Mid-Infrared Diagnostics of LINERs

We report results from the first mid-infrared spectroscopic study of a comprehensive sample of 33 LINERs, observed with the Spitzer Space Telescope. We compare the properties of two different LINER populations: infrared-faint LINERs, with LINER emission arising mostly in compact nuclear regions, and infrared-luminous LINERs, which often show spatially extended (non-AGN) LINER emission. We show that these two populations can be easily distinguished by their mid-infrared spectra in three different ways: (i) their mid-IR spectral energy distributions (SEDs), (ii) the emission features of polycyclic aromatic hydrocarbons (PAHs), and (iii) various combinations of IR fine-structure line ratios. 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 are significantly different in the two groups. Fine structure emission lines from highly excited gas, such as [O IV], are detected in both populations, suggesting the presence of an additional AGN also in a large fraction of IR-bright LINERs, which contributes little to the combined mid-IR light. The two LINER groups occupy different regions of mid-infrared emission-line excitation diagrams. The positions of the various LINER types in our diagnostic diagrams provide important clues regarding the power source of each LINER type. Most of these mid-infrared diagnostics can be applied at low spectral resolution, making AGNand starburst-excited LINERs distinguishable also at high redshifts. Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, D-85741 Garching, Germany; [email protected] Department of Astronomy, University of Maryland, College Park, MD 20742, USA School of Physics and Astronomy, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel IPAC/Caltech, MS 100-22, Pasadena, CA 91125, USA Institute for Astronomy, University, of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA