Title: Molecular spectroscopy in local and high-redshift galaxies: What do “dense gas tracers” mean?
Speaker: 西村優里（天文学教室）／Yuri Nishimura (Department of Astronomy, The University of Tokyo)
Thanks to the technical advancement of (sub)millimeter observing facilities, a great number of molecular lines are now routinely observed with high sensitivity and high angular resolution. These molecular lines and line ratios are useful tools for studying physical, kinematic, and chemical properties of extragalactic systems. To fully exploit line diagnostics, it is important to relate different size scale observations: detailed understanding of nearby systems by spatially-resolved observations is essential to better interpretation of more distant objects which cannot be observed at the same physical resolution.
In this talk, I will present (1) 10 pc-scale multiline mapping toward a Galactic star-forming region W3(OH); (2) spatially- and spectrally-resolved HCN and HCO+ observations of local (ultra)luminous infrared galaxies from the CONquest sample (for more detail on CONquest, see Falstad+2021); and (3) ALMA Band 3 (rest-frame ~350 GHz) line survey toward the Cloverleaf, a gravitationally-lensed quasar at z=2.56. I will first discuss which molecular emission arises from which part of the molecular cloud, and then focus on how galactic-scale dynamics, such as outflows, can alter line ratios.
Title: Evaluation of magnitudes of the Starlink satellites by simultaneous multicolor observations
Speaker: 堀内貴史（天文センター）／ Takashi Horiuchi（Institute of Astronomy, The University of Tokyo)
The U.S. company, SpaceX plans to launch 42,000 Starlink satellites by the mid-2020s. However, these satellites orbit at relatively low altitudes (e.g. 550 km), and there are concerns that the light pollution from sunlight reflection likely affects observations. In January 2020, SpaceX launched a test satellite, Darksat with a black coating on its surface to reduce the reflection flux. In order to verify the effect of its black coating, we observed Darksat and unpainted Starlink satellites by simultaneous multicolor observations (g’: green, Rc: red, and Ic:near-infrared) with the 105 cm Murikabushi telescope/MITSuME. While the magnitude of Darksat is about 7, being difficult to see with the naked eyes, this brightness is sufficient to affect any observations. In June 2020, SpaceX launched Visorsat, which is a satellite with a sun visor to reduce the reflected sunlight. At the present stage, there is no sufficient verification on whether the sun visor is effective at various wavelengths. In this study, we are therefore conducting the simultaneous multicolor observations of Visorsat and the ordinary Starlink satellites through the OISTER campaign. The preliminary observations of Visorsat with the Murikabushi telescope have already shown that the apparent magnitude of Visorsat is not fainter than that of Darksat, ranging from 6.5 mag (g’ band) to 5.7 mag (Ic band). Since most of the satellites to be launched by SpaceX in the future will be visor satellites, it is very important to investigate the effects of the Starlink satellites as soon as possible to reduce the light pollution in astronomical observations.
Title: Cosmic-ray Acceleration & Escape from Intermediate-Aged Supernova Remnant Kes 79
Speaker: Paul K. H. Yeung（Department of Physics, The University of Tokyo)
Hadronic γ-ray sources of supernova remnant — molecular cloud (SNR–MC) interactions can serve as stopwatches for the escape of cosmic rays (CRs) from SNRs, which gradually develops from highest-energy particles to lowest-energy particles with time. Recent multiwavelength studies confirmed the interaction of the intermediate-aged SNR Kes 79 with molecular clouds. In this work, we analyze the 11.5 yr Fermi-LAT data to investigate the γ-ray feature in/around the Kes 79 region. With ≥5 GeV data, we detect two extended sources: Src-N (the brighter one; radius ≈0.31°) concentrated at the north of the SNR while enclosing a powerful pulsar — PSR J1853+0056, and Src-S (radius ≈0.58°) concentrated at the south of the SNR. Their spectra have distinct peak energies (≈1.0 GeV for Src-N and ≲0.5 GeV for Src-S), suggesting different origins for them. In our hadronic model that includes the leaked cosmic rays (CRs) from the shock-cloud collision, even with extreme values of parameters, SNR Kes 79 can by no means provide enough CRs reaching clouds at Src-N to explain the
local GeV spectrum. We propose that the Src-N emission could be predominantly reproduced by a putative pulsar wind nebula powered by PSR J1853+0056. On the other hand, our same hadronic model can reproduce a majority of the GeV emission at Src-S with typical values of parameters. Also, SNR Kes 79’s relatively softer γ-ray spectrum peaked at a relatively lower energy is in agreement with its intermediate age, based on the modelled evolution of cosmic-ray escape from an SNR.
