談話会
Colloquium

内山 久和 （愛媛大学）
活動銀河核と周辺銀河の共進化
遠方宇宙には、銀河全体を凌駕するほどの非常に強い電磁波を放射している中心核 (活動銀河核) を持つ銀河が存在している。それらの電磁波の強さは絶大で、周辺銀河の形成・進化に影響を与えるほどであると期待されている。ゆえに、宇宙の進化や今後辿るその運命を正確に予想するためには、活動銀河核が発現する場所、ならびにそれらの放射の周辺 銀河への影響を定量的に理解することが必要不可欠である。しかしながら、これらの天体は稀な天体であるために、統計的な議論には (特に遠方 z>2 では) 全く至っていないのが現状である。我々はすばる望遠鏡搭載の Hyper Suprime-Cam (HSC) 等の可視光帯の広視野撮像観測装置によって遠方活動銀河核の周辺環境を統計的に特徴付けることにより、これらの問題に挑んだ。結果として、遠方の明るい活動銀河核は周辺密度環境によらず発現し、周辺の軽い銀河の形成を阻害する傾向にあるという示唆を得た。本講演では、これらの結果と、強力なジェットを発射している活動銀河核の環境についての最近の結果とを併せて紹介したい。

高橋 亘 （東北大学）
Novel modeling of magneto-rotational stellar evolution
While magnetic fields have been considered to influence the evolution of non-degenerate and compact stars, it has become clear in recent years that actually all stars are deeply affected. We propose a new framework of stellar evolution simulation, in which intertwined evolution between the magnetic fields, the stellar rotation, and the stellar structure is treated self-consistently.
In this talk, I will report the results of the magneto-rotational evolution of 1.5 M stars, which have radiatively stratified envelopes during their main-sequence. We have found that the Lorentz force aided by the Omega-effect imposes torsional Alfven waves propagating through the magnetized medium, leading to near-rigid rotation within ~Alfven time. Our results of the hydrogen-burning stage can reproduce the main observed properties of Ap/Bp stars, and moreover, calculations continued to the red-giant regime show a pronounced core-envelope coupling, which reproduces the core and surface rotation periods determined by asteroseismic observations.
In addition, I will report the recent progress of developing a 1D scheme for representing the convective dynamo, which accounts for the magnetic activity of convective stars including the sun. Our new approach reproduces the most fundamental properties of this phenomenon, namely the magnetic field amplification as well as the cyclic behavior. We aim to provide the first comprehensive rotational evolution of solar-type stars ranging from the pre-main-sequence to the red-giant stages.

樫山 和己 （東北大学）
Fast radio bursts and related phenomena
Fast radio bursts (FRBs) are an enigmatic phenomenon; trains of coherent radio waves are arriving from a cosmological distance for a millisecond. In this talk I will first review the evolution of FRB research since its discovery in 2007. The most promising source of FRB is neutron star (NS). In fact, a low-luminosity FRB has been detected from a strongly magnetized NS, SGR 1935+2154, simultaneously with an X-ray flare. Focusing on the NS model, I will describe the theoretical insights and challenges that recent observational achievements have implied : “What is the emission mechanism of FRB?”, “What type of NSs can be the source of FRB?”, and “How such NSs are formed with what type of explosive transients?”. Finally I will discuss prospects and strategies to provide a comprehensive answer to these questions.

Hei Yin Jowett Chan (Tohoku University)
Diversity of the core-halo structure from the simulation of the Fuzzy Dark Matter Model
In comparison to the Cold Dark Matter, there are much less research efforts being invested in studying the structure formation of the Fuzzy Dark Matter (FDM) models. To advance our understanding of the model, our group developed a FDM simulation code, and performed soliton merger and cosmological simulation of the FDM model. Our results have improved the understanding of the core structure, as one of the unique feature of the FDM halo, by discovering a diversity in the core-halo structure. We will also discuss how the diversity could explain the inconsistency among different simulation groups, and its implication on nearby dwarf galaxies.

