2023ApJ...951...87O

We investigated the inner buried nucleus of a nearby luminous infrared galaxy, NGC 4418, using high-resolution spectroscopy of fundamental carbon monoxide (CO) rovibrational absorptions around 4.67 μm 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) ¹²CO and moderately deep (optically thin) ¹³CO absorption lines and inferred a large column density (N_H2 = (5 ± 3) × 10²³ cm^–2 in front of the 5 μm photosphere) of warm (T_ex ≃ 170 K) molecular gas by assuming an isothermal plane-parallel slab illuminated by a compact background mid-infrared-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 μm photosphere (at r = several parsecs) of a compact shroud of gas and dust (d ∼ 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.