Title: Environment and AGN activity of distant quiescent galaxies revealed by multi-wavelength surveys in the COSMOS field
Speaker: 伊藤 慧 (天文学専攻) /Kei Ito（Department of Astronomy, The University of Tokyo)
Recent multi-band observations have found that galaxies with suppressed star formation activity exist even in the high redshift universe. The state-of-art spectrograph has now confirmed them up to z~4. On the other hand, it is not well understood why they get quenched at such a high redshift, even though the cold streams are expected to supply gas. One of the preferable quenching mechanisms is the feedback from active galactic nuclei (AGNs). In addition, it has been challenging to see how quenching is connected to their living environment inside the large-scale structure of the distant universe. The cosmic evolution survey (COSMOS) is one of the preferable datasets to reveal these points since it has both a large survey area and deep multi-band photometry. In this talk, I will introduce our recent two papers on quiescent galaxies at high redshift. The first explores the spatial distribution differences among massive quiescent galaxies, mass star-forming galaxies, and Lyα emitters at 2<z<4.5. The second conducts a stacking analysis of X-ray and radio images for massive quiescent galaxies up to z~5 to explore the contribution of AGNs to quenching.
Title: NASA Astrophysics: From the past to the future
Speaker: Dr. Paul Hertz (NASA Astrophysics Division Director)
Venue: 東京大学本郷キャンパス 理学部化学本館５階講堂
Dr. Paul Hertz, the director of the Astrophysics Division in the Science Mission Directorate at NASA is visiting Japan. A special seminar by Dr. Paul Hertz is scheduled on this Thursday 7 July from 3:00pm at Hongo campus of University of Tokyo. We are now just in front of the entrance of the new world explored by the James Web Space Telescope, whose first light image is going to be public on 12 July. Dr. Paul Hertz gives us a talk on NASA’s astronomy and astrophysics researches including the future perspective of science missions. We would welcome participation of students and young scientists in wide research areas including not just astronomy, physics, earth and planetary science but also other science fields
Title: Imaging of the Supermassive Black Hole in Our Galaxy, Sgr A* with the Event Horizon Telescope
Speaker: 小藤由太郎 (天文学専攻) /Yutaro Kofuji（Department of Astronomy, The University of Tokyo)
The first event horizon scale image of the supermassive black hole in our Galaxy, Sgr A* was captured by the Event Horizon Telescope(EHT) Collaboration. EHT is the Very Long Baseline Interferometry (VLBI) that links radio dishes around the world to create an Earth-sized telescope virtually. High-resolution observations of EHT enable us to see the vicinity of the black hole. We analyzed the reconstructed images and concluded that it is highly likely that Sgr A* has ~50 uas ring structure and this is consistent with the shadow of the Kerr black hole which weighs ~4 million solar masses. In this seminar, I would like to summarize the major results, then explain the details of our imaging process and briefly introduce the theoretical interpretations of our images.
