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  第1752回 天文学教室談話会

2023年4月11日(火) 16:15-17:15

Title: X-ray study on the synchrotron emission in Kepler’s SNR
Speaker: Vincenzo SAPIENZA (University of Palermo, staying at Department of Physics, UTokyo)
Language: English
Abstract:
Synchrotron X-ray emission in young supernova remnants (SNRs) is a powerful diagnostic tool to study the population of high energy electrons accelerated at the shock front. We performed a spatially resolved spectral analysis of the young Kepler’s SNR, where we identified two different regimes of particle acceleration. In the north, where the shock interacts with a dense circumstellar medium (CSM), we found a more efficient acceleration than in the south, where the shock velocity is higher and there are no signs of shock interaction with dense CSM. We also studied the temporal evolution of the synchrotron flux, from 2006 to 2014. A number of regions show a steady synchrotron flux and equal cooling and acceleration times. However, we found some regions where we measured a significant decrease in flux from 2006 to 2014. Our results display a coherent picture of the different regimes of electron acceleration observed in Kepler’s SNR.

 

  第1753回 天文学教室談話会

2023年4月18日(火) 16:15-17:15

Title: Atomistic insights on the chemistry of the interstellar medium
Speaker: German Molpeceres (天文学教室)
Language: English
Abstract:
Despite the harsh physical conditions in the dense interstellar medium, there is plenty of evidence that these regions act as the onset of organic chemistry. This chemistry arises from the interplay between gas-phase reactions and reactions on ice-coated dust particles. In most cases, the chemistry that takes place in these regions is unconventional for Earth-based standards and is hardly predictable. Hence, astrophysical models require of abundant input from experiments and theoretical simulations. In this talk, I will review my contributions to the theoretical investigation of gas phase and surface chemistry operating in dense molecular clouds, emphasising the latter. Finally, I will connect my past investigations with our recent efforts to understand the chemistry occurring on ices different from water ice.

 

  第1754回 天文学教室談話会

2023年4月25日(火) 16:15-17:15

Title: Initial mass function and star cluster dynamics: the impact of massive stars
Speaker: Long Wang (Sun Yat-sen University 中山大学、中国)
Link(s): http://orcid.org/0000-0001-8713-0366
Language: English
Abstract:
Massive stars have a significant impact on the dynamical evolution of star clusters. They play a crucial role during star formation, as their radiation can push surrounding gas away and inhibit further star formation. Additionally, strong mass loss from massive stars via strong winds can rapidly reduce the gravitational potential of star clusters and trigger their fast expansion. Once these massive stars evolve into black holes, they continue to drive the expansion of the cluster by forming binary black holes at the center. Recent observations from Gaia have provided abundant kinetic data on stars, which can be used to identify the long stellar streams of star clusters. In this talk, we will explore how these observations can help us to constrain the initial conditions of star clusters, particularly the properties of initial mass functions, by analyzing the dynamical impact of massive stars.

 

  第1755回 天文学教室談話会

2023年5月9日(火) 16:15-17:15

Title: Chemical abundances of red supergiants over a large area of the Galactic disk
Speaker: Daisuke Taniguchi 谷口大輔 (National Astronomical Observatory of Japan 国立天文台)
Link(s): https://d-taniguchi-astro.github.io/homepage/index_en.html
Language: Japanese 日本語
Abstract:
Our Galaxy, also referred to as the Milky Way, is the “nearest” galaxy in the Universe, making it an ideal laboratory for investigating the (chemical) evolution of galaxies. This advantage has enabled us to measure elemental abundances, such as [Fe/H] and [Mg/Fe], of individual stars within several kpc from our Solar system. A natural step forward is to expand observations to more distant objects, such as those located at the tip of the Galactic bar (bar end), at the Galactic center, or even in nearby galaxies. To observe stars located in such far regions (up to ~1 Mpc) with high-resolution spectrographs, it has recently been proposed to use red supergiants (RSGs) as a luminous (>10^4 Lsun) tracer of chemical abundances. In this talk, I will present our recent study to measure chemical abundances of RSGs, utilizing the near-infrared high-resolution, high-throughput spectrograph WINERED (YJ bands; 0.91-1.35 micron). This wavelength range has an advantage of the weakest strengths of molecular lines contaminating to atomic lines. We have developed and tested a method to determine effective temperatures and chemical abundances of RSGs including amongst others Fe, Mg, and Y. Moreover, we have measured the chemical abundances of four RSGs located at around the Galactic bar-end region, and confirmed that their metallicities are lower than expected from the radial metallicity gradient of the Galactic disk.

