談話会
Colloquium

山田 智史（東北大学）
Accretion and Multi-scale Ejection Resolved by X-ray Observations from 1999 to 2025
To make a database of multiphase (e.g., ionized/dusty/neutral/molecular) outflows, we have launched a new project, X-ray Winds In Nearby-to-distant Galaxies (X-WING). As the first study of the X-WING project, we constructed a sample of 132 AGNs in z ~ 0-4 exhibiting blueshifted absorption lines of X-ray winds reported by the end of 2023. With a thorough investigation of the previous works, we created the database of outflow properties of 583 X-ray winds, including outflow velocities (Vout), outflow radii (Rout), and mass/kinetic outflow rates (Mout/Eout). The ultrafast outflows (UFOs) and slower warm absorbers cover the Vout range of ~100 to 100,000 km/s. Interestingly, we found a clear velocity gap around Vout ~ 10,000 km/s. Although the gap can be an artifact due to the confusion of the emission/absorption lines and Fe K edge in the 6-7 keV band, there is another possibility that the UFOs and galactic-scale outflows are physically disconnected. Moreover, we introduce our unprecedented high-energy-resolution spectra with XRISM operated from 2023 and provide new insights into the origin of the Vout gap and the plausible multi-scale structure of X-ray winds (e.g., Yamada+24b, ApJS; XRISM Collaboration+). Finally, we will also report the latest results of the UV-to-radio SED fittings and studies on multiphase outflows for the X-WING AGNs and discuss the accretion and multi-scale ejection in AGNs.

Colton Hill（Chiba Univ.）
South Pole Science - Neutrino Oscillations and the IceCube Neutrino Detector Upgrade
Of all the Standard Model particles, neutrinos are the least well understood. While significant global progress has been made in characterising neutrino oscillations, several key questions remain unanswered: what are the masses of the neutrinos, and do neutrino oscillations truly follow the standard three-flavour model? The cubic-kilometre IceCube Neutrino Detector located at the geographic South Pole is capable of precision measurements of neutrino oscillation properties by observing neutrinos produced from particle interactions in the atmosphere across a broad range of energies (GeV-scale) and path lengths, often travelling through the Earth. To increase IceCube's sensitivity in the GeV-range, the IceCube Upgrade will involve deploying a dense array of high-sensitivity optical modules up to 3 km deep into the Antarctic glacier at the end of this year. One of these flagship modules, the "D-Egg", was developed and tested in Japan as part of an international effort for the Upgrade, and features a factor 2.8 per-device improvement in sensitivity over the current generation detectors. With enhanced direction reconstruction and a lower energy threshold, the Upgrade is expected to probe the neutrino mass ordering at the 3 sigma level within 5 years, and observe tau-neutrino appearance after just one year of data taking. As of January 2025, all 292 D-Eggs have arrived at the South Pole, with approx. 30% having already completed pre-deployment testing. This seminar with focus on the progress of the IceCube Upgrade, including the South Pole activities of the 2024/2025 Antarctic On-Ice Season, and share the latest sensitivities for the IceCube Upgrade ahead of data-taking later next year.

堀米 俊一（東北大学）
Dwarf spheroidal galaxies as probes of dark matter
The presence of dark matter in our universe is one of the biggest open questions in particle physics, astronomy, and cosmology. Among several detection methods, astronomical observations can explore interesting parameter regions that are not easily accessible by collider or direct detection experiments. In this talk, we focus on one of the most promising targets: dwarf spheroidal galaxies (dSphs), which are dark-matter dominated satellite galaxies of the Milky Way. We show recent results of dynamical analyses using the spherical Jeans equation, a standard tool in stellar dynamics, to constrain the dark matter density profiles of dSphs. To interpret these results in a cosmological context, we use a semi-analytical cosmological model called SASHIMI (Semi-Analytical SubHalo Inference ModelIng), which predicts subhalo properties such as mass and density structure. These predictions are used as priors in a Bayesian analysis to connect observations with theoretical dark matter models. For cold dark matter (CDM), we apply this framework to estimate the J-factors of dSphs, which are important for indirect detection studies. For self-interacting dark matter (SIDM), we extend the model to SASHIMI-SIDM, which includes the effects of self-scattering on subhalo evolution. By comparing with observed satellite galaxies, we derive quantitative constraints on the self-interaction cross section, showing how small-scale structures in dSphs can inform the fundamental properties of dark matter.

