An Infrared and Optical Analysis of a Sample of XBONGs and Optically Elusive AGN

TL;DR

This study investigates the nature of optically elusive AGN and XBONGs using near-infrared and infrared data, revealing diverse causes for optical normalcy and identifying hidden AGN features.

Contribution

The paper provides a comprehensive NIR and MIR analysis of optically elusive AGN and XBONGs, highlighting the variability in their properties and the limitations of current diagnostics.

Findings

Optical normalcy arises from diverse causes, not a single explanation.

Some objects exhibit hidden broad line regions in NIR.

XBONGs tend to have more significant hot dust components.

Abstract

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.