 

  第1756回 天文学教室談話会

2023年5月16日(火) 16:15-17:15

Title: The molecular composition of shadowed proto-solar disk midplanes beyond the water snowline
Speaker: Shota Notsu 野津翔太 (Department of Astronomy 天文学教室)
Link(s): Notsu et al. 2022, ApJ, 936, 188, Website of Shota Notsu:
Language: English
Abstract:
Planets form and obtain their compositions in protoplanetary disks around young stars. The chemical compositions of gas and solid dust grains in these planet-forming disks will decide planetary compositions. The thermal structure in the protoplanetary disk plays an important role in controlling the disk chemical structure. The disk midplane temperature is potentially affected by the disk substructures such as dust traps/rings, which have been detected with recent ALMA observations. The dust depletion beyond the water snowline will cast a shadow (Ohno & Ueda 2021).
In our study (Notsu et al. 2022, ApJ, 936, 188), we adopted a detailed gas-grain chemical reaction network, and investigated the radial gas and ice abundance distributions of dominant carbon-, oxygen-, and nitrogen-bearing molecules in disks with shadow structures beyond the water snowline around a protosolar-like star. We also investigated the dependance of the disk chemical structures on ionisation rates and initial abundances.
In shadowed disks, the dust grains at r∼3−8 au are predicted to have more than ∼5−10 times amounts of ices of organic molecules such as H2CO, CH3OH, and NH2CHO, saturated hydrocarbon ices (such as CH4 and C2H6), in addition to H2O, CO, CO2, NH3, N2, and HCN ices, compared with those in non-shadowed disks which are composed mostly H2O, CO2, and NH3 ices. In the shadowed regions, we find that hydrogenation (especially of CO ice) is the dominant formation mechanism of complex organic molecules, rather than radical-radical reactions and gas-phase reactions. The gas-phase N/O ratios show much larger spatial variations than the gas-phase C/O ratios, and thus the N/O ratio is predicted to be a useful tracer of the shadowed region. N2H+ line emission is a potential tracer of the shadowed regions beyond the water snowline. We conclude that a shadowed region allows the recondensation of key volatiles onto dust grains, provides a region of chemical enrichment of ices that is much closer to the star than within a non-shadowed disk, and may explain to some degree the trapping of O2 ice in dust grains that formed comet 67P/Churyumov-Gerasimenko. We discuss that in the shadowed disks, Jupiter does not need to have migrated vast distances, and complex organic molecules can be formed in situ rather than being fully inherited from molecular clouds.
In this talk, first I will briefly review recent topics about astrochemistry in star and planet forming regions, and then present our recent modeling studies about molecular composition of shadowed disk midplanes (Notsu et al. 2022). We also shortly discuss future prospects for molecular line observations of protoplanetary disks with much higher spatial resolutions (ALMA and ngVLA).

 

  第1757回 天文学教室談話会

2023年5月23日(火) 16:15-17:15

Title: Star Formation Quenching in Galaxy Clusters and Their Progenitors
Speaker: Makoto Ando 安藤誠 (Department of Astronomy 天文学教室)
Language: Japanese 日本語
Abstract:
It is well known that galaxies that dwell in denser environments are more likely to be quenched (i.e. star formation is stopped), suggesting that surrounding environments influence galaxy evolution, referred to environmental effect. Galaxy clusters, the densest environments in the Universe, are dominated by the quenched population, which is thought to be built up through many environmental effects. Therefore, they are good laboratories to test our understanding of how dense environments quench galaxy star formation. In this talk, I will present our research on galaxy evolution in clusters and their progenitors, referred to protoclusters.
In the first part, I will present the results of a systematic search for protoclusters at z∼2 to reveal when the quenched population emerges in (proto)clusters. We find the excess of the quenched galaxy fraction in protoclusters compared to the general field at z∼2. Still, the excess is insignificant at z>1, suggesting that significant quenching may occur at the later epoch when whole protocluster structures violently collapse to the core regions. In the second part, I will discuss detecting the anisotropic distribution of quenched galaxies in matured clusters up to z∼1. The detected anisotropy infers that some fast quenching mechanisms, like ram-pressure stripping, work anisotropically within clusters.

 

  第1758回 天文学教室談話会

2023年5月30日(火) 16:15-17:15

Title: Chemical Evolution of Complex Organic Molecules in Turbulent Disks
Speaker: 鈴木大輝 Taiki Suzuki (天文学教室)
Language: Japanese 日本語
Abstract:
One hypothesis for the origin of life is that life on Earth originated from organic molecules formed in space and transported to Earth by meteorites and comets. In the past, complex organic molecules (COMs) were detected mainly in high-mass star-forming regions, but advances in observational instruments such as the ALMA telescope have made it possible to detect complex organic molecules in protoplanetary disks.
However, when and how COMs form in disks is not well understood. We have therefore studied the formation of COMs in protoplanetary disks using a chemical reaction network model, in which the time evolution of molecular abundances is numerically solved from experimental and theoretical reaction rate constants and a given physical environment. The disk of a protoplanetary system is turbulent, and dust, which is the site of COMs formation, moves radially and vertically within the disk. The physical environment changes with time as the dust moves, which is a very important factor in the formation of COMs. We first reproduced the turbulent motion of the dust in the disk and obtained the time variation of physical quantities experienced by the dust, such as temperature and UV intensity. Next, we solved theoretically for the chemical evolution along the changes in these physical quantities. By performing such calculations on approximately 1000 dust particles, we investigated the statistical characteristics of COMs formation.
In this seminar, we will discuss the features of chemical evolution suggested by the chemical model of protoplanetary disks, and in particular the differences between O- and N- bearing COMs.

