談話会
Colloquium

野田 博文（東北大学）
X-ray Observations and Detector Developments to Study the AGN Central Engine
The central engine which consists of an X-ray corona and accretion disk in the vicinity of a supermassive black hole is the origin of powerful radiation from an Active Galactic Nucleus (AGN). However, even its fundamental properties, such as its accretion state, geometric structure, and how these change with varying mass accretion rates have not yet been well understood. To establish the picture of the AGN central engine, we have performed X-ray and multi-wavelength observations of highly variable sources including changing-look AGNs which change their types in short timescales and are under much debate. In parallel, we have developed X-ray detectors onboard Japanese X-ray astronomical satellites, Hitomi and XRISM, to realize unprecedented X-ray spectral resolution for AGN studies. In this presentation, I introduce our observational studies, detector development, and prospects with XRISM for understanding the AGN central engine.

石崎 渉（東北大学）
Fallback Accretion in Binary Neutron Star Mergers
The gravitational wave event GW170817 with a kilonova shows that a merger of two neutron stars ejects matter with radioactivity including r-process nucleosynthesis. A part of the ejecta inevitably falls back to the central object, possibly powering long-lasting activities of a short gamma-ray burst (sGRB). We investigate the fallback accretion with the r-process heating by performing 1D hydrodynamic simulations and developing a semi-analytical model. We show that the fallback is halted by the heating. The characteristic halting timescale is 10^4-8 sec for the GW170817-like r-process heating, which is long enough to continue the long-lasting emission of sGRBs.
Furthermore, I will present a new interpretation of the recently reported re-brightening in the annual-scale X-ray light curve of GW170817. We model the fallback of the merger ejecta and construct a simple light curve model from the accreting ejecta. We find that the X-ray flux excess can be well explained by the fallback of the post-merger ejecta rather than the dynamical ejecta.
If time allows, I would like to introduce a study on the decay of Alfven waves in strongly magnetized plasmas. This process is relevant not only to binary neutron star mergers but also to the general problem in high-energy astrophysics in the context of the conversion of electromagnetic field energy into matter energy.

Ue-Li Pen（ASIAA）
FRBs: what are they and what are they good for?
Fast Radio Bursts are short, millisecond flashes thousands of times per day, distributed isotropically on the sky. I outline our current state of understanding for their nature and environment and their unique properties of cosmic probes. I describe the new BURSTT FRB initiative for ambitious instantaneous fish-eye coverage with VLBI localization across Asia, including Ogasawara and India.

Ronaldo LAISHRAM（東北大学）
Cosmic Evolution and Environmental Dependence of Star Forming Activities and Morphologies
The distribution of galaxies in the universe is inhomogeneous, representing large-scale structures (LSS) that consist of galaxy clusters, groups, and the filaments that connect them. Understanding how galaxy characteristics are influenced by their environments and how they evolve over cosmic time within LSS is crucial. Utilizing dual narrow-band filters for precise redshift estimation, we investigate the environmental effects on star formation within large-scale structures. We explore star-forming activity and the spatial distribution of Hα and continuum emission at z=0.4, probing the cosmic web. We found that star formation in cluster core galaxies is more centrally concentrated and reduced compared to the field sample, which may imply the influence of ram-pressure stripping. We also explore the morphological features and star formation activities of [OII] emitters in the COSMOS UltraDeep field at z ∼ 1.5 using JWST NIRCam data from the COSMOS-Web survey and Subaru Hyper Suprime-Cam. Furthermore, we report the discovery of large filamentary structures traced by [OII] emitters, surrounding an extremely overdense core with a galaxy number density ∼ 11× higher than the field average. Heightened star-forming activity was observed in dense regions, contrary to z=0.4, suggesting an environmental impact on early galaxy evolution. Additionally, we examine the redshift evolution of star-forming activities and morphology. Future studies will explore into the chemical abundance, gas content, and kinematics to comprehend the underlying processes.