Mochammad Wardana（東北大学）
Dwarf spheroidal galaxies as probes of dark matter, too: a stellar dynamics model refinement and test on the fuzzy dark matter paradigm
The preference for cored dark matter density profiles in dwarf disk galaxies has long stirred tensions over the internal structure of dark matter halos, further complicating the Cold Dark Matter (CDM) crisis at sub-galactic scales. Kinematic measurements of stars in the even more dark matter–dominated dwarf spheroidal (dSph) satellites of the Milky Way offer a promising, less baryon-contaminated avenue to revisit these issues. As data on individual member stars improve in both precision and sample size, dynamical estimates of their underlying mass distributions are becoming increasingly accessible. However, the inferred dark matter density profiles show significant variation. Some galaxies are more consistent with cored profiles, others with cuspy ones, and some analyses report that both core and cusp profiles are equally plausible. This may be a clue that the classic core–cusp problem is just the tip of a larger “diversity” problem, and it certainly signals the need for more refined modeling to distinguish cores and cusps apart more reliably. We, therefore, attempt to extract more information contained in the non-uniform shape of the line-of-sight velocity distribution, which is assumed to be fixed in traditional Jeans modeling. This is achieved by invoking the stars’ higher-order velocity moments alongside their standard velocity dispersion. We demonstrate the model’s improved capacity for recovering dark matter density profiles and discuss its inherent limitations. In addition, we explore the implications of our model within the context of fuzzy dark matter (FDM) theory, an alternative to CDM that gained growing attention for its potential to reconcile observations and theory. We provide preliminary constraints on the FDM particle mass necessary to address the core–cusp tension observed in classical Milky Way dSphs.

Zhaoran Liu（東北大学）
Unveiling Hidden Star-formation within Galaxies at 0.9 < z < 1.7
Galaxies experience their most intense stellar growth at redshifts z = 1–3, yet a significant fraction of this activity is obscured by dust. In this talk, I will present a multi-wavelength, multi-tracer investigation that combines observations from JWST, ALMA, and Subaru to trace both the fuel for star formation and its by-products, interstellar dust, in galaxies at 0.9 < z < 1.7, immediately after the cosmic noon. Some of the key questions I aim to answer include: Where in galaxies and how much is dust involved in the starburst and secular phases? How is it related to the propagation of star formation and quenching within galaxies? And how do local and large-scale environments modulate the relation between star formation and dust properties? To address these questions, I will present our work using a combination of tracers: the 3.3 μm PAH emission observed with JWST/MIRI to map dusty star-forming regions; Paα emission obtained through JWST/NIRCam WFSS to trace total star formation including hidden dusty component at high spatial resolution; CO(2–1) measurements from ALMA to probe the molecular gas reservoir fueling star formation; and Hα/Hβ-based dust extinction mapping across a z = 0.9 supercluster using Subaru narrow-band imaging. Together, these observations provide a comprehensive view of the gas, star formation, and dust cycle and reveal how internal processes and external environments shape galaxy growth during and shortly after the peak epoch

佐藤 優理（東北大学）
Multiwavelength afterglows from two-component jets in very-high-energy gamma-ray bursts
Gamma-ray bursts (GRBs) are the most luminous and violent explosive electromagnetic events in the universe. GRBs also have broadband afterglows, from radio to very-high-energy (VHE) gamma rays, which provide us with extensive physical information about their nature. Currently, it is widely believed that prompt gamma-ray photons are emitted by a relativistic jet toward us, and that the interaction of this jet with the surrounding medium of the source generates the afterglow. However, many fundamental questions remain unanswered: how is the relativistic jet formed, what is the origin of the central engine, and how is the multiwavelength afterglow radiated? In this talk, we explore the phenomena associated with the relativistic two-component jet, which consists of two top-hat jets with angularly equi-distributed energy with different opening angles, to investigate the formation mechanism of relativistic jets. In general, the GRB jets have an angular structure, which is also shown in previous hydrodynamic simulations. Such a structured jet is most simply approximated by a two-component jet. The two-component jet model has been motivated to explain the multiwavelength afterglow emission in some GRB events. Our two-component jet model successfully explains the multiwavelength afterglows of GRBs associated with VHE gamma-ray emission. These findings suggest that a structured jet, such as the two-component jet, is essential for explaining complex multiwavelength afterglows.