Title: Resolving Polar Dust in AGN with JWST: Going Beyond the PSF
Speaker: Mason Leist (University of Texas San Antonio)
The launch of the James Webb Space Telescope (JWST) promises to revolutionize infrared astronomy and our understanding of inflows and outflows in active galactic nuclei (AGN). David Rosario (Newcastle University, U.K.) has been awarded JWST science time to explore and characterize diffuse polar dust emission found in AGN using JWST Mid-infrared Instrument (MIRI) imaging. The Galactic Activity, Torus and Outflow Survey (GATOS2) collaboration will have access to these data, of which I am a member. Thanks to JWST’s exquisite low surface brightness sensitivity in the mid-infrared (MIR; 5-25 μm) observations of the diffuse polar dust emission found in AGN will be enabled unprecedented sensitivity. Relying on JWST’s stable PSF, we plan to use deconvolution to establish the structure of this diffuse emission below the resolution of the telescope. We will explore five different deconvolution techniques and select the best method based on comparisons of the flux conservation, FWHM, and Strehl ratios of the deconvolved images with an input model. To explore each technique, we have used the MIRI simulation software (MIRISim) to simulate JWST’s complex PSF convolved with a toy model of an AGN consisting of a resolved bicone and an unresolved AGN point source. Here I discuss the preliminary results on our assessment of the optimum deconvolution strategy.
Title: High-mass star formation in the early universe and their analogous observational signatures in the present-day universe
Speaker: 細川隆史（京都大学）/ Takashi Hosokawa (Kyoto University)
Title: Lifecycle of molecular clouds driven by HI gas accumulation
Speaker: 小林将人（国立天文台）/ Masato Kobayashi (NAOJ)
Molecular clouds host star formation so that they are the key structure to determine the galactic star formation. Recent ALMA observations reveal the statistics of molecular cloud populations in nearby galaxies (e.g., molecular cloud mass functions), as well as their inner structures on sub-pc scales. The connection between such large-scale statistics and inner structures is still poorly understood.
To investigate the formation and evolution of molecular clouds, we calculate a semi-analytic evolution of molecular cloud mass functions and also perform a series of MHD simulations of supersonic converging HI gas flows. We find that many of the observed properties of molecular clouds are inherited from the cold component of HI gas, so called the cold neutral medium. These results suggest the importance of HI gas accumulation to drive molecular cloud formation and subsequent star / star cluster formation. Our results also indicate that most of molecular clouds die in 5 Myr once they start to host massive stars, but only a limited population without significant massive star formation survive to grow to 10^6 Msun clouds, which takes ~ 100 Myr.
If time allows, we may briefly mention some indications to massive molecular clouds at high-redshift star-forming galaxies, based on the metallicity-dependence measured in our simulations.
Title: The death of massive stars and the birth of various compact stars.
Speaker: 黒田仰生（Max Planck Institute）/ Takami Kuroda (Max Planck Institute)
The final fate of massive stars is the moment of birth of diverse compact stars: neutron stars (NSs), black holes (BHs), magnetars, and exotic stars. Such diversity depends strongly on the explosion (i.e. core-collapse supernova, CCSN) mechanism as well as on the progenitor mass.
Although recent CCSN simulations have gradually revealed the explosion mechanism of less massive stars (<~20 Msun), which are considered to leave NSs behind, our understanding of the fate of more massive stars (>~40 Msun) is remaining patchy and incomplete.
A major reason for this incompleteness is due to the difficulty of CCSN simulations in full general relativity (GR), which is particularly important for more massive stars.
Utilizing our original state-of-the-art GR neutrino-radiation code and employing an up-to-date EOS considering the hadron-quark phase transition, we have recently reported for the first time how various compact stars can be formed.
Our results suggest that less massive stars may successfully explode by the standard neutrino heating mechanism and leave NSs behind, while very massive stars (>~70 Msun) tend to fail and their remnants are BHs.
This picture is basically the same as our previous understanding.
The most noteworthy is that some modest massive stars (~40-50 Msun) could explode when their nascent NS experiences the phase transition and forms a quark core surrounded by normal hadronic mattes, i.e., the so-called hybrid star.
The explosion is so strong that such type of explosion could be a potential candidate for super-luminous SNe.
In this talk, I will present our latest CCSN simulations and explain a possible scenario of various compact star formations and their multi-messenger signals.
