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Abstract: One of the most fundamental questions in galaxy formation and evolution is to understanding the establishment of the Hubble sequence, particularly how blue, star-forming galaxies are transformed into red, quiescent galaxies. In this talk I will present some new insights into this question based on recent observational results at high redshift, primarily at z > 2, and discuss how star-formation is regulated and quenched in massive galaxies and what are the possible roles of environment and supermassive black holes in this process. Specifically, I will show how ALMA unveils a significant population of promising progenitors of the earliest-quenched galaxies at z~4, and how we captured the onset of environmental effects on massive galaxy evolution in the most distant galaxy clusters at z=2.5.
Language: English
Abstract: The two Magellan 6.5 meter telescopes were designed, constructed and are currently being operated at Las Campanas Observatory in Chile by a consortium of US universities and research institutions. This talk discusses some of the thinking that went into their design, notable features, and the transport of the telescope to the observatory and installation on site. The talk concludes with the results of some recent investigations of seismic performance.
Language: English
Abstract: 太陽のおよそ１０倍より重い星は進化の果てに中心に鉄コアを作る。その鉄コアが自身の重力で潰れることから始まる現象が重力崩壊型の超新星爆発である。中心部にはパルサーの元となる原始中性子星が形成され、超新星爆発ではその外側の吹き飛ばされた部分が光る。この中心部の密度、温度は極限的に高くなり、弱い相互作用でしか生まれない粒子であるニュートリノが効率的に放出される。実際、このニュートリノがまさに外側の物質を吹き飛ばすのに一役買い、これをニュートリノによる加熱機構と呼ぶ。この超新星爆発の計算は非常に難しく、安定かつ信頼できる枠組みでシミュレーションできるようになったのは２０００年代の前半である。しかし、その結果は驚くべきことに、爆発が起こらないというものであった。何が問題なのだろうか？実は、それらの計算では簡単のため、球対称という仮定を課しており、対流の効果が入っていなかった。我々はそれを取り除き、３次元計算することで、超新星爆発が成功するという結果を得た。３次元計算することで、ニュートリノや重力波の放出も議論できるようになり、超新星爆発のシミュレーション結果を将来の観測と比較できるような時代がついに訪れる。今回のセミナーでは、超新星爆発の機構から最近の研究までをレビューする。超新星爆発の研究の新時代の到来を感じていただければ幸いである。
Language: Japanese
Abstract: I will overview the young neutron-star model for fast radio bursts (FRBs). Applying the model to a repeating FRB 121102, which has been recently identified as a cosmological event, I will discuss the possible progenitor systems.
Language: English
Abstract: In the Local Group, there are several tens of dwarf galaxies, which have sub-solar metallicity. They provide us a great opportunity to constitute a basic template to study faint low-metallicity galaxies, such as those at high redshift. Because metallicity is an important parameter which significantly affects gas-phase and grain surface chemistry, chemical compositions of molecular clouds in low-metallicity dwarf galaxies are of great interest. To investigate the chemical composition in low-metallicity environments, we have conducted spectral line survey observation in the 3 mm band (85-116 GHz) toward low-metallicity dwarf galaxies, the Large Magellanic Cloud, IC10, and NGC6822. The lines of CCH, HCN, HCO+, HNC, CS, SO, 13CO, and 12CO have been detected. The spectral intensity patterns are found to be similar among three dwarf galaxies. Compared with solar-metallicity environments, the chemical compositions of dwarf galaxies are characterized by deficient nitrogen-bearing molecules, as a direct effect of the lower elemental abundance of nitrogen, and abundant CCH and deficient CH3OH, as an effect of extended photon dominated regions in cloud peripheries due to the lower abundance of dust grains.
Language: Japanese
Abstract: Constraining the grain size in protoplanetary disks is a key to understand the first stage of planet formation. The grain size has been estimated by measuring the spectral index at millimeter wavelengths, while it has huge uncertainties. Here, we propose that millimeter-wave polarization is another method to constrain the grain size. We show that thermal dust emission is scattered off of other dust grains and the residual polarization is up to 2.5 %, which is detectable with ALMA. This self-scattering polarization is efficient only if the maximum grain size is comparable to the wavelengths. Therefore, we can constrain the grain size from millimeter-wave polarization of protoplanetary disks. Furthermore, we have observed the protoplanetary disk around HD 142527 with ALMA polarization mode, and found the evidence of the self-scattering plays a role in the protoplanetary disk.
