談話会
Colloquium

李 建鋒（東北大学）
Probing the connection between radio jets and molecular gas in high-redshift radio galaxies
Radio active galactic nuclei (AGN) feedback is one of the most plausible scenarios to interpret the observational drop at the bright end of the galaxy luminosity function. To understand how radio feedback works in massive systems, high-redshift radio galaxies are good test beds. High-redshift radio galaxies (HzRGs) are the most massive galaxies in the early universe, with characteristic powerful radio jets. In contrast to local quiescent massive galaxies, HzRGs are actively star-forming, lying either on or below the corresponding star-forming main sequence. Compared to other populations of galaxies, HzRGs show high star formation efficiencies and short depletion time. In the literature, studies of CO observations indicate that HzRGs have a low molecular gas fraction up to z~4, suggesting that they are on the road to being quenched. In this talk, I present VLA CO(1-0) observations of seven radio galaxies at 4.5<z<5.7 and further discuss the results of one of our targets, a 'gas-rich' radio galaxy at z=5.2. The non-detection of CO(1-0) in six targets indicates their low molecular gas fraction. In the target at z=5.2, not only is the CO(1-0) emission detected, but the off-center molecular reservoir and alignment between the radio jet axis and the molecular reservoir are observed. I will discuss the interpretation of this phenomenon.

工藤祐己（東北大学）
Hierarchical gas structure in outflows of active galactic nuclei
The multi-phase gas outflows observed in active galactic nuclei (AGN) are important for understanding the supply onto the central supermassive black hole (SMBH). The widely believed AGN unification scheme does not provide a clear picture of the outflow and its variability based on recent observations. Our simulations for the dusty wind and ultra-fast outflow (UFO) in AGN have been carried out with the computational domain of the dynamic range of 4 orders of magnitude (i.e. 10² -10⁶ Rs ~ 10^-4- 1 pc). We have performed the axisymmetric 2D hydrodynamic simulations using the CANS+ code taking into account dust and radiation. We found that dust-free and dusty gases blow out from the dusty disk by radiation force and radiative heating. The viewing angle dependence of column density is determined by dust-free gas which is sublimated. UFO is dominant above the angle of 40 degrees, measured from the equatorial plane, while the dusty outflow is around the disk surface. UFO contributes significantly to the variability of the column density. These outflows are expected to build the AGN gas structure and explain the absorbers and time-varying phenomena observed in X-ray.

Christopher Irwin（University of Tokyo）
Insights on the origin of low-luminosity GRBs from a revised shock breakout picture for GRB 060218
Despite ~two decades since the discovery of low-luminosity gamma-ray bursts (LLGRBs), their origin remains poorly understood. Shock breakout from a progenitor with an extended (~10¹³), low-mass (10^-2 Msun) envelope provides a possible interpretation for the smooth prompt X-ray light curve lasting ~1000 s and the early optical peak at ~0.5 days observed in some events. However, even in the best-observed case, GRB 060218, current shock breakout models have difficulties explaining the unexpectedly strong optical emission at a few hundred seconds, the simultaneous presence of thermal and non-thermal components in the X-ray spectrum, and the rapid evolution of the peak energy. We suggest that these peculiar features can be explained by a previously unexplored regime of shock breakout. Applying recent advances in shock breakout theory which predict more rapid thermalization in the early planar phase of evolution, we show that for sufficiently fast shocks breaking out of especially extended progenitors, a non-standard breakout scenario is expected in which the gas and radiation are initially out of thermal equilibrium, but the time to reach equilibrium is less than the light-crossing time of the envelope. In this case, due to light travel time effects, the observed X-ray spectrum is a multi-temperature blend of blackbody and free-free emission components. The bremsstrahlung component extends down to the optical band, which can account for the excess optical emission observed at early times. As the system thermalizes, the non-thermal component quickly evolves toward lower energies and eventually fades altogether, resulting in a rapid peak energy decay consistent with observations. These results strengthen the case for a shock breakout origin of LLGRBs, and provide further evidence connecting LLGRBs to peculiar progenitors with extended low-mass envelopes.

