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Observational Studies on Physical Properties of Molecular Gas in Nearby Galaxies

矢島, 義之 北海道大学

2022.03.24

概要

星形成は宇宙で最も基本的な物理過程である。星は分子ガスが収縮し高密度となることで誕生するため、分子ガスの物理状態と星形成の関わり合いを研究することは銀河、ひいては宇宙の進化を理解する上で必要不可欠である。本学位論文では星形成活動の理解に近付くため、近傍銀河における分子ガスの観測から以下のことを明らかにした。

1. 12co(/= 2-i)/12co(7= i-o)比の多様性
星の原料となる低温の分子ガスでは、最低エネルギーの遷移で放射される12co(/=1-0)輝線が重要なトレーサーである。しかし近年の観測では、より観測効率の高い12co(/=2-1)を観測し、12 co(/ = 2 - 1 ) / 12 co(/ = 1 - 0 ) 強度比( =2 / 1 ) を一定値と仮定することで分子ガス質量等を求める研究が見られるようになった。そこで我々は野辺山宇宙電波観測所45 m望遠鏡で取得された12 co(/ = 1 - 0 )、ミリ波電波天文学研究所( IRAM) 30 m望遠鏡で取得された12 co(/ = 2 - 1 ) の観測データを用い、24 個の近傍銀河における星/ h及びそれを一定と仮定した場合の影響を調べた。その結果、晟/1を一定とし、12co(/= 2-1)かゝら総分子ガス質量や分子ガス面密度を求めると〜20-30%程見誤ることが判明した。さらに分子ガスと星形成間の基本的な関係であるKennicutt-Schmidt関係の幕は12 CO(/ = 2-1)を用いると多くの銀河で過小評価してしまうと分かった。また、星/1は星形成率、分子ガスの消費時間、赤外線の色指数の様な星形成活動の指標となる物理量や分子ガスの温度に対する強い依存性が示された。これらの結果により、星形成活動に応じてK2〃 は系統的に変化しているため、12co (/=2-1)を分子ガストレーサーとして使用する場合は注意が必要であると結論付けられる。

2. 棒渦巻銀河における分子ガスの状態と星形成に対するガスダイナミクスの影響
棒渦巻銀河の棒状部分(バー)では分子ガスが豊富にも関わらず、星が生まれにくいことが知られている。その原因として、バーにおける激しい分子ガスの運動が分子ガスの状態を変化させ、星形成に影響を与えていると考えられている。しかし、それらの関係性は定量的に示されていなかった。そこで我々は棒渦巻銀河NGC2903、NGC3627. NGC4303について野辺山45m 鏡で得られた12 co(/ = 1 - 0 )、1 3 CO(/ = 1 - 0 )、 James Clerk Maxwell Telescope15m鏡で得られた12co(/= 3-2)の観測データを用い、非局所熱力学平衡解析による分子の励起解析から分子ガスの数密度n(H2)を銀河の構造ごとに求めた。そして星の生まれやすさの指標である星形成効率と比較した。その結果、3銀河全てにおいて星形成効率と77(H2)の間に正の相関が確認された。また分子ガスの大局的な運動による影響を調べるため、72(H2)の速度分散を反映するスペクトルの半値半幅と数密度を比較した。その結果、速度分散が適度に大きい場合では77(H2)は速度分散に対し上昇する一方、速度分散が大きすぎると72(H2)は減少に転じる傾向を発見した。以上の様に、大局的な分子ガスの運動の激しさが分子ガスの状態を変化させ、星形成活動に影響を与えるという一連のシナリオを明らかにした。

3. 棒渦巻銀河Maffei 2 におけるCO輝線の2 成分分離で探る分子ガスの状態と星形成を決定する因子
分子ガスはこれまで分子雲と呼ばれる雲状の構造物として銀河内に存在しているという描像を念頭に考えられてきたが、無視できない量の希薄で広がった分子ガスの存在も確認されている。さらに、広がった成分の分子ガスの割合は星形成に影響を与えている示唆も得られている。一方で、分子雲は星形成の母胎でもあるため、その物理状態も星形成に影響を及ぼし得る。そこで我々は棒渦巻銀河Maffei 2に対し、分子ガス全体をトレースする12c〇(/= 1 - 0 ) と雲状の分子ガスを選択的にトレースする1 3 CO(/ = 1 - 0 ) の2 輝線を野辺山45 m鏡で観測し、広がった成分の分子ガスの割合と分子雲そのものの状態、どちらが星形成に重要な因子となるのか調べた。そのため、1 2 co(/ = i - o) は同じ位置、速度における13 co(/ = 1-0)の検出に基づき、2成分に分離した。その結果、広がった成分の分子ガスの割合は銀河内で大きく異なるのに対し、雲状成分の分子ガスの状態はほぼ一様であった。さらに銀河構造間の星形成効率の差異は、分子ガスの総量から求めたものと比較し、雲状成分のみから求めたものは一定に近づく傾向が見られた。以上より、雲状成分の分子ガスの割合は分子ガスの状態や星形成を特徴付ける重要な因子であることが明らかとなった。さらに銀河の構造は雲状成分の分子ガスの形成、崩壊を促進させる一方、雲状成分の状態を変化させる程強くはないと結論付けられる。

4. スターバースト銀河NGC1808における分子雲と分子ガスの状態
銀河系や局所銀河群の銀河等、現在の宇宙で一般的な銀河における分子雲は類似しており、その性質は銀河の構造ごとに大きな差異はないことが報告されている。一方でスターバースト銀河の様な極端な環境下の分子雲はそれらから大きく逸脱した性質が示されている。そこで強烈な環境下における分子雲の銀河構造に対する依存性を明らかにするため、アタカマ大型ミリ波サブミリ波干渉計( ALMA) によるスターバースト銀河NGC 1808 の高空間分解能が達成された1 2 CO(/ = 1-0)の観測データを用い、銀河の構造ごとに分子雲の状態を調ベた。その結果、渦状腕からバー、中心部のリング構造に移るにつれ、ビリアルパラメータは上昇していた。対して、リング構造から銀河の中心核にかけて急激に分子雲の質量と面密度が増し、ビリアルパラメータは低下する傾向が確認された。また、ALMA12mアレイのミッシングフラックの割合は中心に向かうにつれ、バーでは上昇する一方、リング構造より内側では減少する結果が得られた。これらから分子雲は銀河中心部に落下するにつれ、星を作りにくい状態に変化するが、中心部のリング構造に到達後はその高圧環境や豊富な分子ガスにより、急激に分子雲は成長し、重力的に不安定な状態となり、リング構造下流や中心核におけるスターバーストを誘発していると予測される。この様にスターバースト銀河における分子雲の進化過程を明らかにした。

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参考文献

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