A Spitzer Spectral Atlas of Low-mass Active Galactic Nuclei

We present Spitzer low-resolution Infrared Spectrograph (IRS) spectra (5 μm $\lesssim \lambda \lesssim$ 40 μm) of a sample of 41 Type 1 and Type 2 active galactic nuclei (AGNs) with estimated black hole masses of $M_{BH}$ $\lesssim {10}^6$ $M_{\odot }$, in order to probe the validity of unification models in this mass regime. We find that the Type 2 objects tend to have redder continuum shapes than Type 1 objects and slightly stronger polycyclic aromatic hydrocarbon (PAH) emission, and attribute these differences to a larger contribution from the host galaxies in the spectra of the Type 2 objects. Examination of emission-line ratios, such as [Ne iii]/[Ne v], [O iv]/[Ne v], and [Ne v] at 14 μm/24 μm, for a combination of our sample and higher $M_{BH}$ comparison samples, shows no evidence for structural changes over the range of black hole masses examined. Analysis of the IR to X-ray spectral energy distributions provides further evidence that low-mass AGNs are scaled down versions of more massive AGNs, with no evidence of significant differences in broadband spectral properties. We estimate the star formation rates of the host galaxies with detected PAH features from the PAH luminosities, finding average star formation rates of 0.13 $M_{\odot }$ yr ${}^{-1}$ for Type 1 objects and 0.20 $M_{\odot }$ yr ${}^{-1}$ for Type 2 objects. No silicate features, either in absorption or emission, were detected in any objects. Finally, we confirm that both the [O iv] and [Ne v] luminosities are better indicators than $L_{\left[OIII\right]}$ of an object's bolometric luminosity, regardless of $M_{BH}$.