Extremely Buried Nucleus of IRAS 17208$-$0014 Observed at Sub-Millimeter
  and Near-Infrared Wavelengths
  The ultraluminous infrared galaxy IRAS 17208$-$0014 is a late-stage merger
that hosts a buried active galactic nucleus (AGN). To investigate its nuclear
structure, we performed high spatial resolution
($\sim0.\!\!^{\prime\prime}04\sim32\,\mathrm{pc}$) Atacama Large
Millimeter/submillimeter Array (ALMA) observations in Band 9
($\sim$450\,\micron\ or $\sim$660\,GHz), along with near-infrared AKARI
spectroscopy in 2.5--5.0\,\micron. The Band 9 dust continuum peaks at the AGN
location, and toward this position CO($J$=6--5) and CS($J$=14--13) are detected
in absorption. Comparison with non-local thermal equilibrium calculations
indicates that, within the central beam ($r\sim20\,\mathrm{pc}$), there exists
a concentrated component that is dense ($10^7\,\mathrm{cm}^{-2}$) and warm
($>$200\,K) and has a large column density
($N_\mathrm{H_2}>10^{23}\,\mathrm{cm}^{-2}$). The AKARI spectrum shows deep and
broad CO ro-vibrational absorption at 4.67\,\micron. Its band profile is well
reproduced with a similarly dense and large column but hotter ($\sim$1000\,K)
gas. The region observed through absorption in the near-infrared is highly
likely in the nuclear direction, as in the sub-millimeter, but with a narrower
beam including a region closer to the nucleus. The central component is
considered to possess a hot structure where vibrationally excited HCN emission
originates. The most plausible heating source for the gas is X-rays from the
AGN. The AKARI spectrum does not show other AGN signs in 2.5--4\,\micron, but
this absence may be usual for AGNs buried in a hot mid-infrared core. Besides,
based on our ALMA observations, we relate various nuclear structures of IRAS
17208$-$0014 that have been proposed in the literature.