Smaranika Banerjee (Tohoku University)
Early kilonova emission from neutron star mergers
The origin of heavy elements has long been predicted to be the binary neutron star merger. In the neutron-rich material ejected from the binary neutron star merger, heavy elements are synthesized via rapid neutron capture (r-process). The radioactive decay of such elements produces emissions in the ultraviolet-optical-infrared (UVOIR) range, called a kilonova (Li and Paczynsky 1998). The first kilonova from the binary neutron star merger was detected by the follow-up observation of the gravitational wave event GW170817. Although the radioactive decay of the heavy elements is confirmed to be the source of observation at the late time (t > 1 day), the origin of the emission detected in the early time (t < 1 day) is still unclear. The observational properties of the kilonova (light curve and spectra) depend on the bound-bound opacity of the heavy elements, which in turn, requires atomic calculations. In my talk, I will share some of our recent progress in modelling the early kilonova by focusing on the aspects of atomic opacity calculation.

片桐 拓弥 （東北大学）
Tidal deformation of black holes in general relativity: From hidden symmetry
In binary systems of compact objects, when the orbital separation sufficiently decreases so that the tidal interaction becomes important, the objects will be deformed by the tidal field; this affects their orbital motion. The tidal deformability of the objects is encoded in a set of the so-called tidal Love numbers. These quantities are detectable imprints in gravitational-wave signals from binary systems. With these properties, measurements of the tidal Love numbers will be a novel way to explore extreme environments around black holes and to test theories of gravity in the strong-field regime. It is known that the tidal Love numbers of Schwarzschild and Kerr black holes are precisely zero. A natural question then arises: What is the origin of the lack of tidal deformation? Is that a unique feature of black holes in vacuum in general relativity?
In this talk, I will report our recent work on this problem. I will first explain the basics of black holes in general relativity and their static response to an external tidal field. After that, the previous works on this topic and a ladder operator from spacetime symmetry we use will be reviewed. I will then argue the vanishing of the tidal Love numbers in terms of hidden symmetry governing static tidal perturbations to black holes. If time permits, I will further discuss the symmetry breaking due to a slight modification of effective potentials for perturbations, giving nonzero tidal Love numbers.

Chris Packham (University of Texas at San Antonio / NAOJ)
Thermal-IR Observations of AGN: Why and near/long-term plans
As we excitedly await the opportunities that JWST will provide, and the 30m class telescopes after that, the field of thermal-IR (TIR) astronomy continues to gather attention. In this talk, I discuss AGN TIR work, in both the science and instrumentation arenas. I use this to illustrate the path I hope to follow in the coming years. In the final section I discuss some changes the pandemic has brought to teaching and research at UTSA.

藤林 翔（Max Planck Institute for Gravitational Physics）
Mass ejection and nucleosynthesis in binary neutron star mergers leaving short-lived massive neutron stars
We performed numerical relativity simulations for merger and post-merger phases of mergers of binary neutron stars with different mass ratios to investigate the dependence of the ejecta properties on the binary mass ratio for the case in which the remnant massive neutron stars collapse into black holes 3-20 ms after the merger. It is found that the solar abundance of the r-process nuclei is approximately reproduced irrespective of the mass ratio of the merging binary.
The reason for this is as follows: The dynamical ejecta is more neutron-rich for the merger of more asymmetric binary, which results in more production of heavier r-process nuclei and more severe underproduction of lighter r-process nuclei compared to the solar abundance of r-process nuclei. On the other hand, the post-merger ejecta has only a mild neutron-richness irrespective of the mass ratio, which results in the production of lighter r-process nuclei. Importantly, the post-merger ejecta is more massive for the merger of a more asymmetric binary. As a result, the underproduced lighter r-process nuclei for the asymmetric merger are compensated by the post-merger ejecta.

Janek Pflugradt (Tohoku University)
Finding of a population of active galactic nuclei showing significant luminosity decline in the past ~10³-10⁴ yrs
Active galactic nuclei (AGN) are a key population to understand the growth of supermassive black holes (SMBHs). Recent observations have revealed an interesting AGN subclass that shows strong activity at large scales (1 kpc) but weaker at small scales (<10 pc), suggesting a strong change in the mass accretion rate of the central engine in the past 10^3-4 yr. We systematically search for such so-called declining or fading AGN by cross-matching the SDSS type-1 AGN catalog at z<0.4, covering the [OIII] emission line which is a tracer for the narrow-line region (NLR) emission, with the WISE mid-infrared (MIR) catalog covering the emissions from the dusty tori. Out of the 7,653 sources, we found 57 AGN whose bolometric luminosities estimated from the MIR band are at least one order of magnitude fainter than those estimated from the [OIII] emission line. This declining population shows four important properties: 1) the past AGN activity estimated from the [OIII] line reaches around the Eddington-limit, 2) more than 30% of the luminosity declining AGN candidates show a large absolute variability of Delta W1>0.45 mag in the previous ~10 yr at the WISE 3.4um band, 3) the median [NII] over Ha line ratio of log([NII]/Ha) is -0.52, suggesting a higher contamination from the HII region on the observed emission lines 4) the second epoch spectra of the population indicate a spectral type change (so called a changing-look phenomenon) for the 31% of the sources. This population provides key insights on the possible connection between the luminosity decline which started ~10^3-4 yr ago out of a higher eddington phase into a phase with high variability in a 10 yr time scale.

