Narrow escape: how ionizing photons escape from disc galaxies
Narrow escape: how ionizing photons escape from disc galaxies
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九死一生：电离光子如何从盘状星系中逃逸
DOI：
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发表时间：
2014
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通讯作者：
P. Sharma
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作者：
Arpita Roy;B. Nath;P. Sharma
本文计算了星暴星系电离光子的逃逸分数f（esc）。利用二维轴对称流体动力学模拟，我们研究了超泡重叠超新星在OB协会。计算了电离光子从盘中心沿着不同角度通过超气泡和气体盘的逃逸分数。在与OB协会的光度函数卷积后，我们发现电离光子在近似40度的圆锥内逃逸，与附近星系的观测结果一致。逃逸分数随时间的演化表明，当冷气体聚集在致密壳层中时，逃逸分数开始福尔斯下降。当壳层越过一些尺度高度和碎片后，通过极地区域的逃逸分数再次上升。从几何考虑（使用发射锥开口角）角度平均逃逸分数不能超过类似于1- cos（1 rad）] = 0.5。我们计算了时间平均和角度平均的逃逸分数对中平面圆盘气体密度（在n（0）= 0.15-50 cm（-3）的范围内）和圆盘尺度高度（在z（0）= 10和600 pc之间）的依赖性。我们发现逃逸分数与盘参数（中面盘密度和尺度高度）大致相关，使得f（esc）（alpha）n（0）（2）z（0）（3）（其中alpha近似为2.2）是一个常数.对于具有给定的暖中性介质温度的盘，大质量盘具有比低质量星系更低的逃逸分数。对于银河系ISM参数，我们发现f（esc）近似为5%，对于质量小10倍的星系，它增加到约10%。我们讨论了ISM的结块对逃逸分数估计的可能影响以及我们的结果对宇宙再电离的影响。
In this paper we calculate the escape fraction (f(esc)) of ionizing photons from starburst galaxies. Using 2D axisymmetric hydrodynamic simulations, we study superbubbles created by overlapping supernovae in OB associations. We calculate the escape fraction of ionizing photons from the centre of the disc along different angles through the superbubble and the gas disc. After convolving with the luminosity function of OB associations, we show that the ionizing photons escape within a cone of similar to 40 degrees, consistent with observations of nearby galaxies. The evolution of the escape fraction with time shows that it falls initially as cold gas is accumulated in a dense shell. After the shell crosses a few scaleheights and fragments, the escape fraction through the polar regions rises again. The angle-averaged escape fraction cannot exceed similar to1 - cos (1 rad)] = 0.5 from geometrical considerations (using the emission cone opening angle). We calculate the dependence of the time-and angle-averaged escape fraction on the mid-plane disc gas density (in the range n(0) = 0.15-50 cm(-3)) and the disc scaleheight (between z(0) = 10 and 600 pc). We find that the escape fraction is related to the disc parameters (the mid-plane disc density and scaleheight) roughly so that f(esc)(alpha)n(0)(2)z(0)(3) (with alpha approximate to 2.2) is a constant. For discs with a given warm neutral medium temperature, massive discs have lower escape fraction than low-mass galaxies. For Milky Way ISM parameters, we find f(esc) similar to 5 per cent, and it increases to approximate to 10 per cent for a galaxy 10 times less massive. We discuss the possible effects of clumpiness of the ISM on the estimate of the escape fraction and the implications of our results for the reionization of the Universe.