 

  第1759回 天文学教室談話会

2023年6月20日(火) 16:15-17:15

Title: A Keplerian disk with a four-arm spiral birthing an episodically accreting high-mass protostar
Speaker: Ross Burns (理化学研究所 Riken)
Language: English
Link(s): https://sci.nao.ac.jp/main/preparation/20230227
https://www.miz.nao.ac.jp/content/news/topic/20230228-441.html
Abstract:
Multi-epoch VLBI observations of methanol masers were conducted during a rare accretion burst in high-mass protostar G358-MM1. As the ‘heat-wave’ of accretion energy spread outward through the disk it ignited maser emission in concentric rings at ever increasing radii from the protostar, i.e the multi-epoch campaign provided a multi-radius sampling of the disk. By concatenating images from the individual VLBI epochs a discretely sampled map of the maser emission in the high-mass protostellar disk was revealed, with milliarcsecond scale resolution. This ‘heat-wave mapping’ uncovered a four-arm spiral pattern in a disk which exhibits a Keplerian rotation curve. This result represents the first observervationally confirmed association between accretion disk gravitational instability and episodic accretion in the high-mass star formation framework.

 

  第1760回 天文学教室談話会

2023年6月27日(火) 16:15-17:15

Title: Evolution of the Interstellar Medium Explored by X-ray and Radio Observations
Speaker: Kisetsu Tsuge 柘植 紀節 (物理学専攻)
Language: 日本語 Japanese
Link(s): https://researchmap.jp/ktsuge?lang=en
Galactic tidal interactions play a crucial role in star formation and galaxy evolution. Such interactions and mergers drive starburst, releasing significant energy and heavy elements into interstellar space. However, most interacting galaxies are distant, and it isn’t easy to resolve individual clouds and investigate their physical properties in detail. Thus, it is a long-standing question of what physical conditions of the Interstellar Medium (ISM) is required for the starburst. We studied nearby interacting galaxies with a high spatial resolution of the radio interferometers (e.g., ASKAP, ALMA) and compared the observations with numerical simulations. We identified active star-forming regions triggered by the collision of HI/CO clouds. We found a positive correlation between the pressure of the compressed gas generated by collisions and the total stellar mass of the superstar clusters (SSCs), suggesting that the pressure may be a key parameter in the SSC formations. To establish a detailed picture of the star formation and ISM evolution, we extend the studies by multi-wavelength observations from radio to X-ray. Especially the study is being conducted with the latest X-ray data obtained by eROSITA. We proposed a 3D collision geometry and a scenario in which the diffuse X-ray emissions have been compressed and heated by galactic-scale gas collisions. In this talk, I will also mention the prospects of upcoming missions such as XRISM and next-generation radio observatories like SKA and ngVLA.

 

  第1761回 天文学教室談話会

2023年7月4日(火) 16:15-17:15

Title: First M87 imaging results from the Global Millimeter VLBI Array connected to ALMA and GLT
Speaker: Kazuhiro Hada 秦 和弘（NAOJ 国立天文台・水沢VLBI観測所）
Language: English
Link(s): https://researchmap.jp/ktsuge?lang=en
The nearby radio galaxy M87 offers a privileged opportunity to study the physics of supermassive black holes and powerful relativistic jets. The Event Horizon Telescope (a global VLBI network operated at 1.3mm) detected a gravitationally-lensed ring-like structure at the core of the galaxy, providing compelling evidence for the presence of a supermassive black hole. Due to the limited image dynamic range, however, the EHT images were still inadequate to detect the emission beyond the black hole shadow, preventing us from linking the central black hole to the surrounding structure such as the accretion flow and jet base. Here we present the first imaging results of M87 at 3.5mm obtained with the Global Millimeter VLBI Array (GMVA) connected to the ALMA and GLT. Thanks to the higher imaging dynamic range and sensitivity, the image reveals a ring-like structure apparently connected to the large-scale jet. The ring-like structure observed at 3.5mm is 50% larger in diameter than that at 1.3mm in a higher optical depth regime, suggesting that the 3.5mm ring emission is dominated by the hot accretion flow surrounding the photon ring. The new image also reveals various new features in the extended jet. I will overview the results of this study as well as future prospects.

 

