英仙A星系中冷分子氫性質之研究

Properties of the cool molecular filaments in Perseus A

何宜庭(I-Ting Ho)

指導教授 ： 賀曾樸

共同指導教授 ： 林仁良 ； 丁文忠(Van-Trung Dinh)

國立臺灣大學/理學院/天文物理研究所/碩士(2009年)

https://doi.org/10.6342/NTU.2009.01471

摘要

星系團的X射線觀測顯示星系團中充斥著炙熱(溫度達10^7-10^8 K)的「星系團內介質」(ICM)。為於星系團的中央處的氣體的密度較外圍氣體密度高得多，理論計算顯示此處的氣體會比外圍的氣體冷卻得更快速；冷卻的結果使得熱壓力無法克服外層氣體的重力，因此理論預測會有大量的X射線溫度以下的氣體流入並沈澱累積在星系團的中央，這就是所謂的「X射線冷卻流」。然而觀測上所見的冷氣體數量遠不及理論所預測的，而且對已知的冷氣體的觀測並沒有足夠的證據證明這些冷氣體是由星系團內介質所冷卻而來的，或是由其他可能機制而產生的。

NGC 1275 (又稱之為「英仙A」)是位在英仙星系團中央的巨大橢圓cD星系，也是X射線觀測上被預測會產生X射線冷卻流的星系。我們使用次毫米微波陣列(SubMillimeter Array, SMA)觀測一氧化碳分子J=2-1的譜線來了解冷分子氫在英仙A中央的空間分佈與速度分佈。我們一共執行了三次的觀測(其中後二次為作者的研究);第一次以約3角秒的角分辨率(約1000秒差距)觀測英仙A中心半徑約28角秒(約10,000秒差距)內的區域；第二次我們以同樣的角分辨率觀測，但在東西方向拓展我們所涵蓋的範圍至離英仙A中心約45角秒處(約16,000秒差距)；而第三次的觀測我們以約1.5角秒的分辨率(約500秒差距)觀測與第一次觀測同樣的範圍。

前二次觀測的結果主要讓我們了解到:1)冷分子氫在我們所觀測的範圍中集中為六道排列在徑方向且對準星系中心的絲狀結構體(filament)，其中東西方向各有三道。2)其中二道較長的結構(Western filament and Eastern filament)在徑方向有線性的速度梯度，而我們可以用粒子在英仙A的重力場中自由落體來重現這個梯度，這意味著這二道氣體流正順著英仙A的重力場流向其中心；這是至今最直接的證據證明這些冷氫分子是直接來自X射線冷卻流。也間接的暗示這些氣體可能最終會是提供中央活躍星系核噴流的燃料物質。3)我們還發現這些冷結構體在空間上與亮度上與氫-阿法有很強的相關性；這個結果支持氫-阿法是由X射線所激發而產生的。4)我們的計算顯示這些結構體都是被重力束縛的且會在10^6年內塌縮，然而二條長氣體流的動力學年齡卻是10^7年。潮汐力並無法(最極端情形時可恰好)抵消重力塌縮，而較有可能的機制是雲氣中的紊流或是磁場使得這些氣體可以存在這麼久卻沒有塌縮形成恆星。5)藉由與單碟電波望遠鏡(single-dish radio telescope)的觀測做比較，我們計算了由短缺短基線(baseline)所造成無法觀測到的發射線的量；我們發現在影像的中央以及東側我們觀測到了所有的一氧化碳分子，但在影像的西側，高達1/2的發射線無法被我們觀測到。因此未來必需要以更緊密排列的SMA(subcompact configuration)做觀測或合併單碟電波望遠鏡的資料。

第三次較高解析度的觀測讓我們了解到:1)位於東側原本以為的三道結構體只是二道流入的氣體流，他們的速度分佈皆顯示出由外而內先減速度再加速度的特徵；我們推測這二道流入的氣流被一個正在向外傳播的壓力波所影響，並且我們成功的模擬出他們的速度特徵。這個結果顯示在氣體流入星系中心的過程中，會數次穿透由活躍星系核的噴流所產生的壓力波。2)我們在中心1.5個角秒(500秒差距)內看到一個南北方向的速度梯度，這個梯度的速度方向和空間方向都與活躍星系核的噴流相符合，暗示著這些冷氣體是被中央噴流所拖曳出來的；這個現象在很多恆星形成區域噴流附近也都被觀測到。這間接的證實了冷氣流可以流進次秒差距的尺度，並可能被中心大質量黑洞吞噬然後產生活躍星系核的噴流。

關鍵字

星系 ； 冷卻流

並列摘要

The first Sub-millimeter Array (SMA) observation of Perseus A (Lim et al. 2008), the central elliptical galaxy in the Perseus cluster at a resolution of ~3" (~1 kpc) revealed that its molecular gas is concentrated primarily in three radial filaments extending out to ~10 kpc in the east-west direction. The two outer filaments, the Eastern and Western filament, have kinematics that can be reproduced by free-fall in the gravitational potential of Perseus A, indicating that they originate from an X-ray cooling flow in the cluster. Results from two follow-up observations are presented in this thesis.

Motivated by the single-dish observation (Salome et al. 2006) that shows molecular gas extending out to ~14 kpc east-west, the second observation imaged the entire east-west extension of the molecular gas at the same resolution (Ho et al. 2009). No new features were found outside the previous field of view, and two of the previous features to the north and south were found to be sidelobes artifacts. A total of six genuine features is shown to comprise radially aligned filaments, which are most naturally explained by the infall motion proposed by (Lim et al. 2008). All the detected filaments coincide with locally bright H-alpha features, and have nearly constant ratio of CO(2-1) to H-alpha luminosity of ~10^-3, suggesting a common heating mechanism to the ionize gas and molecular gas. These filaments are found to be gravitationally bound, and should collapse on timescales <10^6 yr, much shorter than the dynamical ages of the two longest filaments of ~10^7 yr. Tidal shear may help delay their collapse, but more likely turbulent velocity of at least tens of km/s or magnetic field with strength of at least several ~10 uG are required to support these filaments.

To study the spatial kinematics of the three short filaments as well as the complex Inner filament, the third observation was conducted with the SMA in its extended configuration, providing images at resolution of ~1.5" (~500 pc). Our images to the eastern region first reveal velocity gradients in the two short filaments E1 and E2. The Eastern filament that radially aligned to E1 motivated us to model their kinematics as a single radial filament. By considering a pressure wave, produced by the central jet-inflated X-ray bubbles, passes through the infalling filament and therefore retards it, we can successfully reproduce the kinematics of not only the Eastern filament and E1, but also E2, suggesting that all observed molecular filaments in the eastern region originate from an X-ray cooling flow. Our images to the inner region reveal that the Inner filament comprise a compact central condensation and other four features aligned to the filaments in the eastern or western regions. The central condensation exhibits a north-south velocity gradient consistent, in both direction and sense, with the behavior of jet-entrained gas, indicating that the molecular gas genuinely accumulated in the nucleus, and therefore may be responsible for fueling the AGN.

並列關鍵字

galaxy ； cooling flow

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