Mochammad Wardana (Tohoku University)
Higher-order velocity moments Jeans dynamical modeling
The core-cusp problem is one of the long-standing sub-galactic challenges faced by Cold Dark Matter (CDM) theory. Although new dark matter theories are being proposed as one of the alternatives to solve the problem, it is, in fact, still unclear whether the cored density profile in dwarf galaxies does exist or is just a consequence of dynamical modeling limitations. The source of difficulties mainly comes from the existence of degeneracy between the mass distribution and velocity anisotropy of tracers. Completely relying on the line-of-sight velocity and projected positions data, breaking the degeneracy requires more constraints in the dynamical modeling. Our group incorporates higher-order line-of-sight velocity moments, in addition to the second-order moments, in order to explore their ability to constrain dark matter density profile in dwarf spheroidal galaxies through Jeans dynamical modeling. We will also discuss the prospects of our model to be applied to Prime Focus Spectrograph (PFS) data.

Yu-Hsuan CHANG (Tohoku University)
銀河系円盤の化学動力学と形成史: 恒星系のRadial migrationの調査
宇宙に存在する銀河の７割以上は我々が住む天の川銀河のような円盤銀河である。したがって、銀河系円盤の形成史を解明することが重要であり、そのためには円盤にある恒星系の化学動力学を調べる必要がある。ここで、近年恒星系の動径移動 (Radial migration)の役割に関して議論されているが、その明確な証拠が観測データから見つかっていなかった。そこで本研究では、APOKASC-2のKepler星震学データに基づいて導出された年齢データ、APOGEE DR14の分光データ、さらにGaia EDR3の運動データを使って太陽近傍にある恒星の化学動力学を調査した。そして、恒星系の年齢と金属量の関係を詳しく調べた結果、Radial migrationの証拠を発見した。さらに、先行研究の銀河系円盤のシミュレーションと比較した結果、銀河系の衛星銀河であるいて座矮小楕円体銀河の二回目の接近がRadial migrationを引き起こした原因であることが示唆された。

Zhaoran LIU (Tohoku University)
Large scale structures and galaxy evolution in a z=0.9 supercluster traced by unique pair narrow-band imaging
In growing large scale structures, galaxies therein are not only regulated by internal processes, but are also affected by external processes such as interactions with other neighboring galaxies or intra-cluster gas. Our goal is to understand the physics behind such environmentally dependent galaxy formation and evolution.However, the largest difficulty that has been hampering us to obtain the fundamental but critical quantity such as star formation rate, is how we correct for dust extinction accurately. Balmer decrement technique is known to be the best indicator of dust extinction. In this work, we take a unique approach that utilizes two narrow-band filters (on SWIMS and HSC) which can neatly capture H-alpha and H-beta lines and thus Balmer decrement only with “imaging” for the first time. Our science target is CL1604 supercluster at z=0.9 stretching over 100Mpc in comoving scale. We have so far completed H-alpha and J-band imaging for one SWIMS pointing, and have detected 12 H-alpha emitter candidates in a galaxy group in the supercluster, and we are comparing the properties of those emitters, including MIPS counterparts (starbursts), with those in a richer cluster in the same supercluster (our previous work, Asano et al. 2020) to investigate the environmental effects. We will also describe the whole concept of this unique program and its future prospects.

日下 暁人 (Lawrence Berkeley National Laboratory / University of Tokyo)
Exploration of Inflation and Dark Universe through CMB Polarization
In the coming decade, we foresee a rich and fruitful evolution in the field of cosmic microwave background (CMB) observations. Polarization measurements at degree angular scales constrain possible primordial gravitational waves, probing inflation and quantum fluctuations of gravity. Polarization fluctuations at arcmin angular scales, on the other hand, trace the growth of large-scale structure via gravitational lensing effects. These data sets are rich for measuring dark energy, the neutrino mass, and the evolution of galaxies and clusters. There also are emerging possibility of observing variability of microwave sky to constrain cosmic Axions. In this talk, I will review these CMB science and some of the recent results from CMB observations. I will also discuss some challenges we are facing with a next-generation project such as Simons Observatory, and how we aim to overcome these challenges.

