Mid-Infrared Diagnostics of LINER s

We report results from the first mid-infrared spectroscopic study of acomprehensive sample of 33 LINERs, observed with the Spitzer Space Telescope.We compare the properties of two different LINER populations: infrared-faintLINERs, with LINER emission arising mostly in compact nuclear regions, andinfrared-luminous LINERs, which often show spatially extended (non-AGN) LINERemission. We show that these two populations can be easily distinguished bytheir mid-infrared spectra in three different ways: (i) their mid-IR spectralenergy distributions (SEDs), (ii) the emission features of polycyclic aromatichydrocarbons (PAHs), and (iii) various combinations of IR fine-structure lineratios. IR-luminous LINERs show mid-IR SEDs typical of starburst galaxies,while the mid-IR SEDs of IR-faint LINERs are much bluer. PAH flux ratios aresignificantly different in the two groups. Fine structure emission lines fromhighly excited gas, such as [O IV], are detected in both populations,suggesting the presence of an additional AGN also in a large fraction ofIR-bright LINERs, which contributes little to the combined mid-IR light. Thetwo LINER groups occupy different regions of mid-infrared emission-lineexcitation diagrams. The positions of the various LINER types in our diagnosticdiagrams provide important clues regarding the power source of each LINER type.Most of these mid-infrared diagnostics can be applied at low spectralresolution, making AGN- and starburst-excited LINERs distinguishable also athigh redshifts.