安部 大晟（東北大学）
Formation & Evolution of Star-forming Filaments in Molecular Clouds
Observations of molecular clouds indicate that dense filaments are the sites for present-day star formation. Therefore, it is crucial to understand filament formation and evolution, as these filaments provide the initial conditions for star formation. The width of filaments is a significant factor as it determines the fragmentation scale through self-gravity and affects the stellar masses to be formed. Observations suggest that the width has a universal value of 0.1 pc, regardless of the filament’s line mass. However, theoretical predictions suggest that the width of supercritical filaments (> 17 solar masses per pc) should contract due to self-gravity. Most simulations show a much narrower width due to strong gravity for massive filaments, and why massive filaments maintain their width of 0.1 pc has remained unexplained for more than ten years. Recent studies suggest that massive filaments (~100 solar masses per pc) are bound by slow mode shocks resulting from accretion flows onto the filaments. The wavefront of the slow mode shock is known to be unstable, and the corrugation of the shock front grows. This corrugation converts the accretion flow’s ram pressure into thermal/turbulent pressure across the shock front, possibly maintaining the filament’s width. In this study, we perform non-ideal MHD simulations to investigate filament evolution via slow shock instability, considering ambipolar diffusion, which is effective in dense filaments. We discovered slow shock instability including ambipolar diffusion drives anisotropic turbulence in the massive filament, named this new mechanism the “bullet mechanism”. We also conducted a simulation considering self-gravity and found that the bullet mechanism can sustain a realistic filament width, even for a filament as massive as ~100 solar masses per pc.

Jeong-Gyu Kim（National Astronomical Observatory of Japan）
Modeling Multiphase, Star-forming ISM with UV Radiation Feedback from Massive Stars
UV radiation emitted by massive stars is the major source of heating and ionization in the interstellar medium (ISM). H II regions created by ionizing photons photoevaporate and expel giant molecular clouds, controlling their star formation efficiency and lifetime. Far-UV photons escaping into the large-scale ISM produce the interstellar radiation field, which in turn sets the thermal pressure of diffuse neutral gas via grain photoelectric heating. The efficiency with which UV radiation (and supernovae) feedback recovers the ISM pressure required to meet the vertical dynamical equilibrium determines the galactic star formation rate. Drawing from recent theoretical models and numerical simulations of multiphase, star-forming ISM with UV radiation feedback, I will overview our current understanding of how stellar UV radiation interacts with the surrounding ISM and how this affects star formation and multiphase structure on both cloud and galactic scales.

Hefan Li（東北大学）
Measuring the influence of the LMC on the Milky Way based on velocity ellipsoid
The Milky Way evolved by accreting nearby dwarf galaxies. In this process, these galaxies are tidally disrupted and leave behind stellar streams and substructures in the Milky Way halo. On the other hand, they are affecting the Milky Way in a number of ways. Here, we study the velocity zero point of the inner halo caused by the Large Magellanic Cloud (LMC), the most massive galaxy near the Milky Way. By collecting data from several large surveys, a large data set of stars is assembled. The distances, ages, initial stellar masses, and reddenings for the stars are computed using the calibrated atmospheric parameters and Gaia-DR3 photometry. We kinematically and chemically select halo stars and measure the reflection motion at different locations based on their velocity ellipsoids. In addition, we find that the reflection motion reduces the radial dominance of the anisotropic parameters, thereby affecting estimates of the Milky Way mass.

和田 知己（東北大学）
Magnetar Bursting Activities: X-ray and Radio Transients
Magnetars are neutron stars whose photospheric luminosities are higher than their spin-down luminosities. They have strong magnetic fields of ∼ 10¹⁵ G, believed to be the source of their high photospheric luminosities. Magnetars frequently exhibit bursting activities in X-rays, such as short bursts, outbursts, and giant flares. Magnetars are also strong candidates for fast radio bursts, mysterious radio transients. There are many observational results about magnetar bursting activities, but many aspects remain unclear, such as the triggers of the bursts, the emission mechanisms of fast radio bursts, and the potential for other signals.
We study the bursting activities of magnetars. First, we study a possible triggering mechanism of the bursts. We focus on the Dzhanibekov effect in magnetars, which is a kind of free precession, and suggest that this effect may trigger bursting activities. Second, we study a plasma outflow that can be formed during the bursts. Our numerical simulations demonstrate that the outflow can be accelerated to ultra-relativistic velocities through radiative acceleration in a strong magnetic field. The ultra-relativistic outflow can be the energy budget for fast radio bursts. Third, we study the neutrino emission from magnetar bursts. We evaluate the neutrino luminosity from the ultra-relativistic outflow in the magnetar bursts and discuss the possibility of detecting high-energy neutrinos from magnetar bursts. Based on these studies, we discuss the origins and consequences of the magnetar bursting activities.