武藤 恭之 (工学院大学)
Substructures of Transitional Disks:Detection Methods and Statistics
ALMA has provided us with the images of protoplanetary disks at ~10au resolution. Spatially resolved observations of protoplanetary disks have revealed that disks generally harbor substructures. Structures in disks provide us with information about physical and chemical environment for planet formation. In the talk, we present an overview of ring structures of protoplanetary disks, with particular focuses on transitional disks, which are disks harboring dust-depleted inner holes. We briefly present the methods to analyze data directly in the visibility domain and show several cases with asymmetric ring structures and/or compact inner disks, which have not been indicated by Spectral Energy Distribution (SED). We present statistics of such structures and discuss the origin of them. Finally, we show that ring structures have been observed in young systems, indicating various origin of disk structures.

市川 幸平 （東北大学）
Multi-wavelength view of active galactic nuclei
Active galactic nuclei (AGN) are a key population to understand the mass assembly phase of supermassive black holes (SMBHs). One beauty point of AGN is their visibility in the wide wavelength bands from radio to X-ray, and each frequency gives unique information on AGN radiation mechanism as well as the emission components with different physical scales. In this talk, I will talk how the combination of multiwavelength information help us to understand the early, rapidly growing, and final quenching phase of SMBHs, especially by focusing two populations. One is a fading AGN population, which have been discovered thanks to the recent multiwavelength coverage giving kpc scale AGN emission component. The other is radio bright AGN, which used to be known as a final phase of SMBH growth with a bright jet and low mass accretion rate. The view of radio AGN is changing thanks to the more findings of optical counterparts of radio sources, which turned out to cover radio AGN residing in low-mass galaxies and high BH accretion rate.

野田 博文（大阪大学）
X-ray studies of the vicinity to supermassive black holes and future prospects with XRISM
X-ray observations of Active Galactic Nuclei (AGNs) are one of the most efficient ways to probe the vicinity to Super Massive Black Holes (SMBHs). A variable X-ray continuum is produced via inverse Compton scattering in a corona formed near a SMBH, and it is reprocessed by surrounding materials such as an accretion disk, a broad-line region, and a dusty torus, causing fluorescence lines and a reflection component. Therefore, by checking X-ray spectra and time variabilities, we can study the physical condition and geometry of hot plasmas and cold materials near SMBHs. In this talk, I present an overview of X-ray observations of AGNs, and introduce our results of SMBH accretion flows and their state transitions in highly-variable AGNs, comparing to Galactic black hole binaries. Furthermore, in this Japanese fiscal year, the next X-ray astronomical satellite XRISM is planned to be launched, and an unprecedentedly high energy resolution will be achieved by the X-ray microcalorimeter in orbit. I also introduce the current status of the development and future prospects with XRISM.

Takahiro Morishita（Caltech）
Too early to claim a z~20 galaxy? Initial results from JWST early release programs
The James Webb Space Telescope (JWST) started its scientific operation in June 2022 and has already given us a new view of the universe. Among several scientific programs carried out so far, the Early Release programs offer opportunities for the community to get familiar with new instruments by releasing data without a proprietary period. In this talk, I will start with an overview of several studies that identified (ultra)high-z galaxies from the JWST Early Release Observations. I will then introduce GLASS-JWST, one of the 13 Early Release Science programs, offering a wide range of science — cluster galaxies, galaxies at cosmic noon, and high-z galaxies. I will present early results obtained with two instruments, NIRISS, focusing on spectroscopic identification and characterization of emission line galaxies with its wide-field slitless spectroscopic mode, and NIRCam on the identification and characterization of early galaxies. Lastly, I will provide future prospects for data processing and upcoming observations.

池田思朗（統計数理研究所）
Data science for future astronomy
The field of machine learning and artificial intelligence have developed significantly in recent years and have attracted attention following the success of deep learning. The fields of data processing, which includes the above fields, statistics, optimisation theory, signal processing, are now called data science. In astronomy, future observations will provide data with different quantity and quality, thus, new methods of information processing are required. Therefore, data science will become undoubtedly important. Furthermore, since astronomy tries to infer a lot of information from limited observations, it is necessary not only to import data science methods but also to modify methods for your own goal. Through lectures, I would like to show examples of data science methods in astronomy and the basic idea of data science.

