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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 AGN- and starburst-excited LINERs distinguishable also at high redshifts.
In this paper I summarize the science motivations, as well as a few mid-infrared spectroscopic methods used to identify the principal mechanisms of energy production in dust enshrouded galactic nuclei. The development of the various techniques is bri
We present new mid-infrared (MIR) diagnostics to distinguish emission of active galactic nuclei (AGN) from that originating in starburst regions. Our method uses empirical spectroscopic criteria based on the fact that MIR emission from star forming o
We present mid-IR spectral decomposition of a sample of 48 Spitzer-selected ULIRGs spanning z~1-3 and likely L_IR~10^12-10^13Lsun. Our study aims at quantifying the star-formation and AGN processes in these sources which recent results suggest have e
In order to examine the relative importance of powerful starbursts and Compton-thick AGNs in NGC 6240, we have obtained mid-infrared images and low-resolution spectra of the galaxy with sub-arcsecond spatial resolution using the Keck Telescopes. Desp
We present Herschel observations of six fine-structure lines in 25 Ultraluminous Infrared Galaxies at z<0.27. The lines, [O III]52, [N III]57, [O I]63, [N II]122, [O I]145, and [C II]158, are mostly single gaussians with widths <600 km s-1 and lumino