared for the first time using the IRS. The AGNs are NGC 4151, Mrk 3, I Zw 1, NGC 1275, Centaurus A, NGC 7469, Mrk 231, and NGC 3079. These sources are used to demonstrate the range of infrared spectra encountered in objects that have widely different classification criteria at other wavelengths but that unquestionably contain AGNs. Overall spectral characteristics, including continuum shape, nebular emission lines, silicate absorption and emission features, and PAH emission features, are considered to understand how spectral classifications based on mid-infrared spectra relate to those previously derived from optical spectra. The AGNs are also compared to the same parameters for starburst galaxies such as NGC 7714 and the compact, low-metallicity starburst SBS 0335-052 previously observed with the IRS.
Result. confirm the much lower strengths of PAH emission features in AGNs, but there are no spectral parameters in this sample that unambiguously distinguish AGNs and starbursts based only on the slopes of the continuous spectra.