Mariko Kubo (Tohoku University)
Massive quiescent galaxies and environments in the early Universe
Giant ellipticals are the dominant population of galaxy clusters, which are characterized by large stellar masses, old stellar population, and suppressed star formation. Protoclusters, which are thought to evolve into clusters of galaxies today, have been studied to understand how and when giant ellipticals have formed large stellar masses and stopped star formation. In this talk, I will start with an overview of the multi-wavelength observations of protoclusters to date. Then I will present our discovery of a massive quiescent galaxy and an AGN with significant ionized gas outflows found from the multi-wavelength imaging/spectroscopic observations of a protocluster at z=3.1; they are interesting snapshots of galaxies around the epoch of star formation quenching. Finally, I will introduce the statistical approach to studying protoclusters using multi-wavelength wide-field surveys.

Gerhard Hensler (University of Vienna)
Shaping galaxies in the Virgo Cluster
Due to its proximity the Virgo Cluster serves as the best test bed for studying environmental effects on the evolution of its galaxies by their mutual gravitational interaction and the influence of the intra-cluster medium (ICM). This talk will focus on 2 large Virgo surveys based on different targets: 1) The SMAKCED (Stellar content, MAss and Kinematics of Cluster Early-type Dwarfs, PI: Thorsten Lisker) survey combines imaging and spectra of dwarf elliptical galaxies. It was dedicated to study their diversity in stellar kinematics, brightness profiles, and chemistry, and tries to understand their various evolutionary paths within the cluster environment. 2) Deep H-alpha images of the Virgo cluster field were taken by VESTIGE (A Virgo Environmental Survey Tracing Ionised Gas Emission, PI: Alessandro Boselli) to trace the ionized gas component of galaxies and decipher their mass loss and the distribution of the warm component within the ICM. For a deeper insight into the processes acting on the galaxies, individual objects are compared with numerical simulations and theoretical concepts.

Hamid Hamidani (Tohoku University)
The Cocoon Breakout and Emission in Binary Neutron Star Mergers
In binary neutron star mergers, short gamma-ray bursts' jets are launched and propagate across the dynamical ejecta. Using 2D numerical simulations, we study the jet cocoon; the byproduct of the jet propagation across the dynamical ejecta. We follow the cocoon at later times after the jet breakout. Our results show that, the fraction of the cocoon that breaks out of the ejecta (in terms of mass and energy) is not substantial, with most of the cocoon being trapped inside the dynamical ejecta even at later times. This is in contrast with the cocoon of collapsar jets. In addition, we present an analytical model for the cocoon breakout. Our model is reasonably consistent with numerical simulations at predicting the properties of the cocoon after the breakout. Finally, we apply our analytic model to estimate the cocoon’s cooling emission, as an electromagnetic counterpart to gravitational waves from neutron star mergers, as well as other observational features.

Hajime Ogane (Tohoku University)
Study for performance improvement of Laser Tomographic Adaptive Optics
Adaptive optics (AO) is a system that maximizes the performance of large ground-based telescopes by measuring and correcting the wavefront distortion of object light caused by turbulence in the Earth's atmosphere. In recent years, laser tomography adaptive optics (LTAO), which tomographically measures atmospheric turbulence using multiple laser guide stars and wavefront sensors has been actively developed and implemented. LTAO will enable higher precision in AO of 8-10 meter class telescopes and be essential in AO of 30-40 meter class telescopes. Our team is developing the ULTIMATE-START, LTAO demonstration system for the Subaru Telescope. The performance goal of this system is RMS<100nm for the corrected wavefront. To achieve this goal, I propose and evaluate two fundamental technologies: (1) tomographic error reduction using prior information on atmospheric turbulence profile, and (2) use of sCMOS sensors as wavefront sensor detectors. In this presentation, I will present details of these two studies and my future work during Ph.D course.

