APEC Seminar (Astronomy - Particle Physics - Experimental Physics - Cosmology)

Speaker: Takao Nakagawa (JAXA/ISAS)
Title: CO ro-vibrational absorption observations through molecular tori in AGN – revealing dynamical nature of the tori -
Date (JST): Thu, Jun 30, 2022, 11:00 - 12:00
Place: Zoom
Abstract: The molecular torus in AGN (active galactic nuclei) is a key element in the unified scheme of AGN. However, the origin of the optically and geometrically thick structure that covers a large solid angle with respect to the AGN and the nature of its three-dimensional structure is still unclear. Moreover, the inner structure is difficult to be spatially resolved because the size of the torus is expected to be a few parsecs, which is beyond the scope of current observational capability. We here present an overview of our attempts to reveal the inner structures and physical conditions of molecular tori using the near-infrared (4.6 micron) ro-vibrational transitions of CO. Our strategy is to observe the molecular tori in the CO absorption with the dust emission from the sublimation layer around AGN as background sources. Many transitions at various energy levels (v = 0-1, delta J = +1 or -1) are expected to enable reliable measurements of the temperatures and the column densities of molecules in the tori. Our observations consist of two parts: one is those with AKARI and the other is those with Subaru. AKARI observation covers various targets, although the spectral resolution is rather limited. AKARI showed that many obscured AGN showed CO features in absorption, but not all the Seyfert 2 galaxies. Our model calculation suggests that this is due to the small difference in the viewing in Seyfert 2 galaxies, although they have been regarded basically as edge-on systems. This indicates that the central region of AGN could have more complicated structures than the unified scheme suggests. On the basis of Subaru observations, we attribute the high temperature and large column densities observed in CO absorption features to the warm molecular clouds excited by the X-ray radiation from the center. We decompose the absorption features into several velocity components and propose a model of torus structures to explain the observations. Our observations have indicated that the inner
structure of the molecular torus is not static but dynamic, as predicted by recent hydrodynamic and magnetohydrodynamic simulations. Furthermore, the results also imply the differences in the inner structure of the torus are attributable to the torus evolution with the widening of polar outflows.