Language: Japanese (slides in English)
Abstract: 2015年にaLIGOの観測が始まり、その中で重力波の初検出が達成されその観測期間中には計２つの重力波が検出された。どちらも連星ブラックホールの合体であった。またつい先日に、２０１７年１月４日に３つ目の連星ブラックホールの合体による重力波観測が報告され、周辺分野をにぎわわせている。今後はそういった観測の天文学的な情報を得るために、仏伊のadVirgo、KAGRAといった重力波望遠鏡ネットワークで観測を行う体制、そして重力波望遠鏡のみでは非常に困難な、重力波源の特定を実現するために様々な波長での天文観測の連携体制が望まれている。本講演では、現在の重力波望遠鏡の観測結果と、各望遠鏡の現状を報告する。また、検出した重力波からどんな物理が引き出せるかという問題について、超新星爆発からの重力波検出を例にその試みの一端を報告したい。
Language: Japanese
Abstract: 宇宙人存在問題とは、「なぜ、我々は宇宙人と出会っていないのか？」と いう素朴だが難解な問題を指し、フェルミの矛盾とも呼ばれる。これに関して 度々引用されるのがドレイク方程式である。しかしながら、自然科学の範疇 内でドレイク方程式の精密化を試みても不毛な議論となる可能性が高い。 その大きな理由は、「知的生命は自然科学的原理だけでは行動しない」から である。大航海時代を引用するまでもなく、文明が未知域への探検に巨額 の投資を行う背景には、軍事・政治・経済・宗教あるいは知的好奇心など 何らかの文化科学上の行動原理がカギとなりうる。一方、最近のＡＩの進展 を見ると、知的生命が生物である必然性は薄らいでいるようにも思える。 実際、「光速の壁」を克服するためには自己複製可能なクローンあるいは ロボットのみが広い宇宙を席巻できるとすると、もっと違った文化的考察が 必要かもしれない。いずれにしろ、宇宙人存在問題を深く議論するためには、 文理双方の知識・技能・方法論を総動員することが必要であろう。本講演では、 これまでの研究を概括するとともに、総研大文化科学研究科の研究者と 議論する機会により得られた新しい回答案を１０個紹介する。 Reference: Webb (2015), If the Universe is teeming with aliens ... Where is everybody?, Springer, Berlin
Language: Japanese
Abstract: The Spitzer Space Telescope is pioneering the exploration of infrared (IR) luminous transients. In our Spitzer InfraRed Intensive Transients Survey (SPIRITS), we are conducting a systematic search of 200 nearby galaxies for IR-luminous outbursts that elude detection in traditional optical surveys. In this talk, I will present two puzzling discoveries from SPIRITS and discuss open questions and the plans to address them.
SPIRITS has revealed a new class of obscured, red transients with mid-IR luminosities between novae and supernovae that do not exhibit optical counterparts. We call them SPRITEs (eSPecially Red Intermediate-luminosity Transient Events). The key to disentangling the various possible physical origins of SPRITEs is IR spectroscopy, which can only be obtained with the unprecedented sensitivity of the upcoming James Webb Space Telescope (JWST). I will discuss possible SPRITE origins and the JWST Guaranteed Time Observation (GTO) program I am leading to determine their nature.
In SPIRITS, we serendipitously discovered variable mid-IR counterparts of ultraluminous X-ray sources (ULXs). A ULX is an off-nuclear point source showing extreme, super-Eddington X-ray luminosities thought to be driven by accretion onto a compact object in a close binary undergoing Roche Lobe overflow. We attribute the mid-IR emission to a circumbinary dust disk produced by the outflow from the supergiant donor star, but the nature of the variability is still uncertain. I will discuss the work I have lead on dusty ULXs and highlight the importance of coordinated multi-wavelength observations.