Naoaki Yamamoto (Tohoku University)
宇宙の銀河形成最盛期以降における銀河団と銀河の進化
近傍宇宙では銀河の諸物理量 (色・年齢・星形成活動・形態など) は、銀河の周辺環境によって大きく異なることが知られている。例えば銀河団領域では進化の進んだ古く赤い銀河の割合が高いのに対して、一般フィールド領域では星形成が活発な若く青い銀河の割合が高いことが知られている。この銀河進化の環境依存性がいつ・どこで・どのように生じてきたのかについて過去に遡って理解を深めることは、銀河の形成や進化の歴史を紐解く上で非常に重要である。しかしながら遠方宇宙における環境依存性の研究は、銀河団のサンプル数が限られており統計的な調査が少ないという問題や、比較的進化の進んだ銀河団にバイアスされているという問題があった。そこで従来よりも視野が広く深い観測データが利用できる、超広視野撮像装置すばるHSCに着目し大規模な遠方銀河団探査を行うことにした。HSCに搭載された狭帯域フィルターで選択された輝線銀河を利用して、z~1.5の時代における若い星形成銀河の密度超過領域を探査した。さらに、これらの領域に付随したメンバー銀河候補を探すために、すばるHSCの可視光撮像データに加えUltra-VISTAなどの近赤外撮像データを使用した。このことにより、遠方宇宙においてさまざまな進化段階の銀河団を発見し、そこに存在する銀河の性質を調べることが可能になる。そして、銀河団の集積・成長と共に、銀河の星形成活動がどのように変遷し、現在の宇宙に見られるような銀河の性質の強い環境依存性が出来上がってきたのかを議論する予定である。

Yuichi Harikane (University of Tokyo)
A Comprehensive Study on Galaxies at z~9-17 Found in the Early JWST Data
We conduct a comprehensive study on dropout galaxy candidates at z∼9−17 using the first 90 arcmin2 JWST/NIRCam images taken by the ERO and ERS programs. We carefully apply a secure photo-z selection criterion and conventional color criteria with confirmations of the ERO NIRSpec spectroscopic redshifts, and obtain a total of ~20 dropout galaxies at z∼9−17, including two candidates at zphot=16.45+0.09−0.32 and 16.66+1.86−0.34. We perform thorough comparisons of dropout galaxies found in our work with recent JWST studies, and conclude that our galaxy sample is reliable enough for statistical analyses. We derive the UV luminosity functions at z∼9−17, and confirm that our UV luminosity functions at z∼9 and 12 agree with those determined by previous HST and JWST studies. The cosmic star-formation rate density decreases from z∼9 to 12, and perhaps to 17, but the densities at z∼12−17 are higher than the constant star formation efficiency model. Interestingly, there are six bright galaxy candidates at z∼11−17 with MUV<−19.5 whose stellar masses are very high, 10⁸⁻⁹Msun. Because a majority (∼70%) of them show no signatures of AGNs in their morphologies, the high cosmic star-formation rate densities and the existence of these stellar massive galaxies are explained by no suppression of star-formation by the UV background radiation at the pre-reionization epoch or an efficient UV radiation production by a top-heavy IMF possibly with Population III-like star formation.

Kazuki Tokuda (Kyushu University)
Investigating the star formation process based on the low-metallicity Magellanic Clouds observations
Newborn stars launch bipolar outflows as their first cry, universally observed in the present star-forming regions. Previous outflow studies have mainly focused on the solar-neighborhood condition, and it is poorly understood whether such outflow-driving is expected in the low-metallicity environment, representing the earlier phase of the universe. Since the early ALMA's scientific operations, we have been actively observing molecular clouds and the associated protostellar objects in the low-metallicity Magellanic Clouds, whose metallicity is 0.2-0.5 times the solar value. In this presentation, I report the detection of molecular outflows for the first time in both Magellanic Clouds as an indicator to verify the universality of the star formation mechanism from dense cores to protostars. I also review recent ALMA studies of molecular clouds in the Magellanic Clouds, along with the outflow detection report.

Rosemary Wyse (Johns Hopkins University)
The Old are Rich: Implications of the Old, Metal-Rich Milky Way
The inner regions of the Milky Way contain old, metal-rich stars, consistent with having formed in an early starburst. Indeed, most Galactic stars older than 8Gyr are more metal-rich than [Fe/H] ~ -0.5. Simulations of Milky Way-like galaxies in standard Cold Dark Matter dominated cosmological models fail to reach sufficiently high metallicities at early epochs, plausibly reflecting the low-mass and shallow potential wells of the sub-structures in which the ancient stars form. I will quantify this comparison between observations and predictions, using public datasets from two different simulation suites, and ages and metallicities for stars in our Galaxy. I will then interpret the results in the context of other indications of the rapid growth of Milky Way-like galaxies and their associated dark matter haloes, and tensions with the predictions of CDM on galactic scales.