喜友名 正樹（東北大学）
The Role of Cold Accretion in the Formation of Supermassive Stars and Heavy Seed BHs
Supermassive black holes (SMBHs), with masses of 10^(6-10) Msun, are located at the center of most galaxies. Since their discovery, the origin of SMBHs has been a significant unresolved problem. Recent discoveries of distant quasars at the redshift z~10 highlight the challenge of their rapid formation. One promising candidate for SMBH progenitors is supermassive stars with masses of approximately 10^5 Msun. After forming, supermassive stars collapse directly into BHs of approximately 10^5 Msun, which can grow into SMBHs within enough time. A key challenge lies in understanding how to form them. Cold accretion has been suggested to play a crucial role in supermassive star formation, but it has not been studied intensively due to poor understanding of cold accretion in high-z low-mass halos. We perform cosmological simulations to investigate supermassive star formation driven by cold accretion. We first clarify that cold accretion emerges in high-z halos with masses of approximately 10^7 Msun. We then follow the subsequent star formation and discover that cold accretion forms dense, compact disc at the halo center, forming multiple supermassive stars. This indicates the ubiquitous formation of heavy seed BHs, ultimately growing into SMBHs.

Nuo Chen（東北大学）
Multiple Emission-Line properties of Hα emitters at Cosmic Noon probed by Broad-band excesses
The rest-frame optical spectrum of high-redshift star-forming galaxies is characterized by a number of crucial emission lines originating from the interstellar medium (ISM) within them, including the hydrogen recombination line and metal lines. These spectral lines serve as powerful indicators of the physical and chemical conditions in galaxies. In contrast to the traditional spectroscopic method, we introduce a new selection method to search for emission line galaxies based on their flux excess in broad-band photometry. More than 1,000 Hα emitters (HAEs) at 2.05 < z < 2.5 have been found in the ZFOURGE survey by our method, along with [OIII] and [OII] emission lines. Our measurements demonstrate good consistency with those obtained from spectroscopic surveys. We investigate the correlations between the equivalent widths (EWs) of these emission lines and various galaxy and ISM properties, providing insights into the nature of intense emitters. To investigate and resolve the rest-frame optical structures of these HAEs in remarkable detail, we further utilize the JWST-JADES data, which is an extremely deep survey overlapping with the ZFOURGE-CDFS field. The data enable us to perform a rest-frame optical, spatially resolved analysis on a subsample of the HAEs. We newly discover a population of kiloparsec-scale compact emission line regions (“Green Seeds”) with high EW[OIII]. Embedded within the host galaxy, many Green Seeds correspond to UV star-forming clumps and HII regions, indicating elevated starburst activity in them. Based on theoretical frameworks, we also discuss the formation and fate of Green Seeds. The systematic study of emission line galaxies conducted in this work has the potential to significantly deepen our understanding of galaxy structure and evolution at Cosmic Noon.

野田 浩司 (千葉大学)
Gamma-ray astronomy with CTAO LST and its status
Astronomy has developed by expanding the wavelengths of the electromagnetic waves. However, the sky observed with very-high-energy (VHE; >100 GeV) gamma rays is not yet understood well. Gamma-ray detection in TeV energies is the realm of the instruments called Imaging Atmospheric Cherenkov Telescopes (IACTs). Currently a few IACT experiments are under operation, which led to discoveries of above 200 astronomical sources. However, they are not sufficient to fully understand the nature of the particle acceleration sites responsible for the gamma-ray emission. In particular, it is difficult to investigate transient objects, due to the complexity in the observations. Cherenkov Telescope Array Observatory (CTAO) is the next-generation project in this field, and Large-Sized Telescope (LST) is one of its key instruments most sensitive in the energy range from 20 GeV to tens of TeV. In this seminar introduced are the physics motivations and prospects using the coming LSTs, as well as the status of the projects such as the construction and commissioning of the telescopes being built in La Palma, Canaries, Spain.

