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Language: English
Abstract: Mass-accreting massive black holes are presumably the most energetic and dynamic sources in the universe, not to mention that they are also an important ingredient of galaxies. However, our understanding of the central engine of active galactic nuclei (AGNs) is mainly limited due to the lack of proper spatial resolution. In this talk I will discuss how we can unveil the structure of black hole engine via time-domain studies. First, I will present a couple of exciting results from the Seoul National University AGN Monitoring Project (SAMP), which was carried out for six years, (2016-2021), and discuss our on-going variability studies for measuring the size of accretion disk and torus. Second, I will discuss the changing-look AGNs, which are currently challenging the orientation-based unification model of AGNs. In particular I will present our new discovery of changing-look AGNs based on the variability characteristic analysis and spectroscopic confirmation.
Language: English
Abstract: Galaxies evolve through the interaction with their surrounding environment. Galaxies are born within the large-scale structure of the universe, and they do not uniformly exist. Consequently, galaxy evolution is expected to differ based on their environment. In fact, in nearby galaxy clusters, it is known that galaxies with similar stellar masses exhibit lower levels of star formation activity compared to typical environments. Conversely, the contribution from galaxies within galaxy clusters or "proto"-clusters to the cosmic star formation rate density rises with increasing redshifts. It is estimated that during the epoch of cosmic reionization, these galaxies could contribute to over 50% of the cosmic star formation rate density. We investigate the reasons behind the low star formation activity in current galaxy clusters and the high star formation activity in high-redshift protoclusters using observations and cosmological galaxy formation simulations. For instance, we discovered that the low star formation activity in galaxies belonging to the Virgo cluster and the Fornax cluster is not due to a decrease in the efficiency of converting molecular gas into stars, but rather because of the lower amount of molecular gas available (Morokuma-Matsui+2021; 2022). In this presentation, I will also introduce our ongoing research on protoclusters at redshifts 6 to 14, utilizing data from a cosmological galaxy formation simulation, FOREVER22 (Yajima+2022).