Bovornpratch Vijarnwannaulk（Tohoku University）
The obscuration of of luminous AGN at high-redshift unvield by deep & wide multiwavelength survey
Nuclear obscuration by a dusty toroidal structure has long been used to unify population of unobscured and obscured active galactic nuclei (AGN). However, it has been shown by statistical X-ray AGN studies that the fraction of obscured increases with redshift, but the trend is not well determined above redshift 2 especially for luminous AGN (log Lx [erg /s]>44.5). These observations indicate that high redshift black hole growth occurs under heavy obscuration and are not fully explained under the nuclear obscuration which implies additional components obscuration from the host galaxy are needed. Using the unique deep & wide multiwavelength imaging datasets in the HSC-Deep XMM-LSS field, we investigated the obscured fraction of luminous AGN with log Lx [erg /s]>44.5 above redshift 2. We find an obscured fraction of 76±4% which is more than twice the fraction in the local universe (~30%). Both the obscured and unobscured z>2 AGN show a broad range of SEDs and morphology, which may reflect the broad variety of host galaxy properties and physical processes associated with the obscuration. We further investigated the incidence of obscuration in samples of Type 1 & 2 AGN at redshift 0.8-1.8 under AGN obscuration scenarios which describe obscuration properties in the local universe. For most of the sample, we find that the Eddington ratio of Type-2 AGN is lower than Type-1 broad-line detected AGN. The distribution is consistent with dusty nuclear obscuration regulated by radiation pressure, but we find evidence of non-nuclear obscuration among the sample of Type-1 AGN. We finally discuss the difference between optical and X-ray obscuration and how each affect the optical SED of AGNs.

Zhiying Mao（Tohoku University）
Understanding galaxy quenching through statistical analysis of recently quenched galaxies
Recently quenched galaxies (RQGs), as a transitional population between quiescent and star-forming galaxies, is crucial in investigating physical mechanisms behind quenching processes. A statistical sample of RQGs is necessary for studying mass and environment dependence of quenching. However, the rarity of RQGs hampers statistical spectroscopic analysis. To prepare for the coming spectroscopic survey of RQGs at intermediate and high redshifts, we used the rest-frame UVJ diagram to select RQG candidates and compared our selection with the PCA selection method. As the UVJ and PCA methods are both photometric methods, we further confirmed these two methods with LEGA-C and UDSz spectroscopic data. In addition to archived spectroscopic data, we also obtained and reduced our own spectral data at z~2 to confirm the photometric methods. The two methods are generally consistent, with a slight preference towards different sub-population. Based on this, we conducted a statistical analysis of UVJ-classified galaxy populations in photometrically selected clusters and X-ray-confirmed groups in the COSMOS field. We use the ratio of different galaxy populations to quantify the quenching efficiency, quenching stage and quenching timescale of galaxies. For galaxies within HST coverage, we also analyzed the mass and environment dependence of their structural parameters. Through these analyses, we gain insights into the quenching scenario at different redshifts. These analyses can also benefit the further spectroscopic RQG survey at intermediate and high redshifts.

木村 和貴（東北大学）
Evolution of Circumstellar Disk and Stellar Structure in the Primordial Star Formation
Primordial stars, also known as Population III stars, are formed in the pristine universe after the Big Bang. Their radiation and supernova explosion affect the interstellar medium dynamically and chemically. Moreover, their remnant black holes are possible seeds of high-redshift supermassive black holes. Elucidating primordial star formation is thus necessary to understand their roles in the subsequent structure formation. In this seminar, we talk about the formation processes of primordial stars. We especially focus on the accretion phase, where circumstellar disks emerge and protostars grow in mass. First, we discuss the evolution of circumstellar disks using our one-dimensional disk model. Our model demonstrates that the accretion rates throughout the disks are lower than the mass supply rates from the infalling envelope. Due to this fact, disks become more massive than central stars and gravitationally unstable in the early stage. Second, we talk about the evolution of rapidly accreting primordial protostars. We have performed three-dimensional radiation hydrodynamic (RHD) simulations resolving stellar interior with a newly developed RHD solver. Our simulations show that protostars rotate rapidly due to the angular momentum of the accreting gas. Such rotation is critical to the stellar evolution and the accretion process via circumstellar disks. We finally discuss the implications of our results and future directions.

前田 龍之介（東北大学）
Formation of Young Massive Clusters by Fast HI Gas Collision
Young massive clusters (YMCs) are dense aggregates of young stars, which are essential to galaxy evolution, owing to their ultraviolet radiation, stellar winds, and supernovae. The typical mass and radius of YMCs are M~10⁴ M_{sun-1, which is the typical velocity of the galaxy-galaxy interaction. In this study, we examine whether the fast HI gas collision triggers YMC formation using three-dimensional magnetohydrodynamics simulations, which include the effects of self-gravity, radiative cooling/heating, chemistry, and the stellar feedback effect. We demonstrate that massive gravitationally bound gas clumps with M > 4*10⁴ Msun and L∼4 pc are formed in the shock compressed region induced by the fast HI gas collision, which massive gas clumps can evolve into YMCs. Our results show that the YMC precursors are formed by the global gravitational collapse of molecular clouds formed by fast HI gas collision. We find that the formed sufficiently compact massive gas clumps have a large escape velocity compared to the sound speed of the HII region, which means that gravity prevents gas evaporation and thus they form stars with a high star formation efficiency keeping high-density structure. We also examined the initial condition dependence of YMC formation by fast HI gas collision. Our results show that YMCs can be formed even in low-metal environments, such as the Magellanic Clouds. Additionally, the very massive YMC precursor cloud, with M > 10⁵ Msun, can be created when we consider the fast collision of HI clouds, which may explain the origin of the very massive stellar cluster R136 system in the Large Magellanic Cloud.}