Title: The formation of the box/peanut bulge of the Milky Way
Speaker: Victor Debattista (University of Central Lancashire)
The bulge of the Milky Way constitutes some 25-30% of its stellar mass. Understanding how it formed therefore is vital to understanding galaxy formation in general. Historically it has been thought that the bulge is comprised of an accreted component and an in-situ formed population, varying in the shape, rotation rate, and metallicities. I will demonstrate how these properties arise naturally in a bulge that formed wholly in situ, with only the minor contamination of the stellar halo. I further show how the chemical bimodality of the bulge can be understood if its chemical evolution includes an episode of high star formation rate density clumps, as observed in high redshift galaxies. Taken together these facts point to a bulge that is substantially just the thickened part of the bar.
Title: Solar and stellar spectroscopy using 3D non-LTE radiative transfer
Speaker: Anish Amarsi (Department of Physics and Astronomy, Uppsala University)
Stars leave their signatures on the light that they emit, in the form of spectral lines. These signatures can be decoded to reveal the fundamental parameters and chemical compositions of stars, which can then shed light on the structure and evolution of stars, and of the Galaxy. However, such spectroscopic analyses are heavily model dependent. Shortcomings in the models often restrict the accuracy of the final results. For late-type stars like our Sun, two of the main problems in present-day methods are that they assume that stellar atmospheres are a) one-dimensional (1D) and hydrostatic, and b) satisfying local thermodynamic equilibrium (LTE).
These assumptions may be relaxed simultaneously, by performing detailed 3D non-LTE radiative transfer post-processing of 3D radiative-hydrodynamic model stellar atmospheres. I shall describe the method, before discussing their application to understand the solar chemical composition, in the context of the solar modelling problem. I shall then present results on carbon, oxygen, and iron abundances in a sample of around 200 stars in the Mlky Way disk and halo, for which we find the 1D LTE and 3D non-LTE analyses to imply different conclusions about the chemical evolution of our Galaxy, and the compositions of planet-hosting stars. Finally, I shall discuss our efforts to use 1D non-LTE and 3D non-LTE models to surveys of millions of stars, including GALAH and 4MOST.
Title: The effects of surface fossil magnetic fields on massive star evolution
Speaker: Zsolt Keszthelyi（National Astronomical Observatory of Japan)
Magnetism plays a vital role in several astrophysical phenomena, from the scale of sub-atomic particles up to galaxy clusters. Studying stellar magnetic fields can help us better understand planetary habitability, stellar variability, and the overall magnetic flux evolution from star formation to compact objects.
There has been renewed interest in this field, partly due to the surprising discovery that some massive stars host strong, globally organized, large-scale magnetic fields. The long-term (years to decades-long) spectropolarimetric monitoring shows a lack of correlation with stellar parameters. This suggests that the observed fields are not produced by a dynamo mechanism. Instead, they are thought to be remnants from the earlier history of the star (from the star formation phase or, in part, produced by stellar mergers).
In a series of publications, we have been studying the long-term, evolutionary impact of such fossil fields on massive star evolution. Two main effects concern i) trapping wind material and thus reducing the mass loss and ii) magnetic braking leading to slowly spinning stars. Both have far-reaching consequences and could affect completely evolutionary pathways, stellar populations, predictions on stellar end products, and gravitational wave progenitors.
We recently computed and scrutinized a grid of models including these effects in three metallicity environments. The library of new stellar models is open source and available for the community via: https://zenodo.org/record/7069766
Title: Discovery of triple collision orbits in the general three-body problem
Speaker: 谷川清隆（国立天文台）/ Kiyotaka Tanikawa（National Astronomical Observatory of Japan)
Abstract: We recently (Tanikawa et al. 2019, 2021) discovered triple collision orbits in the (general) three-body problem. More precisely, we developed a procedure to systematically find triple collision orbits starting at general triangular configurations. Astrophyics in the twenty-first century is confronted with extremely energetic phenomena related to compact cosmological objects. Triple collision orbits or orbits close to triple collision may be used to explain the mechanism of these high energy phenomena since an infinite amount of energy can be extrated from a single triple collision. The availability of the initial conditions of concrete triple collision orbits may profit the study of high energy astrophysics. We briefly introduce the gravitational slingshot effect as one of the possible mechanisms of cosmological jets which is proposed by some celestial mechanist.
