Warm Molecular Gas in the Central Parsecs of the Buried Nucleus of NGC 4418 Traced with the Fundamental CO Ro-vibrational Absorptions

We investigated the inner buried nucleus of a nearby luminous infrared galaxy NGC 4418 using high-resolution spectroscopy of fundamental carbon monoxide (CO) ro-vibrational absorptions around $4.67\mu$m for the first time. This method allowed us to examine the physical and kinematical properties in the hot inner region of this nucleus. We detected a series of both very deep (partly saturated) ${}^{12}$CO and moderately deep (optically thin) ${}^{13}$CO absorption lines and inferred a large column density ($N_{\mathrm{H}2}=(5\pm 3)\times {10}^{23}$ cm${}^{-2}$ in front of the $5\mu$m photosphere) of warm ($T_{\mathrm{e}\mathrm{x}}\simeq 170$ K) molecular gas by assuming an isothermal plane-parallel slab illuminated by a compact background MIR-emitting source. We modeled that the warm CO absorber almost covers the central heating source and that it is an inner layer around the $5\mu$m photosphere (at $r=$several pc) of a compact shroud of gas and dust ($d\sim 100$ pc). The width of the absorption lines ($110$ km s${}^{-1}$) and their small deviation from the systemic velocity ($<10$ km s${}^{-1}$) are consistent with a warm and turbulent layer with little bulk motion in the radial direction.