長谷川 幸彦（東北大学）
Collisional Growth and Fragmentation of Dust Aggregates
One of the key mechanisms in planet formation is the evolution of solid materials (called dust) in protoplanetary disks. The dust grains grow and fragment through collisions between dust grains. In order to clarify the evolution process of dust grains, we need to know the detailed physical properties of collisions between dust grains, which depend on the masses, the collision velocity, the impact parameter, the internal structures, the material properties, and so on of colliding dust grains. In this seminar, we talk about collisional outcomes of water-ice dust aggregates with various mass ratios; we particularly focus on unequal-mass offset collisions. We carried out three-dimensional numerical N-body simulations of collisions between two dust aggregates in a wide range of the mass ratio 1-64. First, we found that the mass transfer from a larger target to a smaller projectile is a dominant process in collisions with a mass ratio higher than 3. As a result, the critical velocity for fragmentation of the largest body is considerably reduced due to the mass transfer for such unequal-mass collisions; the critical velocity of collisions with a mass ratio of 3 is about half of that obtained from equal-mass collisions. This result implies that the reduced critical velocity for unequal-mass collisions would delay growth of dust grains more than previously thought in protoplanetary disks. Next, we derived analytic expressions of the mass distribution of large remnants and small fragments by numerical fitting to the simulation results. Our analytic formulae for masses of the large remnants can reproduce the contribution of mass transfer from a larger target to a smaller projectile. Our fragment model can roughly reproduce the results of our simulations and be applied to statistical simulations of the dust evolution.

Kei Ito（The University of Tokyo）
Environment, AGN activity and morphology of massive quiescent galaxies 1-2 Gyrs from the Big Bang
Recent observations have found that massive galaxies with suppressed star formation activity exist even in the high redshift universe to z~5. On the other hand, many of their properties are still unknown. In this talk, I will introduce our three recent works on these quiescent galaxies at z>2, focusing on their environment, AGN activity, and morphology. First, I report discovering an overdense structure of quiescent galaxies at z=2.77 in the COSMOS field, which is spectroscopically confirmed by Keck/MOSFIRE observation. The four confirmed quiescent members imply that this structure is more than 68 times denser than the general field. This structure is likely a protocluster that will evolve into a Coma-like cluster at z=0. Secondly, I report X-ray (Chandra) and radio (VLA) image stacking analysis of quiescent galaxies to z~5. We detected the average stacked signal of quiescent galaxies in X-ray and radio. Their luminosity could not be explained without AGNs. Moreover, the AGN luminosity of quiescent galaxies is higher than those of star-forming galaxies with the same stellar mass. This could imply that the AGN activity is related to quenching. Lastly, I report our recent study, which constrains the morphological properties of quiescent galaxies at 3<z<5 based on James Webb Space Telescope/NIRCam imaging. We find that the average size of quiescent galaxies at z>3 is ~0.6kpc at logMstar/Msun = 10.5, which is significantly smaller than those at z<3. In addition, most of them have bulge-dominated shapes. This implies that the size evolution and the bulge formation have begun as early as z>3.

Kento Masuda（Osaka University）
Toward understanding the evolution of close-in exoplanets
Exoplanets orbiting close to their host stars may undergo various changes in their orbits and physical properties. It has long been discussed that the shortest-period planets may have orbits unstable to tidal dissipation and spiral into their host stars. Recent transit observations of low-mass planets have also provided evidence for evaporation of their atmospheres, which provides clues to the internal composition of those planets. In this talk, I will describe our ongoing efforts to probe those changes in planetary systems throughout the main-sequence life of their host stars. I will present our recent work to derive the age dependence of the occurrence rate of giant planets orbiting Sun-like stars based on isochronal ages, and also briefly mention insights into gyrochronology from high-resolution spectroscopy of Sun-like stars belonging to twin binaries.

