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.

Rosemary Wyse (Johns Hopkins University)
From Star Counts to Primordial Black Holes
Star counts within galaxies of the Local Group can provide insight into many astrophysical processes. The first aspect I will address is the nature of the low-mass stellar Initial Mass Function (IMF) at high redshift and low metallicity and the second is the possible contribution of asteroid-mass primordial black holes (PBH) to the dark matter within galaxies. The analyses are based on deep images with the Hubble Space Telescope of five Ultra-faint dwarf (UFD) galaxies, four that are satellites of the Milky Way plus one that is likely to be a satellite of the LMC. The stars in these UFD galaxies are extremely old and extremely metal-poor and likely formed long ago in an environment very different from that of the local solar neighbourhood. The inferred dark-matter fractions in these systems are also significantly higher than the values found in larger galaxies. Stars of mass below that of the Sun live for sufficiently long that even in ancient systems, star-count techniques can be used to investigate the low-mass stellar Initial Mass Function (IMF). Comparisons between the low-mass IMF in UFD galaxies and that of the local Milky Way thus provides insight into star-formation processes across a range of conditions and redshifts. We conclude that there is little evidence in favour of a variable low-mass IMF. These results for the low-mass IMF can also be used to limit the contribution to dark matter made by primordial black holes in the as-yet largely unconstrained asteroid mass range, through the predicted destruction of stars by captured PBH.

Novan Haryana (Tohoku Univ.)
Unveiling the formation of massive compact quiescent galaxies and establishment of their black hole-bulge mass relation in the early universe
JWST has revealed that compact massive quiescent galaxies (MQGs) already existed less than two billion years after the Big Bang. These systems are expected to be the progenitors of local massive elliptical galaxies, whose bulge masses are tightly correlated with their central black hole (BH) masses. However, this raises two key questions: (1) how did such compact systems form so rapidly? and (2) how did they evolve toward the local BH–bulge mass relation? I investigate these questions using spatially resolved spectral energy distribution analysis of JWST and HST imaging. First, I study MQGs at z < 4 and find that they already host dense, compact stellar cores comparable to those of local elliptical galaxies. This suggests that their central stellar mass was largely assembled before the cessation of star formation. I then extend the analysis to the most massive galaxies at z < 5 to trace the formation phase of these compact MQGs. I find that their progenitors likely experienced “core compaction” at z ~ 4–5, increasing the central stellar mass by a factor of seven within only ~400 Myr, followed by star formation quenching from inside-out. Using Chandra X-ray data, I also find that this compaction phase coincided with rapid BH accretion, suggesting that core compaction may help establish the connection between massive galaxy bulges and their central BHs. Overall, these results support core compaction as a promising scenario for explaining both the rapid formation of dense stellar cores and the emergence of the BH–bulge mass relation in the early Universe.

Ryo Albert Sutanto (Tohoku Univ.)
How Does the Cosmic Environment Shape Galaxy Populations and Quenching?
Star formation is one of the key processes in galaxy evolution, governing the exchange of material among stellar, gas, and dust reservoirs. To sustain this process, galaxies acquire cold gas from their surroundings through filamentary structures in the cosmic web, making the large-scale environment a fundamental driver of how galaxies evolve over time. In this study, we investigate the role of the environment in shaping distinct galaxy populations and quenching mechanisms, characterized at two scales: the local environment, traced by the galaxy overdensity factor, and the global environment, traced by the total dark matter halo mass of the host system. We utilize the dataset from the BEACON JWST Cycle 2 NIRCam pure-parallel imaging survey, which spans 20 independent fields and substantially mitigates the effect of cosmic variance. We apply a weighted adaptive kernel density estimation method incorporating the full photometric redshift probability distribution function of each galaxy to construct overdensity maps and identify 207 significant (>4σ) overdensities at 1.5 < z < 5. By linking these environmental measurements to distinct galaxy populations, including quiescent galaxies, clumpy galaxies, and dusty star-forming galaxies (DSFGs), we find evidence for a complex interplay between environment and star formation activity across cosmic time. Our results are consistent with a transition in gas accretion mode around cosmic noon, where environmental quenching becomes increasingly efficient in massive halos, while clumpy and dusty populations offer complementary views of how dense environments regulate and transform star-forming activity at high redshift.

Dragan Salak (Tohoku Univ.)
Atomic Carbon in Nearby Galaxies: ALMA View on AGN
The interstellar medium (ISM) in many active galactic nuclei (AGN) contains multi-phase gas concentrated in a molecular torus (size ~10 pc) within a larger circumnuclear disk (CND; ~100 pc). The gas is feeding the supermassive black hole via inflows, whereas feedback from radiation and jets drives outflows. To understand these processes, it is important to study the dynamics and physical conditions of all ISM phases in the CND. Atomic carbon (C), e.g., is a key constituent as it regulates the cooling of neutral gas via the fine-structure lines, [CI] 609 um and 370 um, and plays a major role in the chemistry involving CO. The [CI] lines have been used recently as probes of molecular gas mass, especially at high redshift, where the standard tracers, such as the CO (J=1-0) rotational line, are difficult to observe. While the C/CO abundance ratio is expected to increase in cosmic-ray and X-ray dominated regions (XDRs) in AGN, it remains unconstrained observationally. Following a brief review of previous [CI] observations of nearby galaxies, I will present preliminary results of the first imaging of the [CI] 370 um and CO (J=7-6) lines toward the torii of two nearby Seyfert 2 galaxies using the Atacama Large Millimeter/submillimeter Array at unprecedented resolution of 8-14 pc. The data reveal noncircular motions of neutral gas in the CND including inflows toward the torus and outflows. Since the two AGN have been observed in the ground-state transition [CI] 609 um before, now for the first time we have both [CI] line measurements that fully probe the excitation of atomic carbon. The [CI] 370 um/609 um line-luminosity ratio is ~1.0 in the CND, indicating a high gas temperature (>50 K) and density (>1000/cc) of molecular gas. Based on radiative transfer calculations, the C/CO abundance ratio is estimated to be ~0.6-2.0, higher than the average value in Galactic clouds (~0.1) and Galactic center (~0.5), possibly a result of XDR chemistry. I will discuss the limits of current analysis and what future [CI] observations of AGN at even higher resolution may be able to answer.