An embedded active nucleus in the OH megamaser galaxy IRAS16399-0937

Abstract

We present a multiwavelength study of the OH megamaser galaxy IRAS16399−0937, based on new Hubble Space Telescope (HST)/Advanced Camera for Surveys F814W and Hα+[N ii] images and archive data from HST, Two Micron All Sky Survey, Spitzer, Herschel and the Very Large Array. This system has a double nucleus, whose northern (IRAS16399N) and southern (IRAS16399S) components have a projected separation of ∼6"(3.4 kpc) and have previously been identified based on optical spectra as a low ionization nuclear emission line region (LINER) and starburst nucleus, respectively. The nuclei are embedded in a tidally distorted common envelope, in which star formation is mostly heavily obscured. The infrared spectrum is dominated by strong polycyclic aromatic hydrocarbon, but deep silicate and molecular absorption features are also present, and are strongest in the IRAS16399N nucleus. The 0.435–500 μm spectral energy distribution was fitted with a model including stellar, interstellar medium and active galactic nucleus (AGN) torus components using our newMarkov ChainMonte Carlo code, clumpyDREAM. The results indicate that the IRAS16399N contains an AGN (Lbol ∼ 1044 erg s-ˡ) deeply embedded in a quasi-spherical distribution of optically thick clumps with a covering fraction ≈1. We suggest that these clumps are the source of the OHM emission in IRAS16399−0937. The high torus covering fraction precludes AGN photoionization as the origin of the LINER spectrum, however, the spectrum is consistent with shocks (v ∼ 100–200 km s-ˡ). We infer that the ∼108Mʘ black hole in IRAS16399N is accreting at a small fraction (∼1%) of its Eddington rate. The low accretion rate and modest nuclear star formation rates suggest that while the gas-rich major merger forming the IRAS16399−0937 system has triggered widespread star formation, the massive gas inflows expected from merger simulations have not yet fully developed.