Akio Taniguchi (Nagoya University)
Data-science-combined development for the future large submillimeter telescope
Imaging spectroscopy at the submillimeter wavelength is a promising approach to surveying the huge cosmic volumes, uncovering star formation history and galaxy evolution in the early universe by bright nebular lines (e.g. [O III] 88 μm, [C II] 158 μm) and multi-color continuum emission. Complementary to large interferometers (e.g. ALMA) that are proficient in pinpointing individual targets at high angular resolutions, next-generation large submillimeter single-dish telescopes (e.g. LST 50 m) have been proposed to achieve wide-field (~1 deg^2) and wide-band (> 100 GHz) imaging spectroscopy. However, construction, observation, and analysis are all challenging for such facilities due to the required high surface accuracy (~45 μm r.m.s) and the identification of faint signals from noisy big (~petabytes) data. In this talk, I would like to introduce new approaches in each step combined with data-scientific methods. We developed a noise removal method based on matrix decomposition that avoids direct subtraction between two noisy spectra. It then offers us to improve the observation sensitivity by a factor of √2. For the high surface accuracy, we are developing the millimetric adaptive optics by wave-front modeling and the optimization of the telescope support structure by genetic algorithm. These methods will compensate for dynamic and static surface deformation, respectively. I would like to introduce the early results in both simulation and observations.

Mainak Mukhopadhyay (Pennsylvania State University)
Multi-messenger signatures from core-collapse supernovae: neutrinos and gravitational waves
In the current era of multi-messenger astronomy, gravitational wave (GW), neutrino, photon, and cosmic ray observations are combined to extract information about astrophysical sources and phenomena in the Universe. A core-collapse supernova (CCSN) serves as nature’s very own laboratory. They have been one of the prime focuses of multi-messenger astronomy. In this talk, I will discuss my work on understanding the information that neutrinos from CCSNe can provide us with in the context of multi-messenger astronomy. I will present a new phenomenological description of the neutrino GW memory effect, highlighting its detectability, and physics potential in the present context. I will also present a novel idea of how observations of the neutrino GW memory from CCSNe will enable time-triggered searches of supernova neutrinos at megaton scale neutrino detectors. I will conclude with discussing my work on how neutrino observations at future neutrino detectors may enable us to constrain turbulence in supernovae and the pre-supernova alerts using pre-supernova neutrinos. Overall, the main theme of this talk will be to show how neutrinos from the various phases of a CCSN can be used to gain insights and understanding in a much broader context.

Kimihiko Nakajima (National Astronomical Observatory of Japan)
Unveiling the Interstellar Medium and Stellar Population in High-Redshift Galaxies: Insights from the Early JWST/NIRSpec Observations
The advent of the James Webb Space Telescope (JWST) has opened up a new window to investigate the Interstellar medium (ISM) properties and the stellar population in detail in high-redshift galaxies (z=4 to 10) using the rest-frame optical spectroscopic signatures. In this talk, I will provide an overview of recent advancements in our understanding of ISM properties and stellar populations. Specifically, our research focuses on exploring the evolution of the mass-metallicity (MZ) and star-formation rate-mass-metallicity (SFR-MZ) relations across the redshift range of z=4-10. We have analyzed a sample of 135 galaxies identified in the JWST/NIRSpec data from the three major public spectroscopy programs: ERO, GLASS, and CEERS. Our findings reveal a modest evolution in the MZ relation from z~2-3 to z=4-10. Interestingly, the SFR-MZ relation shows no significant evolution up to z~8 but exhibits a noteworthy decline at z > 8 beyond the error. This decrease in the SFR-MZ relation at z > 8 may indicate a break of the metallicity equilibrium state driven by star formation, gas inflow, and outflow. While our sample reveals a mild evolution in metallicity, it is worth noting that recent studies have reported the existence of extremely metal-poor objects that resemble Pop-III star galaxies. I will review these recent progresses in our understanding of galaxies during the early phase of galaxy evolution.

津久井 遼（株式会社ロジストラボ）
Birefringent PDI Wavefront Sensor for Extreme Adaptive Optics
This talk is based on my presentation at the Ph.D. defense (March 2023). Direct observations of exoplanets are capable of providing important information on the mechanism of planet formation and the environment of potentially habitable planets. Potential targets have small angular separations (≪ 1") from their host stars and high planet-to-star contrasts (> 6 orders of magnitude). Such observations with ground-based telescopes are affected by the Earth’s atmospheric turbulence and resulting wavefront aberration. Thus, an accurate wavefront correction with Extreme Adaptive Optics (ExAO) is crucial. ExAO has much more elements (> 1 k/8-m aperture) and runs faster (> 1 kHz) than conventional AO to achieve more accurate (~ lambda/20) correction. ExAO system requires a high-performance Wavefront Sensor (WFS) that can measure the incoming wavefront fast (> 1kHz) and accurately (< lambda/20) with a weak flux from stars. For this purpose, we proposed and developed a new type of WFS: Birefringent Point-diffraction Interferometer (PDI). We manufactured a key optical device by microprocessing technologies on a birefringent crystal TiO2. We tested the Birefringent PDI in the laboratory and demonstrated accurate wavefront sensing. I will talk about the details of the concept, microprocessing, and demonstration.

