Oral Presentation
Structure of AGN Tori in Ultra-/luminous Infrared Galaxies Unveiled by Mid-infrared and X-ray Spectroscopy
Presenter: Satoshi Yamada (Kyoto University)
Mergers of gas-rich galaxies, which are often observed as ultra-/luminous infrared galaxies (U/LIRGs), play a key role for the co-evolution of supermassive black holes and galaxies. Understanding the nature of their active galactic nuclei (AGNs) is important to complete the cosmic census of obscured black-hole growth triggered by mergers. First, we propose new diagnostics that utilize the [O IV] 25.89 um and subarcsecond-scale 12 um luminosity ratio for identifying deeply "buried" AGNs (i.e., obscured by tori with large covering factors; Yamada+19, ApJ, 876, 96). Second, we perform a systematic X-ray spectroscopic survey of local U/LIRGs in various merger stages, utilizing the broadband data obtained with NuSTAR, Suzaku, XMM-Newton, Chandra and/or Swift XRT. Applying the state-of-art X-ray clumpy torus model (XCLUMPY), we are able to constrain the covering factor of the obscurer for each object (Yamada+20, ApJ, 897, 107; Yamada+21, in prep). Both results suggest that AGNs in late-stage mergers are deeply "buried", whereas those in non-mergers or early-stage mergers are not. On the basis of these findings, we discuss the structures of the AGN tori in U/LIRGs and their relations to the Eddington ratios.
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