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.