Spitzer Uncovers Active Galactic Nuclei Missed by Optical Surveys in 7 Late-type Galaxies

We conducted a high resolution mid-infrared spectroscopic investigation using Spitzer of 32 latetype (Sbc or later) galaxies that show no definitive signatures of Active Galactic Nuclei (AGN) in their optical spectra in order to search for low luminosity and/or embedded AGN. These observations reveal the presence of the high ionization [NeV] 14μm and/or 24μm line in 7 sources, providing strong evidence for AGNs in these galaxies. Taking into account the variable sensitivity of our observations, we find that the AGN detection rate based on mid-infrared diagnostics in optically normal late-type galaxies is ∼ 30%, implying an AGN detection rate in late-type galaxies that is possibly 4 times larger than what optical spectroscopic observations alone suggest. We demonstrate using photoionization models with both an input AGN and an extreme EUV-bright starburst ionizing radiation field that the observed mid-infrared line ratios in our 7 AGN candidates cannot be replicated unless an AGN contribution, in some cases as little as 10% of the total galaxy luminosity, is included. These models show that when the fraction of the total luminosity due to the AGN is low, optical diagnostics are insensitive to the presence of the AGN. In this regime of parameter space, the mid-infrared diagnostics offer a powerful tool for uncovering AGN missed by optical spectroscopy. The AGN bolometric luminosities in our sample inferred using our [NeV] line luminosities range from ∼ 3×10 ergs s to ∼ 2×10 ergs s. Assuming that the AGN is radiating at the Eddington limit, this range corresponds to a lower mass limit for the black hole that ranges from ∼ 3×10M⊙ to as high as ∼ 1.5×10 M⊙. These lower mass limits however do not put a strain on the well-known relationship between the black hole mass and the host galaxy’s stellar velocity dispersion established in predominantly early-type galaxies. Our findings add to the growing evidence that black holes do form and grow in low-bulge environments and that they are significantly more common than optical studies indicate. Subject headings: Galaxies: Active— Galaxies: black hole physics – dark matter – galaxies: spiral: Galaxies — Infrared: Galaxies