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PKS 0405-123对z~0.5 O VI的强吸收：对宇宙网电离和金属性的启示

Strong z~0.5 O VI Absorption Toward PKS 0405-123: Implications for Ionization and Metallicity of the Cosmic Web

J. Christopher Howk, Joseph S. Ribaudo, Nicolas Lehner, J. Xavier Prochaska, Hsiao-Wen Chen

备注：
Accepted for publication in MNRAS. 21 pages. Abstract abridged slightly
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摘要：

我们给出了用FUSE和STIS观测到的O-VI系统在z_abs = 0.495096处朝向PKS 0405-123（z_em = 0.5726）的观测结果。除了强O VI（log N（O VI）= 14.47+/-0.02）和中等H I外，该吸收剂还显示C III、N IV、O IV和O V，上限为另外7种离子。大量的可用离子使我们能够测试电离模型通常采用少得多的约束。我们发现，离子柱密度不能匹配的碰撞电离模型，在平衡或不平衡。光电离模型可以匹配所观察到的柱密度，包括O VI。如果我们假设光电离是由类星体主导的紫外背景造成的，那么气体的金属丰度为[O/H] =-0.15，而一个紫外背景的模型加上从星系逃逸的电离光子的贡献，则得到[O/H] =-0.62。两者均给出[N/O] ~ -0.6和[C/H] ~ -0.2至-0.1。电离光谱的选择受到很大的限制。同时考虑光致电离气体（T~10^4 K）和碰撞电离气体（T~（1-3）x10^5 K）的多相模型也可以与观测结果相匹配，得到非常相似的金属丰度。该系统中的OVI不一定是WHIM物质的可靠示踪剂。我们没有检测到Ne VIII或Mg X吸收。Ne VIII/O VI的极限< 0.21（3 σ），是迄今观察到的最低值。因此，该吸收体并没有显示出T~（0.5-3）x10^6 K处的“温热星系际介质”存在的确凿证据，该介质被认为含有相当大比例的低红移重子。我们目前的限制，在这个吸收器中作为温度的函数的温热气体的总柱。这个系统不太可能在O VII或O VIII离子中提供可检测的X射线吸收，即使它位于天空中较亮的X射线源前面。

英文摘要：

We present observations of the O VI system at z_abs = 0.495096 toward PKS 0405-123 (z_em = 0.5726) obtained with FUSE and STIS. In addition to strong O VI, with log N(O VI) = 14.47+/-0.02, and moderate H I this absorber shows C III, N IV, O IV, and O V, with upper limits for another seven ions. The large number of available ions allows us to test ionization models usually adopted with far less contraints. We find the ionic column densities cannot be matched by collisional ionization models, in or out of equilibrium. Photoionization models can match the observed column densities, including O VI. If one assumes photoionization by a UV background dominated by QSOs, the metallicity of the gas is [O/H] = -0.15, while a model for the UV background with contributions from ionizing photons escaping from galaxies gives [O/H] = -0.62. Both give [N/O] ~ -0.6 and [C/H] ~ -0.2 to -0.1. The choice of ionizing spectrum is poorly constrained. Multiphase models with a contribution from both photoionized gas (at T~10^4 K) and collisionally ionized gas (at T~(1-3)x10^5 K) can also match the observations giving very similar metallicities. The O VI in this system is not necessarily a reliable tracer of WHIM matter. We do not detect Ne VIII or Mg X absorption. The limit on Ne VIII/O VI < 0.21 (3 sigma), the lowest yet observed. Thus this absorber shows no firm evidence of the "warm-hot intergalactic medium" at T~(0.5-3)x10^6 K thought to contain a significant fraction of the baryons at low redshift. We present limits on the total column of warm-hot gas in this absorber as a function of temperature. This system would be unlikely to provide detectable X-ray absorption in the ions O VII or O VIII even if it resided in front of the brighter X-ray sources in the sky.

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