Mergers of gas-rich galaxies, which are often observed as ultra-/luminous infrared
galaxies (U/LIRGs), play a key role for the coevolution of galaxies and supermassive
black holes (SMBHs).  Understanding the nature of their active galactic nuclei (AGNs) is
important to complete the cosmic census of obscured black-hole growth triggered by
mergers.

In this seminar, I will talk about the broadband X-ray and multiwavelength study of AGNs
in 57 local U/LIRGs observed with hard X-ray telescopes (NuSTAR and/or Swift/BAT).  We
find that the hydrogen column densities and bolometric AGN luminosities increase, while
the X-ray to bolometric luminosity ratios decrease with merger stage. The X-ray-weak
AGNs in late mergers show massive outflows at sub-pc to kpc scales. These results
suggest that, in the final phase of mergers, the multi-phase outflows can be produced
due to chaotic quasi-spherical inflows, where the AGNs become X-ray weak and deeply
buried by gas and dust. Finally, we discuss the picture of merger-driven coevolution of
galaxies and SMBHs (Yamada et al. 2021, ApJS, 257, 61).