Study of the Inner Structure of the Molecular Torus in IRAS 08572+3915 NW with Velocity Decomposition of CO Rovibrational Absorption Lines*

Abstract

Understanding the inner structure of the clumpy molecular torus surrounding the active galactic nucleus is essential in revealing the forming mechanism. However, spatially resolving the torus is difficult because of its size of a few parsecs. Thus, to probe the clump conditions in the torus, we performed the velocity decomposition of the CO rovibrational absorption lines (Δ v = 0 →1, Δ J = ±1) at λ ∼ 4.67 μ m observed toward an ultraluminous infrared galaxy IRAS 08572+3915 NW with the high-resolution spectroscopy ( R ∼ 10,000) of Subaru Telescope. Consequently, we found that each transition had two outflowing components, i.e., (a) and (b), both at approximately ∼−160 km s −1 , but with broad and narrow widths, and an inflowing component, i.e., (c), at approximately ∼+100 km s −1 , which were attributed to the torus. The ratios of the velocity dispersions of each component led to those of the rotating radii around the black hole of R rot,a : R rot,b : R rot,c ≈ 1: 5: 17, indicating the torus where clumps are outflowing in the inner regions and inflowing in the outer regions if a hydrostatic disk with σ V ∝ R rot − 0.5 is assumed. Based on the kinetic temperature of components (a) and (b) of ∼720 and ∼25 K, respectively, estimated from the level population, the temperature gradient is T kin ∝ R rot − 2.1 . Magnetohydrodynamic models with large density fluctuations of two orders of magnitude or more are necessary to reproduce this gradient.