長谷川 樹（東北大学）
rプロセス元素を合成する特殊な超新星爆発の観測的特徴
近年、重力波に付随した中性子星合体からの電磁波対応天体が観測されたことで、rプロセス元素の起源天体として、中性子星合体が有力視されている。しかし、中性子星合体だけでは銀河の化学進化を説明できないという指摘があり、rプロセス元素の起源天体が中性子星合体だけではない可能性がある。別の候補天体として考えられているのが、ニュートリノ加熱とは異なるメカニズムで爆発する特殊な超新星爆発である。その中でも、ブラックホールと降着円盤を形成するCollapsarモデルと、磁気圧駆動のジェットにより爆発する磁気駆動型超新星は、近年の数値シミュレーションにより、rプロセス元素を合成する可能性があると予想されている。
そこで本研究は、このようなrプロセス元素を合成する特殊な超新星の観測的特徴を詳らかにするために、特殊な超新星におけるrプロセス元素と56Niの合成量の予想を元に、一次元流体力学計算と元素合成計算によってrプロセス元素を合成する超新星をモデル化し、詳細な輻射輸送計算を行った。その結果、一部のCollapsarモデルでは、rプロセス元素の効果として高いOpacityが強く影響し、光度曲線のタイムスケールが長くなることが分かった。一方で、一部の磁気駆動型超新星モデルでは、rプロセス元素のOpacityとしての効果が現れない代わりに、rプロセス元素の放射性崩壊によるエネルギーで光る超新星として観測され得ることがわかった。rプロセス元素の効果が強く現れる可能性がある磁気駆動型超新星モデルに関しては、将来的にこの天体を観測することができるか、そして観測できた際に普通の超新星と区別できるかについても議論する。

鈴木 善久（東北大学）
HSC-SSPデータを用いた天の川銀河ハローのトモグラフィー
現在の標準シナリオでは、天の川銀河 (MW) のような銀河は小銀河が重 力相互作用による合体・降着を繰り返すことで形成されてきたと考えられている。こ の痕跡の位相空間上の緩和時間は100億年以上と長いため、銀河全体を覆う希薄な球 状の領域であるハローに今なお存在しているのではないかということが理論上示唆さ れてきた。大規模観測の進展により、このような痕跡を観測的に調査可能な時代が到 来している。特に私たちの属するMWが注目されている。何故ならば、小さな銀河が母 銀河に合体・降着する際に潮汐力で破壊されていく現場を恒星ストリームやサブ構造 として直接観測可能だからである。現状 MW 中心から20-30kpc以内のハローの構造お よびサブ構造については化学動力学の観点から理解が進みつつある一方で、MW外縁部 のハロー構造については未解明の部分が多い。
本研究では、広視野（約1.8平方度）かつ深い測光（i < 26等）が特徴的なHSC-SSPの 全データ（約1200平方度）を用いて、MWの外縁部のハロー構造を調査した。MWの外縁 部に残るサブ構造を効率的に検出するために、年齢が古く金属量の少ない恒星系に対 するIsochrone-filterを作成した。その結果、既に発見されているサブ構造が検出さ れるとともに、新たなサブ構造の候補が発見された。本講演では、これらのサブ構造 の起源について議論していく。

天崎 賢至（東北大学）
星間塵の内部エネルギー制限を考慮した星間塵の熱史を追うシミュレーションコードの開発
今後10年間に計画されているさまざまなCMB偏光観測精密測定で得られるミリ波データは、星間塵の物理の理解の深化につながるデータの宝庫でもある。それらを最大限に活用するためには、信頼度の高い星間塵放射モデルが必須である。ミリ波の星間塵放射に対しては、特に星間塵の温度分布が強い影響を与える。そのため、星間塵の温度分布を数値的に求める手法が提案されてきた。しかし、いずれのモデルも大半のVery Small Grain (VSG)の温度がCMB温度を大きく下回るという不自然な結果を示していた。
本研究では、「星間塵が放射する光子のエネルギーは、星間塵自身が持つ内部エネルギーを超えてはならない」という物理的要請（Internal Energy Bound, IEB）により、星間塵が放射できる光子のエネルギーが厳しく制限されることを示した。このIEBの効果は先行研究では考慮されていなかった。今回、IEBを考慮した星間塵の熱史を追うシミュレーションコードの開発を行い、星間塵の温度分布およびSEDに対するIEBの影響を初めて調べた。その結果、温度分布には先行研究で見られた数百Kの成分が確認されたほか、大半のVSGは10 Kに滞留することがわかった。これにより、大半のVSGがCMB温度以下になるという上記の問題が解消された。この10 K成分はミリ波域で超過放射を出す。そのため、今回の結果は星間塵放射のミリ波と中間赤外域での超過放射の強度に相関があることを予言する。