  第1762回 天文学教室談話会

2023年7月11日(火) 16:15-17:15

Title: Developing a stellar evolution code HOSHI and application to the magnetic stellar evolution　恒星進化コードHOSHIの開発と磁気回転星への応用
Speaker: Koh Takahashi 高橋 亘（NAOJ 国立天文台）
Language: English
Stars are fundamental objects in astronomy, and their properties are often used in all types of research. Therefore, the development of stellar evolution codes that enables us to interpret observational data is indispensable and important. In this talk, I will introduce the HOngo Stellar Hydrodynamics Investigator (HOSHI), the modern stellar evolution code that I have been developing since I was a student, its features, and the studies I have conducted using this code. On the numerical side, I will present features incorporated in recent updates, such as a method for dynamically treating spacial resolution by solving a set of differential equations, a method for actively selecting equations to be handled, and a method for seamless handling of hydrostatic/hydrodynamic equations. I will also present our work on the magnetic stellar evolution, in particular our attempts to infer the structure of the internal large-scale magnetic field from the modulation of the surface rotation period.
恒星は天文学における基礎的な天体であり、どんな研究を行う際にも恒星の性質を利用することは多いだろう。そのため恒星の性質を理論的に予言する恒星進化コードの開発は、現代天文学にとって基礎的であり重要だ。この発表では、発表者が学生の頃から開発をつづけてきた恒星進化コード HOngo Steller Hydrodynamics Investigator (HOSHI) について、その特徴と、それを用いた研究を紹介する。数値的な側面としては最近のアップデートで実現した ・座標間隔を微分方程式によって決める（解く）仕組み ・解きたい微分方程式を切り替える仕組み ・静水圧解?流体解をシームレスに扱う仕組み などを紹介し、また実際の天体物理への応用については恒星の内部磁場構造を推定する方法について特に詳しく議論する。

 

  第1763回 天文学教室談話会

2023年7月18日(火) 16:15-17:15

Title: Revealing the cosmic ray activities at the site of the TeV-discovered SNR HESS J1534-571
Speaker: Dang-thanh-nhan Nguyen（University of Tuebingen ドイツ・Tuebingen大学）
Language: English
Supernova remnants (SNRs) are responsible for the production and acceleration of the Galactic cosmic rays. While tracing the high energy cosmic rays directly from the source is challenging owing to the potential interactions between these particles and the surrounding medium, it is feasible to track down the radiation produced by the above processes with TeV telescopes. At the same time, X-ray astronomy opens the window to study the highly energetic electrons produced at the SNR via synchrotron (non-thermal emission) as well as the shock-heated plasma component (thermal emission).
We report the results obtained from XMM-Newton observations of the TeV-detected SNR HESS J1534-571. In this work, we use XMM-Newton observational data to search for X-ray emissions from the source. A broadband spectral analysis, using data ranging from radio to TeV domain is employed to investigate cosmic ray production and acceleration scenarios. No signatures of X-ray synchrotron emission were detected from the SNR and an upper limit for the flux is derived in the energy band 2-10 keV, which places constraints on the relativistic leptonic particle content in the SNR. Furthermore, evidence of a line-like emission is detected at 6.4 keV from localized regions. The result is consistent with earlier results obtained with Suzaku from other regions of the object (Saji et al. 2018). The emission can be interpreted as the result of interactions between lower energy (at MeV) cosmic ray protons with high gas density regions in and around HESS J1534-571, and thus potentially be associated with particles accelerated in the SNR.

 

  第1764回 天文学教室談話会

2023年7月25日(火) 16:15-17:15

Title: The formation epoch of the Galactic bar and JASMINE
Speaker: Daisuke Kawata 河田 大介（英・University College London）
Language: English
Using the numerical simulation results, we first discuss how the age distribution of the Milky Way’s Nuclear Stellar Disk (NSD, Baba & Kawata 2020) and the boxy/peanut/X(BPX)-shaped bulge (Baba et al. 2021) can tell us the formation epoch of the bar structure of the Milky Way. We will then show the age distribution of the NSD, obtained from the Miras variables (Sanders et al. 2022) and the age distribution of the bulge and disk stars from APOGEE DR17 stars inferred with our Bayesian neural network, BINGO (Bayesian INfererence for Galactic archaeOlogy, Ciuca et al. 2023). We find that these observations indicate the early formation of the Galactic bar. The age distribution of the observed disk stars resembles those of the Auriga cosmological simulation of a Milky Way-like galaxy, where the Galactic bar formation is induced by an ancient gas-rich merger. This merger is similar to the so-called Gaia-Sausage-Enceladus (GSE) merger, which is revealed by Gaia and is believed to have happened in the Milky Way at an early epoch. Hence, we argue that the formation of the Galactic bar could be caused by the GSE merger, which is also considered to have induced the thick disk formation (Grand et al. 2020). We will also briefly introduce Japan Astrometry Satellite Mission for INfrared Exploration (JASMINE, planned launch in 2028), which will reveal the Milky Way’s central core structure and its formation history with Gaia-level (~25 uas) astrometry in the NIR Hw-band, (1.0-1.6 um), Galactic centre archaeology survey, and to discover Earth-like habitable exoplanets from the NIR time-series photometry of M-dwarf transits. We will also briefly introduce the future space missions in Europe, the asteroseismology mission, HAYDN, and the all-sky NIR astrometry mission, GaiaNIR, in which I am heavily involved.

