[论文解读] HST study of Lyman-alpha emission in star-forming galaxies: the effect of neutral gas flows

D. Kunth, Mas-Hesse, J. M.+4 more|arXiv (Cornell University)|Feb 19, 1998

Astrophysics and Star Formation Studies参考文献 1被引用 11

一句话总结

本HST GHRS研究对8个HII星系的分析表明，蓝移的中性气体外流（最高达200 km/s）可通过将共振散射向更长波长移动，从而减少吸收，使莱曼-α发射得以逃逸。中性气体的速度结构，而非柱密度或尘埃，是决定莱曼-α可探测性的主导因素，这挑战了基于莱曼-α巡天估算高红移恒星形成率时的既定假设。

ABSTRACT

We present high dispersion HST GHRS UV spectroscopic observations of 8 HII galaxies covering a wide range of metallicities and physical properties. We have found Lyalpha emission in 4 galaxies with blueshifted absorption features, leading to PCygni like profiles in 3 of them. In all these objects the OI and S II absorption lines are also blueshifted with respect to the ionized gas, indicating that the neutral gas is outflowing in these galaxies with velocities up to 200 km/s.The rest of the sample shows broad damped Lyalpha absorption profiles centered at the wavelength corresponding to the redshift of the H II emitting gas. We therefore find that the velocity structure of the neutral gas in these galaxies is the driving factor that determines the detectability of Lyalpha in emission. Relatively small column densities of neutral gas with even very small dust content would destroy the Lyalpha emission if this gas is static with respect to the ionized region where Lyalpha photons originate. The situation changes dramatically when most of the neutral gas is velocity-shifted with respect to the ionized regions because resonant scattering by neutral hydrogen will be most efficient at wavelengths shorter than the Lyalpha emission, allowing the Lyalpha photons to escape (at least partially). This mechanism complements the effect of porosity in the neutral interstellar medium discussed by other authors. The anisotropy of these gas flows and their dependence on the intrinsic properties of the violent star-forming episodes might explain the apparent lack of correlation between metallicity and the frequency of occurence and strength of Lyalpha emission in star-forming galaxies. Attempts to derive the comoving star-formation rate at high redshifts from Lyalpha emission searches are highly questionable.

研究动机与目标

• 研究不同金属丰度的星暴星系中莱曼-α发射的物理条件。
• 确定尽管存在强烈的电离气体发射，莱曼-α发射为何常无法探测到。
• 评估中性气体动力学与尘埃在抑制或促进莱曼-α逃逸中的作用。
• 评估利用莱曼-α发射作为高红移恒星形成示踪剂的可靠性。

提出的方法

• 采用高分辨率HST GHRS紫外光谱仪对8个具有不同金属丰度和物理特性的HII星系进行观测。
• 分析莱曼-α、OI和S II谱线的轮廓，测量电离气体与中性气体组分之间的速度偏移。
• 将中性气体的速度结构与光谱中莱曼-α发射的有无进行关联。
• 通过建模共振散射机制，解释蓝移中性气体如何通过将散射过程红移到发射线长波方向，从而增强莱曼-α逃逸。
• 将结果与现有星际介质孔隙率和尘埃消光模型进行比较。
• 统计分析表明，莱曼-α发射的频率与强度更依赖于中性气体动力学，而非仅金属丰度。

实验结果

研究问题

• RQ1 为何在存在强烈电离气体发射的情况下，莱曼-α发射在星暴星系中常无法探测到？
• RQ2 中性气体的动力学如何影响莱曼-α光子的可探测性？
• RQ3 尘埃含量与中性气体柱密度在静态与外流构型下，对莱曼-α逃逸的影响程度如何？
• RQ4 中性气体外流在多大程度上可解释金属丰度与莱曼-α发射强度之间观测到的无相关性？
• RQ5 莱曼-α光度作为高红移共动恒星形成率示踪剂的可靠性如何？

主要发现

• 在8个星系中的4个中观测到莱曼-α发射，且伴有蓝移吸收，形成类似P Cygni的轮廓，表明存在中性气体外流。
• 在所有4个具有发射特征的星系中，OI和S II吸收线相对于电离气体均呈现蓝移，证实中性气体外流速度最高可达200 km/s。
• 其余4个星系表现出以电离气体系统红移为中心的宽吸收线（damped Lyman-alpha）轮廓，表明中性气体处于静止或低速状态。
• 中性气体的速度结构被确定为决定莱曼-α可探测性的主要因素，其影响超过尘埃和柱密度的影响。
• 在运动气体中的共振散射使莱曼-α光子得以逃逸，其机制是将散射过程红移到发射线长波方向，从而减少再吸收。
• 研究结果表明，由于高红移恒星形成率估算高度依赖于气体动力学，因此基于莱曼-α巡天推导高红移恒星形成率的方法存在严重疑问。