落入磁化星系晕的气体云演化：起源于星系喷泉的高速云
The Evolution of Gas Clouds Falling in the Magnetized Galactic Halo: High Velocity Clouds (HVCs) Originated in the Galactic Fountain
摘要：
在银河喷泉的假设中，超新星和/或恒星风将物质推进星系晕。当物质冷却时，它凝结成云。利用FLASH三维磁流体动力学模拟，我们模拟和研究了这些气体云形成并开始向银道面下落后的动力学演化。在我们的模拟中，我们假设气体云在银河系中平面上方z=5 kpc的高度形成，然后开始从静止状态下降。我们调查云的演化，动力学，和相互作用的星际介质（ISM）的云的初始质量的影响。我们发现，云与足够大的初始密度（> 0.1氢原子每立方厘米）充分加速，并保持足够大的柱密度被观察到，并确定为高速云（HVC），即使ISM是弱磁化（1.3微高斯）。我们还研究了各种可能的磁场配置的影响。正如预期的那样，当磁场很强并且垂直于云的运动时，ISM的阻力最大。在磁场与银河平面成对角线的情况下，云的轨迹由磁场线引导。模型云模拟表明，云和ISM之间的相互作用可以理解通过类比的激波管问题，其中涉及冲击和稀疏波。我们还讨论了加速的环境气体，从云烧蚀的材料的流光，和云的演变从一个球形的磁盘或雪茄形状的对象。
英文摘要：
In the Galactic fountain scenario, supernovae and/or stellar winds propel material into the Galactic halo. As the material cools, it condenses into clouds. By using FLASH three-dimensional magnetohydrodynamic simulations, we model and study the dynamical evolution of these gas clouds after they form and begin to fall toward the Galactic plane. In our simulations, we assume that the gas clouds form at a height of z=5 kpc above the Galactic midplane, then begin to fall from rest. We investigate how the cloud's evolution, dynamics, and interaction with the interstellar medium (ISM) are affected by the initial mass of the cloud. We find that clouds with sufficiently large initial densities (> 0.1 hydrogen atoms per cc) accelerate sufficiently and maintain sufficiently large column densities as to be observed and identified as high-velocity clouds (HVCs) even if the ISM is weakly magnetized (1.3 micro Gauss). We also investigate the effects of various possible magnetic field configurations. As expected, the ISM's resistance is greatest when the magnetic field is strong and perpendicular to the motion of the cloud. The trajectory of the cloud is guided by the magnetic field lines in cases where the magnetic field is oriented diagonal to the Galactic plane. The model cloud simulations show that the interactions between the cloud and the ISM can be understood via analogy to the shock tube problem which involves shock and rarefaction waves. We also discuss accelerated ambient gas, streamers of material ablated from the clouds, and the cloud's evolution from a sphere-shaped to a disk- or cigar-shaped object.
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