研究生: |
蔡安理 Tsai, An-Li |
---|---|
論文名稱: |
藉由超大分子氣泡與分子噴流的研究瞭解星遽增星系的演化 The Evolution of Starburst Galaxies through Molecular Superbubbles and Outflows |
指導教授: |
管一政
Kuan, Yi-Jehng 松下聡樹 Satoki Matsushita |
學位類別: |
博士 Doctor |
系所名稱: |
地球科學系 Department of Earth Sciences |
論文出版年: | 2011 |
畢業學年度: | 99 |
語文別: | 英文 |
論文頁數: | 147 |
中文關鍵詞: | 星系 、分子氣體 、星遽增星系 、噴流 、超大氣泡 、演化 、恆星形成 、天文 |
英文關鍵詞: | Galaxy, Molecular gas, Starburst, Outflow, Superbubble, Evolution, Star Formation, Astronomy |
論文種類: | 學術論文 |
相關次數: | 點閱:128 下載:5 |
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為了瞭解星系的演化,我們研究星系如何在星系活動中消耗他們的燃料,分子氣體。星遽增星系是研究分子氣體如何被消耗的一個很好的研究對象,它是一種在星系中心正在進行劇烈恆星形成的星系,它因為劇烈恆星形成而產生星系尺度大的噴流並拋出大量的分子氣體。然而,對於星系失去分子氣體的精確測量卻非常少,這是因為分子噴流或巨大分子氣泡的分佈很迷散以致於很稀薄,因此不容易被過去敏感度較低的儀器偵測到。
雖然單天線電波望遠鏡沒有觀測通量損失的問題,但是它的低解析度不足以解析分子噴流或者巨大分子氣泡的細微結構。最近發展的干涉陣列電波望遠鏡觀測,雖然可藉由分子光譜的寬翼譜線偵測到高速分子噴流,但是直接觀測到分子噴流的影像仍然非常少,這對於精確定出分子噴流或巨大分子氣泡的性質為一大缺失。目前僅有極為少數的分子噴流與超大分子氣泡被以高解析度干涉陣列電波望遠鏡觀測到。其中一個最有名的星系就是星遽增星系M82。分子噴流的性質,譬如大小、速度、質量等,皆可被直接觀測到。本研究增加了另外兩個目標星系,NGC 2146以及NGC 3628,它們與M82有非常相似的特性。
我們使用日本野邊山干涉陣列電波望遠鏡(Nobeyama Millimeter Array, NMA)觀測這兩個星系。我們的觀測成功地偵測到星系盤外,數百至數千pc大小的分子噴流以及超大分子氣泡。它們的膨脹速度約每秒35-200公里,拋出的質量約107-8個太陽質量,需要動能約1053-55爾格。NGC 2146的分子噴流速率每年損失約17-34太陽質量,NGC 3628則每年損失約4-7太陽質量。
為了瞭解星系如何藉由分子噴流與超大分子氣泡損失分子氣體質量,我們加入了Chandra X-ray太空望遠鏡資料庫的觀測資料。X-ray觀測資料的分佈位置與分子噴流或超大分子氣泡有很好的空間分佈相關性。我們發現較強的低能量X-ray譜線,分佈在分子噴流或超大分子氣泡的裡面,或者分佈在大致相同的位置。此外,電漿氣體產生的熱壓力比分子噴流的壓力大上一到兩個數量級。這顯示分子噴流正被電漿氣體向外推擠。
由於我們可以精確測量分子氣體的損失,我們得以研究分子氣體的消耗在星系演化中所造成的影響。為了這個目的,我們更加入了NMA的三釐米連續譜線觀測資料,這是一個在電波波段偵測大質量恆星形成區很好的指標。結果顯示,星系中星遽增區域的恆星形成速率在NGC 2146為每年16個太陽質量,在NGC 3628為每年2個太陽質量。這顯示星系藉由分子噴流或者巨大分子氣泡損失分子質量比恆星形成速率還要更快。分子氣體質量損失約為恆星形成率的1-3倍。這提供了一個很好的證據,分子噴流或巨大分子氣泡造成的分子質量損失,可阻止恆星繼續形成。
在比較NGC 2146、NGC 3628、與最具代表性的星遽增星系M82三個星系的三種觀測資料的分子噴流與超大分子氣泡數據之後,我們發現它們分別處於星遽增活動的不同演化階段。NGC 3628最年輕,NGC 2146居中,M82最老。NGC 3628的星遽增現象正被增強中,被拋出的分子噴流氣體回到星系盤的時間比星系盤分子氣體完全消耗完的時間大約短了一個數量級,這顯示這些回到星系盤的分子氣體很快就能成為補充恆星形成的燃料。NGC 2146的星遽增現象也正被增強中,但是被拋出的分子氣體回到星系盤時,星系盤分子氣體也快要消耗完了,這顯示回到星系盤的分子氣體對於延長星遽增活動的時間的影響較小。M82的星遽增現象正在降低,被拋出的分子氣體回到星系盤的時間比星系盤分子氣體完全消耗完的時間長約一個數量級,這顯示被拋出的分子氣體對於延長星遽增活動的時間沒有幫助。這三個星系顯示分子噴流在星遽增活動早期,因為分子氣體來得及回到星系盤繼續提供恆星行成的燃料,而延長星遽增的活動時間,但是在星遽增活動晚期卻會抑制星系的恆星形成現象。
關鍵字:星系、分子氣體、星遽增星系、噴流、超大氣泡、演化、恆星形成、天文
In order to understand the galaxies evolution, we study how galaxies loss their fuel, molecular gas, through galaxy activities. Starburst galaxies are good candidates to study the molecular gas consumption in galaxies. They are currently undergoing intense star formation in the central region of the galaxies, and their strong star formation produced galactic-scale outflow and ejected large amount of molecular gas. However, the accurate measurement of losing molecular gas are very rare because it is difficult to directly observe molecular outflows or superbubbles
due to their diffuse/extended nature and the poor instrumental sensitivities.