 

  第1765回 天文学教室談話会

2023年8月1日(火) 16:15-17:15

Title: New Surprises in Extragalactic Astronomy from JWST, a Personal Account
Speaker: Matt Malkan（米UCLA）
Language: English
JWST’s unmatched sensitivity across the full range of infrared wavelengths, with unprecedented spatial resolution, is revolutionizing every area of astronomy in its first year – as we expected. I will present a personally biased list of several JWST surprises in extragalactic astronomy that most of us did NOT predict. I will start with unexpected results on galaxies JWST is finding at very high redshifts, including how and where they rapidly formed stars, metals and dust, and re-ionized the Universe. Then I will discuss JWST observations of nearby active galactic nuclei. We may have missed a lot of them previously due to their amazingly heavy dust obscuration. Close-up views are suggesting that the high-speed gas outflows they launch may not always be simple two-sided jets. And some of our ideas about how their outflows might have shut down star formation – known as “AGN feedback” – may also need revision. In fact, we may have to re-think some of our ideas about what a “galaxy” and an “AGN” actually are. While trying to answer these questions, we should look at the incredible new data with open minds about what we thought we knew previously.

 

  第1766回 天文学教室談話会

2023年9月29日(火) 14:00-15:00

Title: Exploring the signatures of cosmic ray feedback effects in galaxy ecosystems
Speaker: Ellis Owen (Osaka University)
Language: English
Link(s): https://www.ellisowen.org

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.

 

  第1767回 天文学教室談話会

2023年10月3日(火) 16:00-17:00

Title: Protostars and protoplanetary disks with JWST: first results from the JOYS and MINDS programs
Speaker: Ewine van Dishoeck (Leiden Observatory, the Netherlands)
Language: English
This talk will present first results of the MIRI-MRS spectra of a number of protostars, T Tauri and brown dwarf disks resulting from the JOYS and MINDS GTO programs as well as a number of open time programs. Rich ice spectra are seen in dark clouds and the cold outer envelopes of protostars hinting at the presence of complex molecules in ices formed very early in the evolution. In young and mature disks, many lines from gaseous CO, H2O, CO2, C2H2 and HCN are found, but also surprising new molecules are detected. Together they point to a rich chemistry that is linked to the physical structure of the inner regions of these disks and thereby provides a unique diagnostic of it.
Background information: the 55 hr JWST Observations of Young protoStars (JOYS) GTO program (PI: van Dishoeck) uses MIRI to investigate the physical and chemical properties of two dozen protostars and their immediate environment. JOYS studies the physical characteristics of embedded disks, accretion signatures onto the protostars, feedback from the young protostars on their environment through primordial jets and outflows, as well as the chemical gas and ice constituents of the protostellar envelopes to feed the disks.
The goal of the 120 hr MIRI Mid INfrared Disk Survey (MINDS) GTO program (Th. Henning, I. Kamp co-PIs) is to use JWST to (1) investigate the chemical inventory of the terrestrial planet forming zone, (2) to follow the gas evolution into the disk dispersal stage, and to (3) study the structure of protoplanetary and debris disks in the thermal mid-IR. The program builds a bridge between the chemical inventory of planet-forming disks and the properties of exoplanets. In total, about 50 targets (Herbig Ae stars, T Tauri stars, brown dwarfs and young debris disks) are being observed.

 

  第1768回 天文学教室談話会

2023年10月17日(火) 16:15-17:15

Title: CGM observations in emission
Speaker: Haruka Kusakabe 日下部晴香 (NAOJ/JSPS特別研究員)
Language: English
The evolution of galaxies is directly linked to the gas reservoirs surrounding them, so-called, “the circum-galactic medium (CGM)”. Gas and metals are exchanged via inflows and outflows through the CGM, which is an interface between the interstellar medium (i.e., galaxy) and intergalactic medium (the rest of the Universe). The CGM has been studied with absorption lines imprinted in spectra of bright background quasars (tomography), but this method is limited to line of sight and cannot provide the 2D spatial distribution of the CGM. Recent sensitive, wide-FoV integral-field spectrographs (such as VLT/MUSE and Keck/KCWI) make it possible to individually detect the CGM in emission, which allows us to map the gas and metals around host galaxies. In this talk, I will review the recent progress in observations and understanding of hydrogen gas and metals around z>∼2 galaxies, which are detected as extended Lyα emission (Lyα haloes) and metal-line haloes. I will also connect the CGM observations in emission and absorption and introduce comparisons with simulations.

 

  第1769回 天文学教室談話会

2023年11月7日(火) 16:15-17:15

Title: N-body galaxies as theoretical counterparts of the Milky Way
Speaker: 朝野哲郎 (天文学教室)
Language: 日本語
The astrometry data from ESA’s Gaia mission have revealed a detailed phase-space distribution of the disc stars in the Milky Way (MW). This distribution provides insights into the dynamical structure of the MW both in the present and the past. However, extracting such information directly from observational data alone is challenging, because Gaia’s observation is biased in the solar neighbourhood. To interpret the observation data, we need to compare them with theoretical models, and N-body simulations stand out as one of the most suitable modelling tools. We have performed high-resolution N-body simulations using the GPU-based tree code Bonsai to construct our MW model. It consists of a classical bulge, a stellar disc and a dark matter halo, represented by 5 billion equal-mass particles.
First, I will demonstrate that the bar and spiral arms form characteristic phase space structures based on the detailed analysis of an isolated MW model (Fujii et al. 2019). I will discuss what we can infer about the dynamical properties of the Galactic bar and spiral arms by comparing the phase space stellar distribution in the Gaia data and that in the model. I will then present a simulation of a MW-like galaxy externally perturbed by a Sagittarius dwarf-like galaxy. The close encounter of the dwarf excites the vertical oscillation of the disc, and a snapshot of the subsequentent phase mixing is observed as a phase spiral (phase snail). Simultaneously, the perturbation tidally induces spiral arms, making the phase mixing process complex.

