在0.12 < z < 0.5 II下对OVI吸收系统的STIS调查：电离气体的物理条件
A STIS Survey for OVI Absorption Systems at 0.12 < z < 0.5 II.: Physical Conditions of the Ionised Gas
摘要：
本文通过对HST/STIS数据库中16个类星体阶梯光谱的盲测，给出了27个低红移（0.12 < z < 0.5）OVI吸收体的完整目录。这些吸收体仅基于匹配的谱线轮廓和λλ1031、1037跃迁之间的预期双峰比来识别。执行后续搜索以识别其相关联的转换。在这里，我们介绍了所有相关的吸收特性。通过考虑吸收成分的不同物种，以及在速度空间对齐，我们得到气体温度和非热加宽值，b_{nt}。我们发现，在所有16种情况下，观测到的谱线宽度都是由非热运动决定的，并且气体温度远低于碰撞电离平衡时O^5+的预期温度。这一结果证实了之前对个体视线的研究结果，但与对WHIM起源的预期不一致。至少有一半的吸收体可以用一个简单的光电离模型来解释。此外，在一些吸收器中，我们发现了单个吸收器中各组件之间气体密度/金属度变化很大的证据。多个伴生金属物种的比较进一步表明，在假设的气体被光电离的变钙铝榴岩背景辐射场，吸收云的气体密度为<n_H>< -2.9 and sizes L >1 \kpc。最后，我们比较我们的吸收剂的选择与其他独立研究的结果。
英文摘要：
We present a complete catalogue of 27 O VI absorbers at low redshift (0.12 < z < 0.5) from a blind survey of 16 QSO echelle spectra in the HST/STIS data archive. These absorbers are identified based only upon matching line profiles and the expected doublet ratio between the λλ1031, 1037 transitions. Subsequent searches are carried out to identify their associated transitions. Here we present all relevant absorption properties. By considering absorption components of different species which are well-aligned in velocity-space, we derive gas temperatures and non-thermal broadening values, b_{nt}. We show that in all 16 cases considered the observed line width is dominated by non-thermal motion and that gas temperatures are well below those expected for O^5+ in collisional ionization equilibrium. This result reaffirms previous findings from studies of individual lines of sight, but are at odds with expectations for a WHIM origin. At least half of the absorbers can be explained by a simple photoionization model. In addition, in some absorbers we find evidence for large variation in gas density/metallicity across components in individual absorbers. Comparisons of multiple associated metal species further show that under the assumption of the gas being photoionized by the metagalacitic background radiation field, the absorbing clouds have gas densities <n_H> < -2.9 and sizes L > 1\kpc. Finally, we compare our absorber selection with the results of other independent studies.
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