Strong Evidence for a Buried Active Galactic Nucleus in UGC 5101: Implications for LINER-type Ultraluminous Infrared Galaxies

Abstract

We report on the results of 3-4 μm spectroscopy of the ultraluminous infrared galaxy (ULIRG) UGC 5101. It has a cool far-infrared color and a LINER-type optical spectrum and so, based on a view gaining some currency, would be regarded as dominated by star formation. However, we find that it has strong 3.4 μm carbonaceous dust absorption, low equivalent width 3.3 μm polycyclic aromatic hydrocarbon (PAH) emission, and a small 3.3 μm PAH-to-far-infrared luminosity ratio. This favors an alternative scenario, in which an energetically dominant active galactic nucleus (AGN) is present behind obscuring dust. The AGN is plausibly obscured along all lines of sight (a "buried AGN") rather than merely obscured along our particular line of sight. Such buried AGNs have previously been found in thermal infrared studies of the ULIRGs IRAS 08572+3915 and IRAS F00183-7111, both classified optically as LINERs. We argue that buried AGNs can produce LINER-type optical spectra and that at least some fraction of LINER-type ULIRGs are predominantly powered by buried AGNs.