Chris Packham (University of Texas)
Future Flagships to Revolutionize Astronomy: Habitable Worlds Observatory & TMT/GMT
The coming decades will afford the chance to transform our knowledge of the universe and perhaps will reveal compelling evidence of life outside of the Earth. In this presentation I explore the facilities that will enable such advances, advancing the successes of the JWST and 8m ground based telescopes. Uniquely achieved through multi-national collaborations, leveraging the technical and scientific skills of those partners, as well as spreading the costs and risks, a revolutionary scientific future beckons. Biography: See the WWW link here: https://chrispackham495e.myportfolio.com/about

Li Siyang (Johns Hopkins University)
Testing Hubble Tension Systematics with Cepheids, TRGB & JAGB
The Hubble Tension refers to a >5 σ discrepancy between local and cosmological measurements of the Hubble constant (H0) and suggests the possibility of undiscovered early-universe physics or underestimated systematics. Probing the Hubble Tension necessitates intense scrutiny of the Cepheid-based distance scale, which currently provides the strongest constraints on local measurements of H0. One powerful approach to crosscheck systematics in the Hubble Space Telescope (HST) Cepheid section of the distance ladder is to develop and improve independent routes to measure distances to the same set of galaxies and H0, such as those using the tip of the red giant branch (TRGB) and J-region asymptotic giant branch (JAGB), or by adding an independent anchor. I will present a sub-2% Cepheid distance to M31, which prepares an additional anchor galaxy that can be used to construct the distance ladder once a high-precision geometric detached eclipsing binary observation is feasible, as well as work improving standardization and calibration of the TRGB, including a 2D maximum likelihood formalism enabling Milky Way field star calibrations and contrast ratio standardization in the maser host NGC 4258. I will also present recent James Webb Space Telescope (JWST) TRGB distances to 8 hosts of 10 Type Ia supernovae (SNe Ia), later expanded to 25 hosts, which provides no evidence of a HST Cepheid systematic resolving the Hubble Tension, in addition to a H0 measurement using JWST observations of the JAGB with an expansion to 15 galaxies hosting 18 SNe Ia. While the JAGB remains promising as a tool that can crosscheck Cepheid systematics, non-uniform asymmetry in its luminosity function, caused in part by stellar contamination, presently limits the precision of this candle.

馬場 淳一 (鹿児島大学)
天の川銀河の非軸対称構造と太陽系の軌道移動
Bar/Spiral Structures in the Milky Way and the Radial Migration of the Sun
私たちが住む銀河「天の川銀河」は、宇宙に広く存在する「棒渦巻銀河」の一種 である。しかし、この事実が確立するまでには時間がかかった。これは、天の川 銀河が他の系外銀河では到達不可能な高い分解能で詳細に調べることが可能であ る一方で、我々がその内部に位置し、天の川銀河の全体像を見渡すことが容易で はないためである。しかし、位置天文観測衛星「Gaia」の革新的なデータによ り、天の川銀河の恒星系成分の理解が飛躍的に進展した。また、大規模シミュ レーションも発展し、最新の観測データの解釈に大きく貢献している。本講演で は、講演者らによるGaiaデータと大規模数値シミュレーションデータを用いた研 究に基づき、天の川銀河の基幹構造である渦状腕構造や棒状構造などに関する理 解の現状を紹介する。また、天の川銀河化学動力学進化の観点から太陽系の誕生 半径の推定、太陽系軌道移動に関する講演者による最近の研究成果を紹介する。
The Milky Way Galaxy, our home galaxy, is classified as a barred spiral galaxy—a type commonly found throughout the Universe. However, this classification was not established until relatively recently. One major reason is that, while the Milky Way can be studied with far higher spatial resolution than any external galaxy, we reside within it and thus lack a global, outside view. This limitation has long hindered our ability to understand its overall structure. Thanks to the revolutionary astrometric data provided by the Gaia satellite, our knowledge of the stellar components of the Milky Way has advanced significantly. At the same time, large-scale numerical simulations have also progressed and now play a crucial role in interpreting recent observations. In this talk, I will present our current understanding of the Milky Way’s fundamental structures—such as its spiral arms and central bar—based on studies using Gaia data and state-of-the-art simulations. I will also introduce recent research by our group on the birth radius of the solar system and its subsequent radial migration, discussed in the context of the chemo-dynamical evolution of the Milky Way.