室越 琳生（東北大学）
原始重力波起源のCMB Bモード観測のための前景除去における、星間物質によるCMBモノポールの吸収の影響
原始重力波起源のCMBのBモード偏光はインフレーションを検証するための最も強力なツールであるが、未だ検出されていない。その検出において、データに混入する星間放射などの前景成分の除去精度向上が最も大きな課題の一つとなっている。Nashimoto et al.(2020)は星間物質によるCMBモノポールの吸収がその検出に無視できない影響を及ぼすことを初めて指摘し、この効果をCMB shadowと名付けた。しかしながら彼らの結論は、オーダー評価に基づいた研究から引き出されている。CMB shadowが原始重力波起源のCMBのBモード偏光検出に与える影響を定量的に調べるには、シミュレーションで生成された擬似CMB観測マップに対して既存の成分分離コードを適応して、より現実的な状況で影響を定量評価することが必須である。そこで本研究では、まずCMB shadowが原始重力波起源のCMBのBモード検出にどのくらい影響を与えるかをマップベースで評価した。成分分離コードには、再電離バンプの測定で次世代CMB観測衛星LiteBIRDの測定目標が達成できることが報告されているDelta-map法を用いる。前景除去の過程でCMB shadowの影響を考慮しないと、原始重力波起源のCMBのBモード偏光が存在しない場合でも、有為な原始重力波起源のCMBのBモード偏光検出を報告してしまい、誤検出に繋がることを示した。また原始重力波起源のCMBのBモード偏光が存在する場合は、その強度を大幅に過大評価してしまうことも示した。さらに、Delta-map法を改良し、CMB shadowの影響を除去する具体的な前景除去方法を提示した。この手法によりCMB shadowの影響を適切に除去でき、LiteBIRDなどの次世代CMB偏光観測実験が目指す原始重力波起源のCMBのBモード偏光検出感度を達成できることを示した。本研究ではLiteBIRD実験を想定した大角度スケールに現れる再電離バンプの検出に焦点を当てて調べたが、地上観測を想定したより小さい角度スケールに現れる再結合バンプの検出に適用できるよう、Delta-map法に改良を加えることが今後の課題である。

福地 輝（東北大学）
電波で明るいdust obscured galaxiesで探る超巨大ブラックホールへの超臨界ガス降着流とAGNフィードバックの理解
超巨大ブラックホール (SMBHs) が急速に成長する過程は活動銀河核 (AGN) として観測される。AGN活動は母銀河の成長に大きな影響を及ぼすと考えられており、電波で明るいAGNジェットによって星生成が抑制される可能性が指摘されている。これまでAGNジェットはブラックホール成長の最後のフェーズである低質量降着流に付随すると考えられてきたが、ブラックホールへの降着がエディントン限界を超えるような高降着率の系でも電波で明るいジェットやアウトフローが効率的に生成されると理論的に示唆されている。このエディントン限界を超えての成長はz>6で発見されている重たいクエーサーの起源の理解にも重要である。
可視-赤外線で非常に赤いカラーを持つdust obscured galaxies (DOGs) には、銀河合体に伴い急速に成長する SMBHs が存在すると予想され、一部のDOGsはエディントン限界を超えた降着を実現することでジェット活動を持つ可能性がある。そこで我々は Subaru-HSCと赤外線衛星WISEで得られたi-W4カラーを用いて得られたDOGs 約1300 天体とVLA/FIRST 電波カタログとのクロスマッチングを行うことで電波で明るい DOGs (radio-loud DOGs) 29 天体を発見した。SEDフィッティングを行い、エディントン比を見積もった結果、30%のradio-loud DOGsがエディントン限界に達し、その割合は電波で検出されていない DOGs (7%) とくらべて高いことがわかった。さらに、SDSS で分光観測されているradio-loud DOG 1天体については分光フィッティングを行い、エディントン限界を超えた降着流を実現していること、母銀河からの脱出速度を上回る高速の[OIII]アウトフローを持つことがわかった。よって、radio-loud DOGs はエディントン限界付近での急激な質量降着やAGNフィードバックについて理解するための有力天体と言える。