Yohei Kawazura (Tohoku University)
Ion vs electron heating by turbulence in hot accretion flows
Accretion of matter onto a compact object is triggered by turbulence driven by magnetorotational instability. The energy of turbulent fluctuations cascades down to microscopic scales and dissipates as heat. In low-luminosity accretion flows, such as those observed at M87 and Sgr A* --- the primary targets of the Event Horizon Telescope (EHT) ---, the heat is not radiated away. Instead, it accumulates as the thermal and nonthermal energy of particles, making the plasma into a collisionless state, where ions and electrons can exhibit different temperatures. Understanding what dictates the energy partition between ions and electrons is a pivotal question in accretion flow studies as it plays a critical role in the theoretical interpretation of observations made by EHT. In this colloquium, I will demonstrate how we estimated the differential heating of ions versus electrons caused by the cascade of magnetorotational turbulence. In this methodology, we utilize reduced MHD and gyrokinetics, two models that are commonly employed in the studies of magnetic confinement fusion. With the help of these models, we, for the first time, bridged the gap between the injection and dissipation processes of magnetorotational turbulence, whose spatial scales are usually separated by nearly ten orders of magnitude.

Yoshinobu Fudamoto (Waseda University)
Observations of FIR [CII]158um emission lines in the epoch of reionization
Over the past decades, several optical/near-infrared surveys have built large samples of high-redshift galaxies at z > 6. Further, the arrival of the James Webb Space Telescope (JWST) has opened a completely new window to study the very high redshift Universe. While JWST observations provides unprecedented information about star-forming ionized media via its rest-frame optical spectroscopy, however observations of neutral gas, such as in photo-dissociation regions and molecular gas regions, is challenging. To understand conditions and kinematics of neutral gas ISM in star-forming galaxies, deep mm/submm observations are, thus, essential. In this talk, I present FIR emission line observations of star-forming galaxies at z~6 to z~10. These observations allow us to study the neutral gas and dust formation of galaxies at very high-redshift, to which JWST does not have full access. I will further discuss the importance of future mm/submm observations of very high-redshift galaxies using NOEMA and ALMA.

Daisuke Yonetoku (Kanazawa University)
HiZ-GUNDAM 衛星計画：ガンマ線バーストで探る「宇宙の一番星」と「赤ちゃんブラックホール」
宇宙最大の爆発現象であるガンマ線バーストを用いて、遠方宇宙の物理状態を観測や初代星起源の ガンマ線バーストを探査したり、重力波観測と同期した観測でブラックホールの誕生現場を理解する ための人工衛星「HiZ-GUNDAM」計画を推進しています。いずれの観測も、広視野Ｘ線モニターに よるガンマ線バーストの発見と、近赤外線望遠鏡による追観測を同時に実現する必要があり、 分野横断型で推進する人工衛星計画です。数多のガンマ線バーストの中から重要な観測ターゲットを 即座に選出し、大型望遠鏡に観測の機会を提供していきます。
本講演では、この計画の立案に至るまでの経緯を含めて、HiZ-GUNDAM 衛星計画の現状をお伝えします。また、将来の大型望遠鏡での分光追観測に対するご協力のお願いもさせていただこうと思いますので、ご聴講いただければ幸いです。

Ellis Owen (Osaka University)
Exploring the signatures of cosmic ray feedback effects in galaxy ecosystems
Cosmic rays go hand-in-hand with violent and energetic astrophysical conditions. They are an active agent within galactic and circumgalactic ecosystems, where they can deposit energy and momentum, modify the circulation of baryons, and even have the potential to regulate star-formation on local and galactic scales. Their influence in galaxies can be probed using observable signatures across the electromagnetic spectrum, with high energy radiation being particularly important to determine their energy budget, feedback power and hydrodynamic effects. In this talk, I will discuss some of the astrophysical impacts hadronic and leptonic cosmic rays can have in and around galaxies, how their influence can be probed using signatures in X-rays and gamma-rays, and the opportunities soon to open-up that will allow us to map-out the multi-scale effects of cosmic rays in galaxies near and far.