横山 哲也 (東京科学大学)
Chemical and isotopic analyses of samples returned by the Hayabusa2 mission from the asteroid Ryugu
The recent success of asteroid sample return missions has led to significant advances in Solar System science. JAXA's Hayabusa2 successfully retrieved and returned to Earth a total of 5.4 grams of samples from the C-type asteroid Ryugu. Sample return missions are critical to the scientific community, particularly in the fields of planetary science and cosmochemistry. These missions provide pristine, terrestrially unaltered extraterrestrial materials, allowing detailed laboratory analyses that are not possible with remote sensing. The only other access to extraterrestrial materials is meteorites, but meteorites may have been contaminated by terrestrial materials during impact, during residence time prior to collection (meteorite finds), and during storage in meteorite collections prior to changes in curation protocols. Thus, the analytical data obtained in laboratories for samples collected by these missions will facilitate the understanding of the formation and evolution of the Solar System without bias from potentially contaminated data. For the Hayabusa2 mission, six initial analysis teams (Chemistry, Rock, Sand, Gas, SOM, and IOM) and two Phase 2 curation analysis teams were established, each consisting of up to several dozen individual researchers. I was appointed deputy leader of the Chemistry team and was heavily involved in analyzing the chemical and isotopic compositions of the Ryugu materials. A series of analyses of these samples indicated that the mineral, chemical, and isotopic compositions of Ryugu bear a strong resemblance to those of the Ivuna-type (CI) carbonaceous chondrites. CI chondrites have been recognized as a unique group of meteorites with a chemical composition similar to that of the solar photosphere except for highly volatile elements (noble gases, H, C, N, and O) and Li, which was destroyed in the Sun by nuclear reactions. In the seminar, I will present the meaning and significance of the compositional similarity between Ryugu and CI chondrites. I will also present our recent activities in a new project called the Ryugu Reference Project (RRP), which was initiated to maximize the potential value of the returned sample, aiming to establish an international standard for elemental and isotopic abundances in the solar system using the Ryugu samples and other related extraterrestrial materials.

津久井 崇史 (東北大学)
Understanding Thin and Thick Disk Formation by JWST and ALMA
Present-day disk galaxies, including the Milky Way, exhibit two distinct structures: an older thick disk and a younger thin disk. However, the timing and mechanisms of their formation remain open questions, primarily due to observational limitations ― these structures had not been identified beyond z=0 until recently. Leveraging JWST’s superb capabilities, we have, for the first time, identified and investigated thin and thick disks across cosmic time, up to 10 Gyr ago. Our study reveals a consistent trend: in the earlier universe, more galaxies appear to have had only a single thick disk, while in later epochs, more galaxies showed a two-layered structure with an additional thin disk component. This suggests a sequential formation, in which galaxies first formed a thick disk, followed by the development of a thin disk embedded within it. Notably, thin disks formed earlier in more massive galaxies (disk downsizing). By linking disk structural properties with gas kinematics, we propose that the Toomre Q regulated disk formation naturally explains disk downsizing and disk geometric properties. Complementing this, ALMA observations probing the thick disk formation epoch z~4-5 allow us to investigate how thick disks may have formed in gas-rich systems. I will present how such formation likely proceeded through dynamical processes, such as stellar scattering driven by disk substructures common in gas-rich disks, as supported by observations and numerical simulations.

志達 めぐみ (愛媛大学)
High Resolution X-ray Spectroscopy and Multi-Wavelength Monitoring of Accretion and Outflows in X-ray Binaries
Accreting compact objects often launch powerful outflows: uncollimated disk winds and highly collimated relativistic jets. Studying these outflows is essential to understand the physics of accretion and the feedback processes to their environment. Galactic X-ray binaries are ideal targets for such studies. Thanks to their proximity and brightness, especially during active phases in the X-ray band, they provide valuable opportunities for sensitive observations close to the compact object. In addition, their strong variability and short characteristic timescales enable us to trace the evolution of the outflows across a wide range of mass accretion rates. Disk winds in X-ray binaries are often observed as blueshifted, highly ionized absorption lines in X-ray spectra. Resolve, the cutting-edge X-ray spectrometer aboard XRISM, is dramatically advancing our understanding of these winds through its unprecedented energy resolution. Moreover, the combination of XRISM’s high-resolution spectroscopy with multi-wavelength monitoring, allows us to study disk winds, accretion flows, and jets simultaneously, and to explore how these components may be connected. In this talk, I will present results from the latest XRISM and findings from our multi-wavelength observations of X-ray binaries, and discuss future directions and prospects for deepening our understanding of accretion flows and outflows.

稲吉 恒平 (北京大学)
TBA

千葉 凛平 (東京大学)
TBA