Mid-Infrared Galaxy Classification Based on Silicate Obscuration and PAH Equivalent Width

We present a new diagnostic diagram for mid-infrared spectra of infraredgalaxies based on the equivalent width of the 6.2 micron PAH emission featureand the strength of the 9.7 micron silicate feature. Based on the position inthis diagram we classify galaxies into 9 classes ranging fromcontinuum-dominated AGN hot dust spectra and PAH-dominated starburst spectra toabsorption-dominated spectra of deeply obscured galactic nuclei. We find thatgalaxies are systematically distributed along two distinct branches: one of AGNand starburst-dominated spectra and one of deeply obscured nuclei andstarburst-dominated spectra. The separation into two branches likely reflects afundamental difference in the dust geometry in the two sets of sources: clumpyversus non-clumpy obscuration. Spectra of ULIRGs are found along the fulllength of both branches, reflecting the diverse nature of the ULIRG family.Comment: 5 pages, 3 figures. Accepted for publication in ApJ Letter