Cooling gas outflows from galaxies
Cooling gas outflows from galaxies
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DOI：
10.1086/175633
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发表时间：
1994-12
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影响因子：
--
通讯作者：
Boqi Wang
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文献类型：
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作者：
Boqi Wang
我们研究稳定的，径向气体流出星系的努力，以了解如何稀薄和热气体被输送到大距离远离星系。特别是，我们获得的外流问题的解决方案，并研究外流拓扑结构，星系质量的影响，外流区域的大小，辐射冷却的效率，以及冷却气体的命运。在星系冷却函数和引力场的一般幂律形式下，我们证明了外流解由两参数初始条件决定。在与恒星风或吸积问题的类比中，我们证明了不存在跨音速流，但可以获得亚音速或超音速流。超音速外流的解决方案进行了详细的研究，因为它们是最有可能携带气体到大距离远离星系。我们发现，如果重力是弱的，外流的特征在于辐射冷却时间的流动时间的比值，$t_c/t_f$。星系引力场的重要性在于，在流时间内，流出速度等于星系圆周速度的辐射损失的部分能量;如果这部分能量很小，引力会在气体有机会辐射冷却之前阻止流出。在气体确实辐射冷却的情况下，冷却的气体最有可能通过各种不稳定性形成云。这些云由于继承了有限的动能，所以离银河系越来越远。我们发现矮星系中的热气体既可以作为星系风流出，也可以通过辐射冷却形成云。在后一种情况下，云逃离了星系。相比之下，像我们这样的大质量星系倾向于限制气体。我们提出的表面亮度在不同的X射线能量带。我们还估计了平均值
We study steady, radial gas outflows from galaxies in an effort to understand the way tenuous and hot gas is transported to large distances away from galaxies. In particular, we obtain solutions for outflow problems, and study the outflow topology, effects of the galaxy mass, the size of outflow regions, the efficiency of radiative cooling, and the fate of the cooled gas. Under general power-law forms for the cooling function and the gravitational field of galaxies, we show that the outflow solutions are determined by the two-parameter initial conditions. In an analogy with stellar wind or accretion problems, we demonstrate that there exists no transonic flow, but either subsonic or supersonic flows are obtainable. Solutions of the supersonic outflows are studied in detail as they are most likely to carry gas to large distances away from galaxies. We find that if gravity is weak, the outflow is characterized by the ratio of the radiative cooling time to the flow time, $t_c/t_f$. The importance of the galactic gravitational field is characterized by the fractional energy lost radiatively within the flow time in outflows with velocity equal to the circular velocity of the galaxy; if the fraction is small, gravity stops the outflow before the gas has a chance to cool radiatively. In the case the gas does cool radiatively, the cooled gas is most likely to form clouds via various instabilities. The clouds coast farther away from the galaxy because of the finite kinetic energy they inherit. We find that the hot gas in dwarf galaxies can either flow out as galactic winds, or cools radiatively to form clouds. In the latter case, the clouds escape the galaxies. In contrast, massive galaxies like our own tend to confine the gas. We present the surface brightness in various x-ray energy bands. We also estimate the mean