Juan Pablo Alfonzo（東北大学）
Katachi (形): Decoding the Imprints of Past Star Formation on Present-Day Morphology in Galaxies with Interpretable CNNs
The physical processes responsible for shaping how galaxies form and quench over time leave imprints on both the spatial (galaxy morphology) and temporal (star formation history; SFH) tracers that we use to study galaxies. While the morphology-SFR connection is well studied, the correlation with past star formation activity is not as well understood. To quantify this we present Katachi (形), an interpretable convolutional neural network (CNN) framework that learns the connection between the factors regulating star formation in galaxies on different spatial and temporal scales. Katachi is trained on 9904 galaxies at 0.02<z<0.1 in the SDSS-IV MaNGA DR17 sample to predict stellar mass (M∗; RMSE 0.22 dex), current star formation rate (SFR; RMSE 0.31 dex) and half-mass time (t50; RMSE 0.23 dex). This information allows us to reconstruct non-parametric SFHs for each galaxy from gri imaging alone. To quantify the morphological features informing the SFH predictions we use SHAP (SHapley Additive exPlanations). We recover the expected trends of M∗ governed by the growth of galaxy bulges, and SFR correlating with spiral arms and other star-forming regions. We also find the SHAP maps of D4000 are more complex than those of M∗ and SFR, and that morphology is correlated with t50 even at fixed mass and SFR. Katachi serves as a scalable public framework to predict galaxy properties from large imaging surveys including Rubin, Roman, and Euclid, with large datasets of high SNR imaging across limited photometric bands.

Ryuji Suzuki (National Astronomical Observatory of Japan)
Personal views of astronomical instrumentation and how to build/plan new instruments
This talk picks up some aspects of instrumentation for the ground-based opt-IR astronomy with a focus on the instrumentation in the carrier path, how to build an instrument, and how to plan a new instrument. The talk aims to introduce the ground-based opt-IR instrumentation to those who are not familiar with it, including researchers in different fields, students, and young researchers who are interested in the instrumentation as a part of their career.
I will start the talk by defining the instrumentation in the field of astronomy in relation to the other fields, and emphasize the importance of instrumentation in historical perspective as well as in the career of a researcher. The project size, one of the important aspects of instrumentation, is discussed in the context of an early stage of the carrier path. Some examples are given to illustrate typical instrumentation project phases and milestones as well as the possible team structure scenarios that you may need to build an instrument.
The last part of this talk is devoted to the instrumentation in a big project, how scientists are involved in the project, and how to plan the new instrument which is one of the goals of the ongoing TMT-ACCESS workshop. I will walk the audience through a requirement flow from science cases to the instrument in case of TMT.

Evan Kirby (University of Notre Dame)
The Extremes of Resolved Stellar Spectroscopy
The study of dwarf galaxies with existing spectroscopic instruments and techniques has reached the point of diminishing returns. I will describe the measurement of radial velocities and abundances of individual stars in dwarf galaxies at the threshold of spectral resolution and signal-to-noise ratio. First, I will address measurements from Keck/DEIMOS. I will discuss the case of Andromeda XVIII, at a distance of 1.3 Mpc, which is a possible backsplash galaxy after passing through M31's halo. I will also describe chemical measures of star formation histories in classical dSphs with neutron-capture elements. Finally, I will discuss the promise of the Subaru Prime Focus Spectrograph (PFS) in using dwarf galaxies to learn the nature of dark matter and to study chemical evolution at galactic subscales.

Benedetta Vulcani (Istituto Nazionale di Astrofisica)
Not all the action is in clusters: environmental effects on field galaxies
Galaxies inhabit a wide range of environments and therefore are affected by different physical mechanisms. While environmental effects are typically deemed significant mainly within galaxy clusters, even outside clusters, galaxies can be significantly affected by external processes connected to their position within the cosmic web. First, I will discuss the role of filaments in affecting galaxy properties in the local Universe, by focusing on the filaments flowing onto Virgo, the closest cluster to us. Next, exploiting the data from the GAs Stripping Phenomena in galaxies (GASP) survey I will discuss the multitude of mechanisms that can affect galaxies in isolation, groups and filaments. Spatially resolved maps combined with the knowledge of the hosting environment are indeed very powerful for classifying galaxies by physical process. I will show how a single group can host galaxies undergoing many different processes, such as starvation, ram pressure stripping and gas accretion. I will also show how filaments can assist gas cooling and increase the star formation in the densest regions in the circumgalactic gas of the galaxies flowing through them. I will then show examples of galaxy-galaxy interactions, mergers, and cosmic web stripping. I will emphasise the successes and limitations of a visual optical selection in identifying the processes that deplete galaxies of their gas content and probes the power of IFU data in pinning down the acting mechanism. Future facilities will allow to perform similar analyses at higher redshifts.