 

  第1770回 天文学教室談話会

2023年11月14日(火) 16:15-17:15

Title: Disk driven galaxy transformation at z∼4: insights from spatially resolved 4D (?) ALMA data
Speaker: 津久井崇史 Takafumi Tsukui (オーストラリア国立大 ANU)
Language: English 英語
Recent observations from ALMA and JWST have revealed numerous disk galaxies with redshifts from z=4 to z=6. Archaeological studies of the Milky Way suggest its disk may have formed as early as z~4. ALMA observations provide detailed kinematics and energetics of the interstellar medium, allowing us to directly probe galaxy formation in the gas-rich early universe. We present new findings from high-resolution ALMA data on BRI 1335-0417 at z~4.4, the brightest unlensed galaxy in the submillimeter. Our results include the identification of a bulge-disk structure with bar spirals, evidence of seismic ripples on the disk that show similarities to those seen in Milky Way simulations, and a star formation law after correcting for a central AGN based on spatially resolved dust modelling and point source decomposition. Such detailed investigation at this epoch has only become possible by the spatially resolved data. By studying the brightest galaxies, we can overcome the interferometer’s inherent poor sensitivity, providing a unique benchmark for our understanding along with numerical simulations.
*Atacama Large Millimeter/submillimeter Array
*James Webb Space Telescope
*Active Galactic nucleus

 

Links:
-Web page: https://takafumi291.github.io/
-The latest paper the talk will partly be based on: https://arxiv.org/abs/2308.14798

 

  第1771回 天文学教室談話会

2023年11月17日(金) 17:00-18:00

Title: A double degenerate merger and its remnant
Speaker: 樫山和己 Kazumi Kashiyama (東北大学 Tohoku University)
Language: English 英語
A recently discovered peculiar white dwarf, WD J005311 is likely a remnant of a binary white dwarf merger occurred ~ 840 yr ago, accompanying a weak thermonuclear supernova reported in historical records, e.g., the Azuma Kagami. In my talk, I will describe the observed properties of WD J005311 and the theoretical models used to explain them, and explore the past and future of WD J005311, considering their significance in the context of multi-messenger time-domain astronomy.

 

  第1772回 天文学教室談話会

2023年11月28日(火) 15:00-16:00

Title: Jets, shocks and cosmic rays: computer simulations and laboratory plasmas experiments
Speaker: Andrea Ciardi (Sorbonne University and Paris Observatory)
Language: English
Plasmas generated by high power lasers offer the opportunity to study flows and matter under conditions relevant to many astrophysical environments. Recent experimental advances are now allowing to couple laser produced plasmas with externally imposed Mega-Gauss-level magnetic fields. In the first part of the talk I will present an introductory-level overview of recent and ongoing research efforts to study astrophysically relevant shocks, magnetised flows, and energetic particles in the laboratory. In the second part, I will present our recent work on the collimation of young stellar jets by a large scale magnetic field and on the cosmic rays non-resonant streaming instability.

 

  第1773回 天文学教室談話会

2023年12月5日(火) 16:15-17:15

Title: Band profile alignment of diffuse interstellar bands
Speaker: Alexander Ebenbichler (University of Innsbruck)
Language: English
Context. There have been many attempts to identify families of diffuse interstellar bands (DIBs) with perfectly correlating band strengths. However, although major efforts have been made to classify broadly based DIB families and important insights have been gained so far, no family has been identified with sufficient accuracy or statistical significance to prove that a series of selected DIBs originates from the same carrier. This can be attributed in part to the exclusive use of equivalent widths to establish DIB families.
Aims. In a change of strategy we search for DIBs highly correlated in both band strength and profile shape. This approach increases the chance that correlating DIBs are members of one family, and originate from the same carrier molecule. Subsequently, we search for correlations between DIB profile families and atomic interstellar lines, with the goal to further constrain possible DIB carriers.
Methods. We adapt the well known method of time-series alignment to perform spectral alignment, that is, DIB alignment. In a second step, we analyse the alignment results using clustering analysis. This method requires a statistically significant data set of DIB sight lines. The ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES) data are perfectly suited for this application.
Results. We report eight DIB families with correlating strengths and profiles, as well as four previously unreported DIBs in the visual range using DIB alignment. All profile family members show Pearson correlation coefficients in band strength higher than 0.9.

