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

Abstract

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 μ 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 _H2 = (5 ± 3) × 10 ^23 cm ^−2 in front of the 5 μ m 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 μ m 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.

Keywords

• Luminous infrared galaxies
• Infrared spectroscopy