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

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 & 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) (CO)-C-12 and moderately deep (optically thin) (CO)-C-13 absorption lines and inferred a large column density (N (H2) = (5 & PLUSMN; 3) x 10(23) cm(-2) in front of the 5 & mu;m photosphere) of warm (T (ex) & SIME; 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 & mu;m photosphere (at r = several parsecs) 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.

Automated summary

We investigated the inner nucleus of the nearby luminous infrared galaxy NGC 4418 using high-resolution spectroscopy. By analyzing carbon monoxide (CO) absorptions, we were able to examine the physical and kinematic properties in the hot inner region. Our findings suggest the presence of a significant amount of warm molecular gas surrounding a compact shroud of gas and dust.