The purpose of the talk is (1) to explain the procedure to find triple collision orbits with zero intial velocities; and (2) to apply the procedure to find triple collision orbits with non-zero initial velocities.
In the above, the procedure comprises (a) giving symbols at particular instants of time to the trajectories of the three bodies in the phase space, and (b) forming symbol sequences instead of continuous curves.
In short, we change the geometrical objects (continous curves) to the algebraic objects (sequences of symbols). These allow us to divide the initial condition surface into non-overlapping areas whose boundary curves have a special character. As a cross point of three boundary curves, the initial point of a triple collision orbit is obtained.
Title: The VINTERGATAN project: towards understanding the origins of the Milky Way
Speaker: Oscar Agertz (Department of Astronomy and Theoretical Physics, Lund Observatory)
In this talk I will highlight recent advances in the field of cosmological galaxy formation simulations. I will present a cosmological `zoom’ simulation of a Milky Way-mass galaxy called VINTERGATAN, and how this can be used to improve our understanding of how chemically, kinematically and structurally distinct stellar discs form. The role of gas accretion, mergers and secular processes, in establishing features of the simulated thin and thick disc, will be presented and contrasted to Milky Way observations. Finally, I will discuss a novel way of performing modified cosmological simulation, in which details of the merger history can be controlled, and how this technology can better our understanding of the Milky Way’s origins.
Title: Development of Optical Instruments for Seimei Telescope: KOOLS-IFU and TriCCS
Speaker: 松林 和也 (東京大学大学院理学系研究科附属天文学教育研究センター)/Kazuya Matsubayashi(Institute of Astronomy, The University of Tokyo)
Follow-up observations for rapid transients found by other telescopes are essential in revealing their nature. For follow-up and monitor observations of transients, we developed two optical instruments for the Seimei telescope, the largest aperture general-purpuse optical telescope in East Asia. One is a low-dispersion spectrograph with a fiber-type integral field unit, KOOLS-IFU, and the other is a tricolor CMOS camera and spectrograph, TriCCS. These instruments perform scientific observations in NAOJ open-use and Kyoto University time. In this seminar, I will introduce the Seimei telescope and the details of KOOLS-IFU and TriCCS.
Title: Clues to galaxy evolution from chemical abundances of stars in the Galactic center
Speaker: Brian Thorsbro (Department of Astronomy, Graduate School of Science, University of Tokyo)
I present a detailed study of the composition of 20 M giants in the Galactic center with 15 of them confirmed to be in the nuclear star cluster. As a control sample 7 M giants in the Milky Way disk with similar stellar parameters have also observed. The observations show the first silicon abundance trends versus [Fe/H] for stars in the Galactic center. While finding a disk/bulge like trend at subsolar metallicities, the [Si/Fe] abundance is found to be enhanced at supersolar metallicities. It is possible to speculate on enrichment scenarios to explain such a trend, which can provide clues to galaxy evolution scenarios. Further, high resolution spectroscopy performed on stars in the nuclear star clusters that have been identified by Nishiyama et al. (2016) as possible intermediate-age stars will be presented.
Title: Impact of Interstellar Dust on Galactic Science and CMB Foreground
Speaker: 梨本真志（天文教室）/ Masashi Nashimoto (Department of Astronomy, Graduate School of Science, University of Tokyo)
Abstract: Current and next-generation cosmic microwave background (CMB) experiments will provide multi-frequency high-sensitivity intensity and polarization maps. While these data are essential for precision cosmological measurements, it is crucial to characterize the foreground emission from the interstellar medium in detail to elicit its potential fully. In this talk, I will present our recent and ongoing works on the interstellar medium, especially dust, toward CMB experiments.
The first topic is dust emission. We show that a physically motivated model based on amorphous properties of dust, the two-level systems (TLS) model, can explain observations from far-infrared to microwave intensity and polarization dust spectra, including anomalous microwave emission (AME). Besides, a dust emission model based on a soft-potential model, which is an extension of the TLS model, is under construction. The second topic is dust absorption. We evaluate the effect of non-spherical dust absorbing CMB and causing pseudo-polarization, named CMB shadow, and show that the CMB shadow may be non-negligible in the next-generation CMB experiments.
