Clarifying Dead Quasars and Downsizing through the Evolution of Star-Forming Galaxies and Active Galactic Nuclei

From the catalogs of visible emission lines of galaxies measured by the Subaru Fiber Multi-Object Spectrograph (Subaru-FMOS) and the KECK Multi-Object Spectrograph For InfraRed Exploration (KECK-MOSFIRE), galaxies were classified into Star-Forming Galaxies (SFGs) and Active Galactic Nuclei (AGNs) based on the Baldwin-Phillips-Terlevich (BPT) diagram. In the redshift (z) range 0.8 <= z <= 2.6, z dependence of their number densities was examined, and it was found that SFGs dominate at lower redshifts, whereas AGNs are distributed at higher redshifts. Infrared analyses using the James Webb Space Telescope (JWST) NIRSpec and MIRI for galaxies up to z 6.6 show similar trends, and the number densities of SFGs and AGNs were found to cross at z 1-3. Combined with the fact that the luminosity of galaxies decreases monotonically with the age of the universe, these phenomena can be interpreted by an evolutionary scenario in which SFGs merged to form quasars in the early universe, whose activity weakened and they changed into SFGs later, and SFGs merged again to form Seyferts in the nearby universe. Within this framework, the longstanding issues of "dead quasars" and "downsizing" can be naturally explained.