 

  第1774回 天文学教室談話会

2023年12月12日(火) 16:15-17:15

Title: Galactic Archaeology with Cosmological Simulations
Speaker: Yutaka Hirai 平居悠 (University of Notre Dame/Tohoku University)
Language: English
Galactic archaeology strives to elucidate galaxy formation by examining the chemo-dynamical properties exhibited by stars. Thanks to Gaia, several substructures related to accreted satellite galaxies have been identified in the Milky Way. Follow-up spectroscopic observations found that these substructures would have different chemical abundances from stars formed in situ. However, we still do not know how the assembly history of the Milky Way and its satellites affect the chemical abundances of stars. Appropriate comparison with high-resolution cosmological zoom-in simulations and large samples of spectroscopic observations of metal-poor stars are demanded to detail the Milky Way’s formation in the earliest era. In this talk, I will show the recent progress in understanding the Milky Way’s halo and satellites based on our cosmological zoom-in simulations. I will show that over 90% of very metal-poor stars enhanced in r-process elements come from accreted satellites. I will then discuss the relationship between star formation histories and chemical abundances of the Milky Way’s satellites. We find a group of stars formed in bursts of star formation separated by ~100 Myr. From the mock observations using our simulation, we expect these groups of stars can be observed in upcoming Subaru Prime Focus Spectrograph observations.

 

  第1775回 天文学教室談話会

2023年12月14日(木) 17:00-18:00

Title: Deciphering the nature of dark matter from stellar systems in the Galaxy
Speaker: Masashi Chiba 千葉柾司 (Tohoku University 東北大)
Language: English
The nature of dark matter is still enigmatic. CDM models have been a theoretical paradigm for many years, because the models are successful for reproducing the observed large-scale structure of the Universe, say at scales larger than ~ 1 Mpc. However, there are various unsolved issues at smaller scales, i.e., scales of galaxies and their satellites, such as the missing satellites problem, core/cusp problem, and more, so that several alternative dark matter models are also proposed. Here, I’d like to present our works to get insight into these small-scale issues: the search for new ultra-faint dwarf satellites in the Galaxy, the constraint on the dark matter profile in Galactic dwarf spheroids, and more. Future prospects for settling these small-scale issues are also presented.

 

  第1776回 天文学教室談話会

2023年12月19日(火) 16:15-17:15

Title: AI時代の理系英語コミュニケーション: 論文執筆、履歴書からレフェリー交渉まで
Speaker: Masa Sakano 坂野正明 (Wise Babel Ltd ワイズバベル)
Language: 日本語 Japanese
ちゃんとした英語文章を書くとき、自分の英語が本当に意図通り伝わるのか、不安感がなかなか拭えない経験がありませんか。母国語でさえ書き言葉は難しいところ、英語ならばなおさらなのはもっともです。その時、明日からでも実行できる最大の対策は、理路整然と書くことにつきます。それは論文であれあるいは交渉事の通信であれ共通の大原則です。
近年はChatGPTなどAIベースの優れたツールも登場してきました。しかし、元々筋が間違っていたりあるいは入り組んでしまっている文章を正確で読みやすいものにしてもらうことは期待できません。実はAI時代だからこそ、今まで以上に書き手の資質が問われるとも言えます。むしろ、AI生成の十把一絡げの文章があふれかねない近未来こそ、書き手として何か光るものを出したいものです。
本講演では、理路整然とした筋の立て方と、それを英文として確実に伝達する作文方法について、本質的でかつ具体的、実戦的なアドバイスをまとめます。近年の補助ツールの発展の功罪にも触れます。過去データ機械学習をベースとするAIに特有の欠点であるコンプライアンス的問題についても言及し、グローバル時代を生きる人々のコミュニケーションに一つの指針を提供します。英国在住20年超、論文他の英文校閲を主業とし、日本語を母語とする理系研究者や学生の英文に見られる典型的傾向を熟知する講演者によるセミナーです。

 

  第1777回 天文学教室談話会

2023年12月21日(木) 10:30-11:30

Title: Quasar modeling and parameter extraction from time variable light curves
Speaker: Henry Best (City University of New York and American Museum of Natural History)
Language: English
Quasars are supermassive black holes at the center of galaxies that are actively accreting material and radiating across the electromagnetic spectrum. They are observable at very high redshifts (z_s < 7.0) and are interesting both as cosmologic tools and as feedback mechanisms for their host. They emit intrinsic, correlated signals across many wavelengths, which allows us to place constraints on size scales through reverberation mapping. Strongly lensed quasars are also interesting as they experience extrinsic correlated signals from microlensing. I will give a brief introduction to these variability models and describe how they have led to the measurements of these black hole masses. I will then briefly introduce my code which self-consistently treats these variability models and discuss some nuances that arise. Finally, I will touch upon how our group used machine learning to infer simulation parameters from a set of correlated light curves.

 

  第1778回 天文学教室談話会

2024年1月16日(火) 16:15-17:15

Title: The role of hub-filament systems in the formation of star-clusters with low- to high-mass stars
Speaker: Doris Arzoumanian (国立天文台)
Language: English
Observations reveal the organization of the interstellar medium into filament networks. In molecular clouds, the densest filaments are identified as the precise birthplaces of solar-mass stars, while high-mass stars form in the hubs where the filaments merge. To understand the formation process of star-clusters, it is thus essential to describe the formation and evolution of filaments and hubs and their fragmentation into pre-stellar cores, the progenitor of stars. I will present theoretical and observational studies indicating the formation of filamentary molecular clouds at the edge of expanding bubbles. In particular, I will show observations suggesting multiple compressions responsible in the formation and evolution of the filament networks. I will discuss the role of filament coalescence and hub-filament systems in the formation process of stars from low to high masses. I will also present our new study proposing the formation of the Sun along a dense molecular filament. We show that the filament may play an important role in shielding the young solar system from a nearby supernova explosion while intercepting the required amount of supernova ejecta to explain the observations of primitive material found in meteorites.

