照亮银河系外流：光电离外流
Shining A Light On Galactic Outflows: Photo-Ionized Outflows
摘要：
利用哈勃太空望远镜上的宇宙起源摄谱仪研究了37个邻近恒星形成星系的星系外流电离结构。我们使用O I，Si II，Si III和Si IV紫外吸收线来表征流出气体的不同电离状态。我们测量的等效宽度，线宽，和流出速度的四个过渡，并找到它们之间的浅标度关系和银河系恒星质量和恒星形成率。无论电离势如何，强度相似的谱线具有相似的速度和线宽，这表明这四个跃迁可以被建模为共同移动的相位。硅的等效宽度比（如硅IV/硅II）具有低色散，并与恒星质量的变化很小，而与O I和硅的比例变化的一个因素2对于一个给定的恒星质量。光电离模型再现了这些等效宽度比，而冲击模型则预测了高电离气体的相对量。光电离模型将电离参数（U）限制在-2.25 < log（U）<-1.5之间，并要求流出金属丰度大于0.5 Z $_\odot$。我们推导出电离分数的过渡，并表明，电离参数和恒星金属丰度的范围导致电离分数的1.15-10的变化的因素。从历史上看，质量流出率是通过使用电离分数将单个金属离子的柱密度测量值转换为总氢柱密度来计算的，因此质量流出率对流出物的假定电离结构敏感。
英文摘要：
We study the ionization structure of galactic outflows in 37 nearby, star forming galaxies with the Cosmic Origins Spectrograph on the Hubble Space Telescope. We use the O I, Si II, Si III, and Si IV ultraviolet absorption lines to characterize the different ionization states of outflowing gas. We measure the equivalent widths, line widths, and outflow velocities of the four transitions, and find shallow scaling relations between them and galactic stellar mass and star formation rate. Regardless of the ionization potential, lines of similar strength have similar velocities and line widths, indicating that the four transitions can be modeled as a co-moving phase. The Si equivalent width ratios (e.g. Si IV/Si II) have low dispersion, and little variation with stellar mass; while ratios with O I and Si vary by a factor of 2 for a given stellar mass. Photo-ionization models reproduce these equivalent width ratios, while shock models under predict the relative amount of high ionization gas. The photo-ionization models constrain the ionization parameter (U) between -2.25 < log(U) < -1.5, and require that the outflow metallicities are greater than 0.5 Z$_\odot$. We derive ionization fractions for the transitions, and show that the range of ionization parameters and stellar metallicities leads to a factor of 1.15-10 variation in the ionization fractions. Historically, mass outflow rates are calculated by converting a column density measurement from a single metal ion into a total Hydrogen column density using an ionization fraction, thus mass outflow rates are sensitive to the assumed ionization structure of the outflow.
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