A SPITZER SPECTROSCOPIC SURVEY OF LOW-IONIZATION NUCLEAR EMISSION-LINE REGIONS: CHARACTERIZATION OF THE CENTRAL SOURCE

We have conducted a comprehensive mid-infrared (IR) spectroscopic investigation of 67 low-ionization nuclear emission line regions (LINERs) using archival observations from the high-resolution modules of the Infrared Spectrograph on board the Spitzer Space Telescope. Using the [Ne V] 14 and 24 mu m lines as active galactic nuclei (AGNs) diagnostics, we detect active black holes in 39% of the galaxies in our sample, many of which show no signs of activity in either the optical or X-ray bands. In particular, a detailed comparison of multiwavelength diagnostics shows that optical studies fail to detect AGNs in galaxies with large far-IR luminosities. These observations emphasize that the nuclear power source in a large percentage of LINERs is obscured in the optical. Indeed, the majority of LINERs show mid-IR [Ne V] 14/[Ne V] 24 mu m flux ratios well below the theoretical low-density limit, suggesting that there is substantial extinction toward even the [Ne V]-emitting region. Combining optical, X-ray, and mid-IR diagnostics, we find an AGN detection rate in LINERs of 74%, higher than previously reported statistics of the fraction of LINERs hosting AGNs. The [Ne V] 24 mu m /[O IV] 26 mu m mid-IR line flux ratio in AGN-LINERs is similar to that of standard AGNs, suggesting that the spectral energy distribution of the intrinsic optical/UV continuum is similar in the two. This result is in contrast to previous suggestions of a UV deficit in the intrinsic broadband continuum emission in AGN-LINERs. Consistent with our finding of extinction to the [Ne V]-emitting region, we propose that extinction may also be responsible for the observed optical/UV deficit seen in at least some AGN-LINERs.