Language: English
Abstract: パルサーはその発見から今年でちょうど50年であり、最近は高速電波バーストの起源の候補天体としても注目されている。しかし、パルサーの周囲に形成される磁気圏において、電磁場からプラズマへのエネルギー変換が、どこで、どのように、どの程度の効率で行われるかは未解明のままである。磁気圏の一部の領域では、プラズマが相対論的エネルギーまで加速され、それらの放射する高エネルギー光子を起点として電磁カスケードが発展し、大量の電子・陽電子が磁気圏全体を満たす。この過程で電波からガンマ線までのエネルギー帯域の電磁波が放射される。近年、幅広いエネルギー帯域で検出された天体の数が飛躍的に増加した。講演では、まずガンマ線、X線、可視光、電波それぞれの帯域で観測される放射の特徴とその意味について紹介する。この多波長の結果を組み合わせることで新たに得られた磁気圏構造に対する理解について報告する。
Language: Japanese
Abstract: 超新星残骸は、熱エネルギーや重元素、さらには宇宙線を宇宙空間に供給する、宇宙の多様性の源である。衝撃波速度は秒速数千km程度であるため衝撃波下流は数百万度の光学的に薄いプラズマとなり、熱的制動放射X線や電離された重元素からの特性X線が放射される。したがってX線観測は、超新星残骸研究に最適の手段の一つである。
近年X線観測の空間分解能やエネルギー分解能の向上に伴い、超新星爆発のタイプやタイプごとの爆発非等方性、宇宙線逃亡の環境情報などが引き出せるようになってきた。また、プラズマそのものも複雑な非平衡状態にあり、プラズマ物理としての理解も進んできている。本講演では、Chandra, 「すざく」, 「ひとみ」といったX線衛星の超新星残骸に関する結果をもとに、これらのテーマに関する最近の進展を紹介する。
Language: Japanese
Abstract: Star formation in the Universe decreased by an order of magnitude in the last ten billion years. Probing the molecular gas reservoirs from which stars are formed is crucial to understand this winding-down of star formation. The PHIBSS2 legacy program at the IRAM NOEMA interferometer surveys the molecular gas properties of galaxies at different redshifts on and around the main sequence, where most of star formation occurs. It notably contributes to show that the cosmic evolution of the star formation rate is mainly driven by that of the molecular gas fraction, albeit with a small inflexion of the star formation efficiency. At z=0.5-0.8, we observe the CO(2-1) line for 61 targets, from which we determine molecular gas masses, gas fractions and depletion times. We separately obtain disk sizes and bulge-to-total luminosity ratios, which enable us to characterize morphological changes. The Kennicutt-Schmidt relation between molecular gas and star formation rate surface densities is strikingly linear, which argues in favor of uniform star formation processes across cosmic time. Our results also suggest an ongoing supply of molecular gas to compensate for star formation and a contribution of disk stars to the growing bulge.
Language: English
high spectral resolution observations with SOFIA/upGREAT
Abstract: The fine structure line of ionized carbon, [CII], at 158 μm is one of the strongest cooling lines of the dense interstellar medium. Located in the core region of the far-infrared spectral range, it is observable in the local universe only from space, balloon or airborne platforms, or at high redshifts in the ALMA bands, where it is used as tracer of star formation activity in early galaxies. The recent technological advances now allow routine highest spectral resolution heterodyne observations with the upGREAT instrument on board SOFIA, that resolve the [13CII] satellite lines with sufficient signal/noise, so that it is possible to measure directly the optical depth of the carbon fine structure line emission. We have performed such observations in seven sources ranging from simple to complex spatial and kinematic morphology. In all cases, we find clear evidence of significant optical depth. In many cases, the line profiles show clear evidence for foreground or self-absorption. The high column densities derived and, in many cases, the low temperatures of the foreground/self-absorption, are difficult to account for in standard model scenarios of photodissociation regions (PDRs). These findings imply, that one has to use with caution integrated line intensity ratios, when interpreting [CII] observations in the context of PDR models, to infer average properties of spectroscopically and spatially unresolved sources.
Language: English
Abstract: Substellar objects such as exoplanets and brown dwarfs rotate and some of them have detectable rapid rotation. Recently, rotation velocities of exoplanets have been detected and their rotation velocities are higher than those of planets in the Solar system. Rotation period of brown dwarfs also have been detected and some brown dwarfs have clearly rapid rotation. Their minimum rotation periods are only a few hours and rotation velocities are much higher than those of planets in the Solar system. If the substellar objects have rapid rotation, their shapes might be largely deformed due to the strong centrifugal forces. Their structures cannot be well described by spherical models and so non-spherical models are required to describe them. In order to consider their evolution and structures, an 1D spherical model clearly becomes invalid. At least a 2D model is required to consider the evolution and structures of such rapidly rotating substellar objects. Almost all evolutionary code for substellar evolution, however, can treat only 1D models and nobody has succeeded in obtaining rapidly rotating 2D evolutionary models consistently. As a first step toward the rapidly rotating 2D models, we calculated quasi-evolutionary sequences of rapidly rotating substellar objects using both 1D evolutionary calculation by MESA and 2D rapidly rotating equilibrium calculation by our new numerical scheme (Fujisawa 2015). As a result, we have found that some young rapidly rotating substellar objects are spinning up due to the gravitational contraction as they cool. They might exceed critical rotation and break-up rotation velocity within a few 100 Myrs if the angular momentum is conserved during the evolution. Such rapidly rotating substellar objects would loss their angular momentum by wind or flare to avoid brake-up and the angular momentum loss timescale should be of a few Myrs.
Language: English
