The Extremely Buried Nucleus of IRAS 17208-0014 Observed at Submillimeter and Near-infrared Wavelengths

Abstract

The ultraluminous infrared galaxy IRAS 17208-0014 is a late-stage merger that hosts a buried active galactic nucleus (AGN). To investigate its nuclear structure, we performed high-spatial-resolution ( ~ 0.″04 ~ 32 pc) Atacama Large Millimeter/submillimeter Array (ALMA) observations in Band 9 (~450 μm or ~660 GHz), along with near-infrared AKARI spectroscopy in 2.5-5.0 μm. The Band 9 dust continuum peaks at the AGN location, and toward this position CO(J = 6 - 5) and CS(J = 14 - 13) are detected in absorption. Comparison with nonlocal thermal equilibrium calculations indicates that, within the central beam (r ~ 20 pc), there exists a concentrated component that is dense (10⁷ cm^-3) and warm (>200 K) and has a large column density ( ${N}_{{{\rm{H}}}_{2}}\gt {10}^{23}\,{\mathrm{cm}}^{-2}$ ). The AKARI spectrum shows deep and broad CO rovibrational absorption at 4.67 μm. Its band profile is well reproduced with a similarly dense and large column but hotter (~1000 K) gas. The region observed through absorption in the near-infrared is highly likely in the nuclear direction, as in the submillimeter, but with a narrower beam including a region closer to the nucleus. The central component is considered to possess a hot structure where vibrationally excited HCN emission originates. The most plausible heating source for the gas is X-rays from the AGN. The AKARI spectrum does not show other AGN signs in 2.5-4 μm, but this absence may be usual for AGNs buried in a hot mid-infrared core. Further, based on our ALMA observations, we relate the various nuclear structures of IRAS 17208-0014 that have been proposed in the literature.