北島 歓大（名古屋大学）
An Origin of Narrow Extended Structure in the Interstellar Medium: An Interstellar Contrail Created by a Fast-moving Massive Object
The filamentary structure is the general morphology of the interstellar medium. Their length varies from less than 1 pc to over 100 pc. The formation process of long filaments is still unknown, although the formation process of filaments has been extensively studied. We investigate the thermal condensation caused by a massive object such as a black hole that passes through the interstellar medium with high velocity, and propose a mechanism for creating a filamentary gaseous object, or interstellar contrail. Our main result shows that a long interstellar contrail can form with a certain parameter; a compact object more massive than 10⁴ solar masses can make a filament whose length is larger than 100 pc. Observation of interstellar contrails may provide information on the number, masses, and velocities of fast-moving massive objects, and can be a new method for probing invisible gravitating sources such as intermediate-mass black holes.

藤林 翔（東北大学）
Mass ejection and nucleosynthesis in binary neutron star mergers
Binary neutron star merger is a fascinating astrophysical phenomenon in many aspects: It is one of the primary targets of gravitational wave detectors, it triggers various astrophysical transients in association with its mass ejection activities, and it is one of the promising sites for heavy element synthesis in the Universe. The merger is a highly non-linear process, including general relativistic gravity, hydrodynamics, magnetism, and microphysics. Therefore, numerical simulation is a crucial way to understand the merger process quantitatively. In this talk, I will present current understandings of binary neutron star mergers based on recent numerical simulations, mainly focusing on mass ejection and nucleosynthesis.

鳥羽 儀樹（National Astronomical Observatory of Japan）
Insights into AGN activity in galaxy groups/clusters: a statistical study with Subaru HSC survey
In this talk, I will present our study on the properties of active galactic nuclei (AGNs) in approximately one million member galaxies of galaxy groups and clusters with redshift z_cl < 1.4, selected using the Subaru Hyper Suprime-Cam (HSC) survey. We defined the AGN fraction in two ways: the AGN number fraction and the AGN energy fraction and investigated their dependence on the cluster redshift and the distance from the cluster center. Our findings reveal a positive correlation between the AGN number/energy fraction, with higher values for IR-AGNs at the outskirts of the clusters. We also have tentative evidence that cluster–cluster mergers could enhance AGN activity in the outskirts of massive galaxy clusters. These findings highlight the significant role of high-redshift galaxy groups and clusters in the evolution of AGNs. The seminar will delve into the implications of these results to understand the emergence of AGNs from a multi-wavelength and cluster dynamics perspective (Hashiguchi, Toba et al. 2023, PASJ, 75, 1246; Toba et al. 2024, ApJ, 967, 65).

鈴木 昭宏（東京大学）
GRB jets colliding with massive circum-stellar materials and associated electromagnetic transients
Gamma-ray bursts (GRBs) are the most energetic stellar explosion in the universe. Long-duration GRBs are supposed to arise from collapsing massive stars with an ultra-relativistic.jet. The launched jet penetrates the star and then emit electromagnetic signals from radio to gamma-ray. But, their progenitor, jet production, and radiation mechanisms are still largely unclear. Some underluminous GRBs (low-luminosity GRBs) are supposed to be produced by GRB jets deeply embedded in massive circum-stellar materials (CSMs). The presence of such CSMs can dissipate a considerable fraction the jet kinetic energy, which is channeled to less collimated ejecta component traveling at mildly relativistic speeds. The mildly relativistic ejecta is also expected to outshine radiation across electromagnetic spectrum. Such systems with less gamma-ray brightness could also be a neutrino source and are now paid a lot of attention in the context of multi-messenger astronomy. Recently, we carried out a series of 3D hydrodynamic simulations of GRB jet with massive CSMs and investigate the dynamical property of the mildly relativistic ejecta. In this talk, I introduce our results and discuss expected electromagnetic signals from GRB jet with massive CSM.

Jose Carpio（University of Nevada, Las Vegas）
High-energy astrophysical transients as probes of neutrino physics
Neutrinos are a unique probe of high-energy astrophysical processes. Being able to escape dense environments due to their weak interactions with matter, they help us understand the microphysics and mechanisms of the sources. In particular, high-energy neutrino transients play an important role as signatures of hadronic interactions and particle acceleration. In this talk, I will discuss the particle physics processes involved in neutrino production and how we can use them to study astrophysical sources. We first cover the case of newborn magnetars, which can accelerate cosmic rays to ultrahigh energies. Hadronic interactions between cosmic rays and surrounding material can produce not only pions, but heavier mesons as well, resulting in a unique neutrino signal beyond PeV energies. We then consider the neutrino emission from the next galactic supernova. Such an event can be accompanied by GeV neutrinos from neutron-loaded outflows, which can be observed by the next-generation neutrino detectors.

