談話会
Colloquium

Salvatore Taibi（EPFL）
The dwarf galaxy tale: from the Large Magellanic Cloud to the ultra-faint systems
Dwarf galaxies are the smallest galactic systems, yet they are key probes of how galaxies form and evolve. Abundant throughout the Universe, they are often found as satellites of larger galaxies such as the Milky Way. Our galactic neighbourhood offers a unique laboratory: nearby dwarf galaxies can be resolved into individual stars and studied in exquisite detail through spectroscopic and photometric observations. From their dynamical and chemical properties, we can reconstruct not only their internal evolution, but also how they have contributed to the hierarchical growth of our Galaxy. In this talk, I will provide an overview of the Milky Way satellite system and its orbital properties, using the Magellanic Clouds as an example of ongoing hierarchical accretion. I will then turn to the internal properties of dwarf galaxies, showing how the comparison between satellites and isolated systems allows us to disentangle the effects of environment (including tidal interactions and ram pressure stripping) from intrinsic evolutionary processes. Finally, I will focus on the most dark-matter dominated and chemically pristine systems known, the ultra-faint dwarf galaxies. Their star formation having been halted by reionization, they provide a valuable insight into galaxy formation in the early Universe. Moreover, their very existence allows us to test predictions of the ΛCDM model and its small-scale tensions. I will discuss the observational challenges they present, how we can overcome them, and the open questions that future facilities will be able to answer.

Takahiro Morishita（Tohoku Univ.）
The Dawn of a New Era in Early Galaxy Exploration with JWST
The launch of the James Webb Space Telescope (JWST) has inaugurated a transformative era in our exploration of the early universe. With its unprecedented sensitivity in the near- and mid-infrared, JWST enables direct observations of galaxies within the first few hundred million years after the Big Bang, extending beyond the limits previously established by the Hubble Space Telescope. In this talk, I will review recent observational advances enabled by JWST, including the identification of candidate first galaxies at extreme redshifts, evidence for rapid chemical enrichment, and the emergence of large-scale structure in the early universe. I will discuss the implications of these findings for our understanding of the formation of the first stars and the buildup of massive galaxies. Finally, I will highlight ongoing efforts and future prospects for spectroscopic confirmation and detailed characterization of these primordial systems.

Kohei Ichikawa（Tohoku Univ.）
Interesting radio AGN populations found by wide-field radio and multi-wavelength surveys
Wide-field radio surveys, combined with deep and wide optical, infrared, and X-ray surveys are last frontier opening a window of elusive phase of active galactic nuclei (AGN) and quasars largely missed by previous AGN/quasar surveys. In this talk, I will introduce three interesting populations of radio AGN uncovered through such multi-wavelength approaches, especially emphasizing on SMBH growth, obscuration, and AGN feedback across cosmic time. First, I will introduce super-Eddington AGN with bright radio and X-ray emission, requesting a strong radio jet eve in the super-Eddington phase. The second population is high-z radio AGN population searches at 3<z<9, utilizing the Subaru/HSC, VLASS, and Euclid NIR data set, opening a radio AGN population in the early cosmic epoch. The third population is a newly identified blackbody quasar that may represent a transitional phase between little red dots (LRDs) and typical blue quasars.

Rhythm Shimakawa（Waseda Univ.）
From Cosmic Metropolis to Cosmic Desert: The Environmental Dependence of Galaxies and Gravity
The large-scale structure of the universe is characterized by density contrasts, ranging from galaxy clusters to voids. This talk will discuss environmental dependence from galaxy formation to fundamental physics based on recent studies including our results. In the first half, I will review the formation of massive galaxies in high-redshift protoclusters, the formation sites of the cosmic metropolises. I will also present our recent findings that capture star formation quenching in massive Hα emitters that host X-ray AGNs in the Spiderweb protocluster at z=2.2 based on the multi-wavelength data and recent spatially resolved Paβ-line imaging with JWST/NIRCam. In the latter half, I will shift the focus to the opposite extreme: the cosmic desert. I will report on our recent, first discovery of an unprecedented supervoid candidate with a radius of ~300 Mpc/h at z=1.2, which may uncover the origin of the CMB Cold Spot and challenge a concordance ΛCDM model. This extreme structure is a unique site to tackle the current cosmological problems, including the environmental dependence of gravity, f(R) gravity, by revealing the breakdown of gravitational screening effects with the Roman Space Telescope.