Title: Massive spheroidal galaxies and their central super massive black holes under a violent formation phase
Speaker: 秋山正幸（東北大学）/ Masayuki Akiyama (Tohoku University)
Abstract: Extreme starburst galaxies with star formation rate 100 times higher than the Milky Way galaxy are thought to represent the formation phase of the massive spheroidal galaxies and their central super massive black holes (SMBHs). Therefore, they are a key population of galaxies to unveil the physical mechanism behind the tight correlation between the spheroids and SMBHs. Now we are constructing a statistical sample of extreme starburst galaxies at z<1 utilizing a unique sample from AKARI-WISE-SDSS cross-matching and spectroscopic follow-up programs with Subaru and Seimei telescopes. Utilizing their closeness to us, we are trying to answer the questions with the physical mechanism; what are the driving and maintaining mechanism of the extremely large star formation rate ?, how is the central SMBH fed during the extreme starburst phase ?, and what are the feedback effects associated with the AGN activity.
Title: From turbulent teenagehood to quiet retirement: what drives the evolution of galaxies across cosmic time?
Speaker: Florent Renaud (Lund Observatory)
The formation and evolution of galaxies across cosmic time proceeds in different phases, paced by their internal evolution and external factors like gas accretion and mergers. The complex and always changing interplay between these mechanisms drives the assembly of galaxies and the physical conditions for star formation, which leaves observable imprints on the stellar populations. The signatures of these past events are encrypted in the data from large astrometric and spectroscopic surveys of the Milky Way. Decoding this, with the help of simulations and models, will open new perspectives on the formation and evolution of galaxies in general. In this talk, I will present recent results from a series of hydrodynamical simulations of Milky Way-like galaxies, both in isolation and in cosmological context using the VINTERGATAN simulation. I will show how the transition from a violent early phase of assembly toward a more quiet evolution is driven jointly (and possibly simultaneously) by internal and external factors. I will expose the mergers and the lowering of the gas fraction as the main culprits of this transition.
Title: Exoplanet Observations with TMT: Expectations
Speaker: 田村元秀 (東京大学/アストロバイオロジーセンター/国立天文台) / Motohide Tamura (UTokyo/Astrobiology Center/NAOJ)
Exoplanet observations have enormously expanded in the last ~30 years. More than 5000 exoplanets have been discovered and their candidates can even double this number. Not only detections but also characterizations of exoplanets have been explored. In fact, this field is regarded as one of the major science drivers for the future large telescopes and mission. In this talk, I will highlight the current exoplanet observations, especially using the indirect methods of the Doppler and transit as well as the direct imaging and characterizations with Subaru and other telescopes. Then, with great expectations I will discuss what could be the key exoplanet sciences of TMT.
Title: Exploring visible and obscured sides of the early Universe – Today and Beyond –
Speaker: Seiji Fujimoto (UT Austin)
Understanding galaxy formation and evolution requires comprehensive observations of radiation from distant galaxies composed of stars, gas, and dust. In this talk, I will overview my recent and ongoing studies on distant Universe in scales of the cosmic structure, circumgalactic and interstellar media, and central black holes, with the optical/NIR and submm/mm facilities such as Hubble, ALMA, and JWST. First, I will present the results from the 100-hrs large program of the ALMA Lensing Cluster Survey and its implication for the origin of the CIB and the hidden side of the cosmic star-formation rate density. Next, I will talk about the Baryon cycle in and outside of early galaxies revealed by deep ALMA and JWST observations, and young quasars at z~5-9 reported one after another since the JWST began its operation, along with the discovery of a rapidly growing young quasar in a dust-enshrouded starburst at z=7.2. Finally, I will introduce ongoing JWST and ALMA programs that I have been leading as PI and a JWST large Lensing Cluster Survey in planning.