藤本征史（University of Texas, Austin）
Quest for the Most Distant Universe
Finding and characterizing the earliest systems are crucial for answering fundamental cosmological questions such as the emergence of the first galaxies and black holes (BHs), as well as the cosmic reionization process. The advent of JWST has advanced our capability to detect and analyze systems from the early universe, with current candidates reaching out to z~17. Simultaneously, submm/mm observations have gained prominence, providing complementary insights into the cold dust and gas characteristics and helping to verify the authenticity of ultra-high-redshift galaxy candidates against dusty lower-redshift interlopers. In this talk, I will provide an overview of my recent quest for the most distant universe. My strategy synergizes JWST, ALMA, and Gravitational Lensing to enable the most sensitive and comprehensive investigations of the earliest epochs. My systematic JWST spectroscopic survey corroborates the high abundance of z > 9 galaxies recently claimed from photometric measurements, and I will discuss its possible origins based on their initial characterizations. I have also derived the infrared luminosity function by establishing the largest faint ALMA mm sample to date. This enables the first direct attempt to quantify the total (=unobscured + dust-obscured) cosmic star formation history up to z~8, unveiling an additional star-forming component that contributes ~60% more than previously estimated from optical-NIR surveys. The underlying physics taking place in early galaxies, which is responsible for these statistical results, will be discussed based on the initial results from a total of ~160hrs of my ongoing JWST+ALMA+MUSE programs dedicated to a single lensed galaxy at z=6. Looking ahead, I will also introduce the most efficient survey design for further exploration of the most distant universe in the future.

平松大地（Center for Astrophysics）
Tracing the Final Moments of Massive Stars through Supernovae
Supernovae are the terminal explosions of massive stars with influences on every astrophysical scale. Advanced wide-field and high-cadence transient surveys routinely discover supernovae near the moment of explosion. Coupled with prompt and continuous follow-up facilities, these observations have revealed unprecedented features of dense circumstellar medium in various spatial scales as traced by the expanding supernova ejecta. Such circumstellar medium is thought to originate from mass-loss activities in the final years to decades of stellar evolution; however, their inferred densities exceed the expectations from standard theory by many orders of magnitude. In this talk, I will first introduce standard stellar evolution and supernova explosion mechanisms, and then describe novel observational probes and modeling techniques of supernovae (interacting with circumstellar medium) to reconstruct their explosion properties and progenitor mass-loss histories. Finally, I will discuss our on-going largest sample study of interacting supernovae and emerging pictures of dramatic dying breaths of massive stars.

尾上匡房（東京大学/Kavli IPMU）
JWST初年度観測で見えてきた超巨大ブラックホール初期成長
本講演ではビッグバン後10億年未満の初期宇宙に存在する活動的な超巨大ブラックホール（クェーサー）の初期成長の様子について、すばる望遠鏡とジェイムズ・ウェッブ宇宙望遠鏡（JWST）を使った最新の観測成果を中心にご紹介する。遠方クェーサーは2000年代のSloan Digital Sky Surveyを皮切りに現在までに300を超える数が報告されているが、サーベイ観測の深さが足りないことによって最も明るい対象にバイアスされていた。2014年より開始したすばる望遠鏡Hyper Suprime-Cam (HSC)による広視野サーベイは、従来より10倍程度暗いクェーサーまで探査することができる、世界的に見ても競争力の高いプロジェクトである。我々の研究チームは本プロジェクトによって発見された低光度のクェーサーをJWSTで追観測することにより、遠方宇宙における母銀河の質量とブラックホール質量の分布を制限することを目指している。JWST / NIRCamで観測された最初のターゲットから空間的に広がった母銀河を検出し、これは遠方クェーサーとしては世界で初めての成果となった。またNIRSpecの分光観測データからは、母銀河の星形成活動が抑制されたポスト・スターバースト銀河の兆候が2天体について観測された。本講演では、まず遠方クェーサー観測の概観から始め、自身が関わるその他のJWST観測の成果についても時間が許す限りご紹介したい。

井上昭雄（早稲田大学）
Exploring the early Universe through far-infrared nebular emission lines of galaxies
To understand physical processes of the galaxy formation and cosmic reionization, observations of nebular emission lines in high redshift galaxies are essentially important. Far-infrared nebular emission lines such as [CII] 158 um and [OIII] 88 um lines are targets with ALMA. I will review ALMA observations for these emission lines in the epoch of reionization, especially emphasizing [OIII] observations conducted by an international team including myself. I will also present recent observational comparisons between ALMA and JWST. The latter observes optical nebular emission lines having complimentary information to far-infrared lines. Finally I will discuss future prospects of galaxy formation studies in the early Universe by using wide-field survey space telescopes such as Euclid, Roman, and GREX-PLUS.