 

  第1779回 天文学教室談話会

2024年1月30日(火) 14:00-15:00

Title: Substellar mass function of young open clusters in low-metallicity environments with JWST
Speaker: 安井千香子 Chikako Yasui (国立天文台)
Language: 日本語 Japanese
Stars are fundamental components of galaxies, and because the stellar mass determines the subsequent evolution of a star, the initial mass function (IMF), i.e., the initial distribution of masses for a population of stars, is one of the fundamental parameters that determines the physical and chemical evolution of a stellar system. The IMF was first introduced by Salpeter (1955), and detailed derivations have been carried out for various regions, including the field, young clusters and associations, and old globular clusters. The results indicate that the vast majority of stellar systems originated from a universal IMF. However, because there are many different physical and chemical environments in the universe, whether the IMF is universal or whether it is sensitive to environmental conditions remains a crucial question.
The outer regions of spiral galaxies, such as our Galaxy, are known to be generally in low-metallicity environments (∼-1 dex), roughly comparable to values in the primordial environments of the universe (z∼2). Nonetheless, the relatively small distance of the outer Galaxy makes it the only environment suitable for spatially resolved observations of individual stars, as is the case in the solar neighborhood. We have been conducting near-infrared imaging with the Subaru Telescope of ∼10 young star-forming clusters in the outer Galaxy down to ∼0.1-M_⊙ stars, showing that the initial mass function (IMF) in such environments does not differ significantly from typical IMFs seen in the solar neighborhood in terms of the high-mass slope.
We performed high-sensitive JWST NIRCam/MIRI imaging observations of two star-forming clusters in Digel Cloud 2, a star-forming region in the outermost Galaxy (R_G > 15 kpc), and succeeded a mass detection limit of ∼0.03 M_⊙. This is the first detection of brown dwarfs in star-formation clusters under such an environment. The underlying IMFs are derived by model fitting of the obtained F200W-band luminosity function, resulting in IMF peak masss log M_C/M_⊙ ∼ -1.2 for both clusters. Although the uncertainties are rather large (~0.5), the results suggest that the M_C is somewhat lower than previously derived values in the solar neighborhood (log M_C/M_⊙ ∼ −0.5 ± 0.5). The possible environmental implications of these results will be discussed in this talk.
参考:https://subarutelescope.org/en/results/2023/07/05/3285.html

 

  第1780回 天文学教室談話会

2024年3月26日(火) 15:00-16:00

Title: HST-dark galaxies: Unveiling the Nature of Infrared Bright, Optically Dark Galaxies with Early JWST Imaging and Spectroscopy
Speaker: Laia Barrufet (University of Edinburgh)
Language: English
Over the past decade, low-resolution infrared observations have revealed a population of bright IR galaxies undetected with HST, termed “HST-dark galaxies. Nevertheless, their photometric redshifts and physical properties were uncertain due to the limited photometry even including ALMA data. This can result in an incomplete galaxy census at earlier times, due to UV-faint galaxy populations such as quiescent or dust-obscured sources. The key question is: how common are such sources at z>3—4 and what is their contribution to the cosmic star formation rate density? With JWST, we can study for the first time the physical properties of this enigmatic population. In this talk, I will summarise the progress in our understanding of HST-dark galaxies facilitated by JWST data, employing both photometry and spectroscopy.
In the first part of the talk, I will expose the progress made with HST-dark galaxies thanks to JWST/NIRcam photometry. I will focus on the results of 30 HST-dark sources selected based on their red colours across 1.6 to 4.4 microns with NIRCam imaging from the Early Release Science Program CEERS. These galaxies are generally highly dust-attenuated, massive, star-forming sources at z∼2—8. Our analysis underscores the unique capability of JWST in uncovering this previously elusive galaxy population and in conducting a more complete census of galaxies at z>3 based on rest-frame optical imaging. Interestingly, HST-dark galaxies present a considerable contribution to the obscured star formation rate density at z∼7, already in the Epoch of Reionization.
In the second part of the talk, I will present the initial NIRSpec spectra of HST-dark galaxies from my GO Cycle-1 program ‘Quiescent or dusty? Unveiling the nature of extremely red galaxies at z>3’. I will showcase spectra for highly attenuated dusty galaxies and quiescent galaxies and their physical characteristics. Despite the prevalence of dust in most galaxies, they exhibit distinct spectral features. Furthermore, HST-dark galaxies harbour hidden gems: quiescent galaxies at redshifts > 3, for which I will present spectra and demonstrate NIRSpec’s capacity to analyze these sources.
Finally, I will explore potential avenues for future research involving HST-dark galaxies, leveraging the synergy between JWST and ALMA.