  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 ($N_\mathrm{H2}=(5\pm3)\times10^{23}$
cm$^{-2}$ in front of the $5 \mu$m photosphere) of warm
($T_\mathrm{ex}\simeq170$ 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 ($d\sim100$ 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.

Astrophysics of Galaxies