An Infrared and Optical Analysis of a Sample of XBONGs and Optically
  Elusive AGN
  We present near-infrared (NIR) spectra of four optically-elusive AGN and four
X- ray bright, optically normal galaxies (XBONGs) from the Swift-BAT survey.
With archival observations from SDSS, 2MASS, Spitzer and WISE, we test a number
of AGN indicators in the NIR and MIR; namely, NIR emission line diagnostic
ratios, the presence of coronal high-ionization lines, and infrared photometry.
Of our eight hard X-ray selected AGN, we find that optical normalcy has a
variety of causes from object to object, and no one explanation applies. Our
objects have normal Eddington ratios, and so are unlikely to host
radiatively-inefficient accretion flows (RIAFs). It is unlikely that star
formation in the host or starlight dilution is contributing to their failure of
optical diagnostics, except perhaps in two cases. The NIR continua are well-fit
by two blackbodies: one at the stellar temperature, and a hot dust component
near the dust sublimation temperature. XBONGs are more likely to have
significant hot dust components, while these components are small relative to
starlight in the optically-elusive AGN. Some of our sample have NIR line ratios
typical of AGN, but NIR diagnostics are unsuccessful in distinguishing HII
regions from AGN in general. In one object, we discover a hidden broad line
region in the NIR. These results have strong relevance to the origin of
optically normal AGN in deep X-ray surveys.