Single dish data, although do not have missing flux problem, the poor resolution is not able to resolve the detail structure of molecular outflows or superbubbles. Recent interferometric observations, although some molecular outflows have been detected from high-velocity broad-wing components, the directly evidence from images is sill missing to constrain their properties. So far, only a few molecular outflows and superbubbles in galaxies have been observed with high accuracy. One famous example is the kpc-scale molecular outflow detected from the typical starburst galaxy M82. The detail properties, such as size, velocity, mass, can be measured directly from the interferometer data. However, to known the common features of how molecular gas are losing from galaxies,
we need more samples. We includes two more samples from nearby edge-on starburst galaxies as our case studies,
NGC 2146 and NGC 3628, which have similar characteristics with M82.
We observed the CO(1-0) line on these two galaxies by using the Nobeyama Millimeter Array (NMA). Our results successfully detected the molecular outflows and superbubbles. Molecular outflows and superbubbles can be clearly seen above or below the galactic plane with size of sub-kpc to kpc scale. The expanding velocity is 35-200 km s^{-1}, their mass is 10^{7-8} M_{\sun}, and the kinetic energy is 10^{53-55} erg. The molecular outflow rate in NGC 2146 is 17-34 M_{\sun} yr^{-1}, that in NGC 3628 is 4-7 M_{\sun} yr^{-1}.
In order to know whether and how galaxies losing their molecular gas through outflows and superbubbles, we include the Chandra X-ray Observatory (CXO) soft X-ray archive data to see how plasma gas affect the molecular outflows. The distribution between soft X-ray data and molecular outflows or superbubbles have good spatial correlation. We can see the strong soft X-ray emission either locates within the molecular outflows or superbubble, or distributes in the same area with molecular outflows or superbubbles. Besides, the thermal pressure of plasma gas are 1-2 order larger than that of molecular outflows. This indicates the molecular outflows are pushing by plasma gas due to the thermal expansion.
Since we have accurate measurement on losing molecular gas, we can precisely study the effect of molecular gas consumption on galaxies evolution. For this purpose, we include our NMA 3 mm continuum data, which is a good tracer of massive star forming region at radio waveband. The results show that the SFR in starburst region in NGC 2146 is 16 M_{\sun} yr^{-1}, and that in NGC 3628 is 2 M_{\sun} yr{-1}. This indicates that galaxies loss their molecular gas through outflows or superbubbles faster than galaxies forming stars. The molecular gas mass loss rate is about 1-3 times larger than the SFR in starburst region. This provides a direct evidence that molecular outflows or superbubbles can quench star formation.
After comparing several parameters derived from molecular outflows among our galaxies and M82, we found that they are in three different evolutionary stages of starburst activities. NGC 3628 is the youngest one, NGC 2146 is the middle-age, and M82 is the oldest one. In NGC 3628, the starburst activity is enhanced, the timescale that molecular outflow returns to the galactic disk is one order of magnitude shorter than the the molecular gas consumption timescale, suggesting that the returned gas will fuel the coming star formation. In NGC 2146, the starburst activity is also enhanced, but the molecular outflow gas fallback timescale is a little bit smaller than the gas consumption timescale, suggesting the gas could extend the starburst timescale, but not too much. In M82, the starburst activity is decreasing, the molecular outflow gas fallback timescale is about one order of magnitude longer than the molecular gas consumption timescale, suggesting the gas do not help to extend the starburst timescale. The three cases indicate the molecular outflow extends starburst timescale in the early and middle stage, but quenches the star formation in the late stage of starburst activity.
Keywords: Galaxy, Molecular gas, Starburst, Outflow, Superbubble, Evolution, Star formation, Astronomy
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