Unveiling Buried AGN in OH Megamaser Galaxies

Ultra-luminous/Luminous Infrared Galaxies, (U)LIRGS, dominate the bright end of the galaxy luminosity function in the nearby universe. They are preferentially found in gas-rich mergers and are known for hosting starbursts (SBs) and active galactic nuclei (AGN), frequently in the same system. Approximately 20\% of (U)LIRGS contain highly luminous hydroxyl masers (OHMs). This OHM emission can be used to trace galaxy merger rates and associated processes of the system. OHMs are produced by the amplification of a background radio continuum source and require high column densities of molecular gas and a source of IR pumping radiation. Both SBs and AGN contain the requirements for OHMs, but it is not well understood if the OHM emission is preferentially associated with one or the other. The spectral energy distribution (SED) of (U)LIRGS includes emissions from sources such as the diffuse interstellar medium (ISM), the stellar population, and possibly an embedded AGN that is shrouded in dust. This makes the presence of an AGN difficult to detect. To investigate the dominant source of dust and gas heating in gas-rich mergers and how the OH maser emission is related to the properties of the heating source, we fit the SEDs of a representative sample of nine OHMGs using clumpyLMFIT. clumpyLMFIT uses the GRASIL spectral evolution models for the stellar component, the CLUMPY dust radiative transfer models for the AGN component, and two separate families of models for the ISM dust emission. The first, by Draine and Li (2007; DL07), is more suitable for normal disk galaxies, and the second, by Siebenmorgen and Krügel (2007; SK07), is best suited for extreme starbursts. The results obtained with clumpyLMFIT showed that two morphologically undisturbed systems are best fitted with DL07, and the disturbed systems are best fitted with SK07. Five systems show evidence for an AGN, including two optically classified as SB. The best fit was obtained with an AGN component in two other systems, an S2 and a composite, but the torus contribution to the total IR luminosity is very low. There is one inconclusive result for one of the systems with an unknown classification, while the other shows that an AGN component does not provide the best fit. Furthermore, the OHM emission can be attributed to a clumpy medium, such as the torus, in three out of seven systems analyzed