A numerical simulation survey on the outflow from the Galactic center

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题名:	A numerical simulation survey on the outflow from the Galactic center
作者:	張逸雯;Chang,Yi-Wen
贡献者:	天文研究所
关键词:	數值模擬;費米泡泡;銀河中心;流體力學;Simulation;Fermi Bubbles;PLUTO code;Galactic center;HD
日期:	2013-07-10
上传时间:	2013-08-22 11:40:46 (UTC+8)
出版者:	國立中央大學
摘要:	在2010 的年尾，費米伽瑪射線空間望遠鏡在銀河的盤面上下各發現了一個巨大的伽瑪射線結構，他們從銀河的中心向外延伸，約到達經度40、緯度50，長得像兩個大型的泡泡。這樣的空間分佈，剛好跟在銀盤附近倫琴衛星估測到的 1.5 keV 波段X 射線圖以及幾年前威爾金森微波各向異性探測器的謎團有一定的關聯性，所以我們認為這些現象應該是來自於相同的來源。關於這來源，雖然有很多不同的猜測，但我們在這邊特別討論銀河中心的大質量黑洞不間斷的恆星捕獲造成的結果。每當有捕獲現象發生，就會有極大的能量在銀河中心被釋放，產生一個類似爆炸的現象，形成我們所觀測到的泡泡。我們利用PLUTO(一個天文物理的程式)去模擬這個現象。在二維圓柱座標下設定不同的初始條件去觀察這個泡泡的形成以及演化，例如用不同的能量或在不同的時間間隔下去產生每個爆炸。同時也考慮當銀河盤面上的氣體分佈是指數遞減，泡泡的演化隨著不同大氣標高變化。我們首先比較不同能量下，發生一次爆炸的例子(有相同的大氣標高)。泡泡的側邊在能量小的例子會發展的比較遠，但是擾動也比較小。當能量比較大的時候，他可以輕易的打穿盤面的大氣，向上發展，但是側邊會被限制，大約只能到達三倍大氣標高的距離。接著我們比較單次以及多次爆炸的例子，雖然他們有相同的總能量但是外表卻差很多。多次爆炸的例子側邊的發展也被限制，但是泡泡的內部比較混亂，而內部混亂的結構會隨著時間間隔的拉長而變得更亂。模擬所產生的X 射線圖顯示了在多次爆炸的情況下在較低緯的地方，泡泡有比較厚的結構。我們同時也把他來拿和倫琴衛星的數據做比較。 In the second half of 2010, Fermi satellite discovered two giant gamma ray bubbles above and below our Galactic plane in the direction of the Galactic center. The bubbles extended 50 degrees in Galactic latitude and 40 degrees in longitude. The spatial distributions correlated with the ROSAT X-ray map at 1.5 keV and the WMAP haze near the Galactic plane. Among many possible origins of the bubbles, we are particular interested in the scenario that stars are repeatedly captured by the supermassive black hole located at the Galactic center. At each capture, a huge amount of energy is release and causes a massive expansion or outflow that forms the bubbles. We adopt the astro-hydrodynamic code PLUTO to study this phenomenon. We carry out 2D (cylindrical coordinates) numerical survey on the formation and evolution of the bubbles under different conditions, such as different energy release at each capture and different time intervals between captures. We also consider the effect of different assumed scale heights of the Galactic gaseous disk. When we compare different single capture cases (with same scale height), we learn that the shape of the bubble from small energy release is rounder and extended further in the lateral direction than the one from large energy release, but the perturbation is weaker. When the energy release is large, the bubble can easily penetrate the disk, but the lateral extend is restricted to about three times the scale heights. The morphology of a single capture case and a repeated captures case with the same total energy is significantly different. The repeated captures case has lesser lateral evolution and a lot more turbulent interior. Moreover, the turbulent level increases when the interval between captures decreases. The X-ray maps from simulations show that the repeated captures cases have slightly thick lower bubble edge than the single capture cases. We also compare the maps with ROSAT data.
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