MHD simulations on the formation of  Fermi Bubbles with magnetic field

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姓名

鄭翰(Han Cheng) 查詢紙本館藏

畢業系所

天文研究所

論文名稱

(MHD simulations on the formation of Fermi Bubbles with magnetic field)

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摘要(中)

以可見光觀測，銀河系看起來是個扁平的盤狀結構，然而，以伽瑪射線波段觀測，除了扁平的盤狀結構外，在銀河中心方向的銀暈，還能觀測到兩個數千個秒差距的巨大泡泡狀結構。這泡泡狀的結構被叫做費米泡泡(Fermi bubbles)，它的形成機制在許多文獻中被探討，而我們對於其中一項機制—恆星被銀河中心大質量黑洞捕獲造成的潮汐裂解事件(tidal disruption events)深感興趣，我們利用數值模擬的方法來研究費米泡泡的形成機制及其演化，銀暈中的氣體被模擬為呈指數衰減的大氣，潮汐裂解事件被模擬為一個巨大的能量釋放(爆炸事件)，連續的爆炸事件發生在靠近銀河中心的小體積內，可以使物質不斷被推出、造成費米泡泡，我們改變每次爆炸的能量釋放、爆炸的時間間隔，系统性地調查費米泡泡的形成機制。從模擬結果發現，在相同的總能量釋放下，多次爆炸在泡泡內部造成的紊亂程度會比單次爆炸更為明顯。此外，周圍磁場的配置也被加以探討，磁場會阻礙泡泡的發展，在磁場壓力大的地方更為明顯。(相同能量釋放、爆炸時間間隔的)多次爆炸事件下，泡泡內部的亂流在有加磁場的狀況下比沒加磁場的狀況下還要明顯。最後，為了舉例說明，我們計算出模擬結果的X光亮度（projected X-ray emission）並與ROSAT在1.5千電子伏特波段的X-ray影像進行了比較。

摘要(英)

In visible light, our Galaxy looks like a flat disk to us. However, in gamma ray, in addition to a flat disk similar to visible light there are two giant bubble-like structures extending several kpc in the Galactic halo in the direction of the Galactic center. The two structures are called Fermi bubbles and numerous mechanisms have been proposed for their formation. We are particular interested in the scenario of a series of tidal disruption events (TDEs) initiated by repeated stellar captures by the supermassive black hole in the Galactic center. We use numerical simulation to study the formation and evolution of the Fermi bubbles. The ambient gas in the halo is modeled as a layered exponential atmosphere, and the TDE is modeled as an outburst of a large amount of energy confined in a small volume enclosing the center (a.k.a. an explosion at the center). The series of explosions can drive an outflow and create the bubbles. A systematic survey of the formation process is performed. We vary the energy of each explosion, the interval between two explosions. With the same total energy, multiple explosion events give a more turbulent inner structure than a single event. Furthermore, we also examine different configurations of ambient magnetic field. In general, magnetic field hinders the development of the bubbles, especially in the high magnetic pressure region. For multiple explosion events with the same energy of each explosion and the same interval between two explosions, the case with magnetic field is more turbulent than the one without magnetic field. Finally, for illustration, we compute the projected X-ray emission from our simulation, and compare with the ROSAT X-ray map at 1.5 keV.

關鍵字(中)

★ 數值模擬
★ 費米泡泡
★ 磁場
★ 銀河中心
★ 磁流體力學
★ 潮汐裂解事件

關鍵字(英)

★ simulation
★ Fermi Bubbles
★ magnetic field
★ Galactic center
★ MHD
★ FLASH
★ tidal disruption event
★ TDE

論文目次

Chapter 1 Introduction p.1
1-1 Overview of Related Observations p.2
1-1-1 Gamma-ray Bubbles p.2
1-2-2 X-ray Map p.3
1-1-3 Microwave Haze p.3
1-1-4 Radio and Microwave Polarization Data p.4
1-2 Overview of Related Models p.5
1-2-1 High Energy Activities at the Galactic Center p.5
1-2-2 Emission Mechanisms and Cosmic Ray Acceleration p.5
1-2-3 Repeated Stellar Capture Model p.10

Chapter 2 Simulation Code p.12
2-1 Simulation Code: FLASH p.12
2-1-2 The Major Differences Between 8Wave and USM MHD Solver p.13
2-2 Tests of The Performance of FLASH: Sedov Explosion Problem p.14

Chapter 3 Models and Initial Conditions p.16
3-1 The Star Captured Model p.16
3-2 Exponential Isothermal Atmosphere p.16
3-3 Magnetic Field Configurations p.17
3-4 Units and Initial Conditions p.19
3-5 Abbreviation in This Article p.21

Chapter 4 Tests of Program p.23
4-1 The Comparisons of Two Different MHD Solvers: 8Wave and USM p.23
4-2 Comparing 2D and 3D Simulation Results p.24
4-3 Grid Size p.26

Chapter 5 Results and Discussions p.27
5-1 Comparison of Analytic and Numerical Solutions p.28
5-2 Simulations of Fermi Bubbles without Magnetic Field p.30
5-2-1 Single Explosion p.30
5-2-2 Comparison of Single Explosion and Multiple Explosions p.31
5-2-3 Special Features of Multiple Explosions p.34
5-3 Simulation of Fermi Bubbles with Magnetic Field p.43
5-3-1 Multiple Explosions with Different Initial Magnetic Field Configurations p.43
5-3-2 Velocity distribution of the fiducial model B_p p.47
5-3-3 Magnetic Pressure at the Mid-Plane of the fiducial model B_p p.50
5-3-4 Scale Height of Magnetic Pressure of the fiducial model B_p p.52
5-4 A Case for Comparing with Observations p.54

Chapter 6 Summary p.57

Appendix A p.60
References p.65

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指導教授

高仲明(Chung-Ming Ko)

審核日期

2019-7-23

推文