Systematic Study of the Inner Structure of Molecular Tori in Nearby U/LIRGs Using Velocity Decomposition of CO Rovibrational Absorption Lines

Abstract

Determining the inner structure of the molecular torus around an active galactic nucleus is essential for understanding its formation mechanism. However, spatially resolving the torus is difficult because of its small size. To probe the clump conditions in the torus, we therefore perform the systematic velocity-decomposition analyses of the gaseous ¹²CO rovibrational absorption lines (v = 0 → 1, ΔJ = ±1) at λ ∼ 4.67 μm observed toward four (ultra)luminous infrared galaxies using the high-resolution (R ∼ 5000–10,000) spectroscopy from the Subaru Telescope. We find that each transition has two to five distinct velocity components with different line-of-sight (LOS) velocities (V _LOS ∼ ‑240 to +100 km s^‑1) and dispersions (σ _V ∼ 15–190 km s^‑1), i.e., the components (a), (b), ⋯, beginning with the broadest one in each target, indicating that the tori have clumpy structures. By assuming a hydrostatic disk ( σV∝Rrot‑0.5 ), we find that the tori has dynamic inner structures, with the innermost component (a) outflowing with velocity ∣V _LOS∣ ∼ 160–240 km s^‑1, and the outer components (b) and (c) outflowing more slowly or infalling with ∣V _LOS∣ ≲ 100 km s^‑1. In addition, we find that the innermost component (a) can be attributed to collisionally excited hot (≳530 K) and dense ( log(nH2/cm‑3)≳6 ) clumps, based on the level populations. Conversely, the outer component (b) can be attributed to cold (∼30–140 K) clumps radiatively excited by a far-infrared-to-submillimeter background with a brightness temperature higher than ∼20–400 K. These observational results demonstrate the clumpy and dynamic structure of tori in the presence of background radiation. *This research is based on data collected at the Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. We are honored by and grateful for the opportunity to observe the universe from Maunakea, which has cultural, historical, and natural significance in Hawaii.