AI 中文总结
研究早期宇宙中隐匿的分子气体，通过新观测发现极端居间分子吸收体，其金属丰度低但分子分数高，为开发探测算法提供基准，揭示高\(H_2\)发生率及相关观测偏差，强调未来巡天和大望远镜观测的重要性。
AI 中文摘要
我们报告了在红移\(z_{abs}=4.1\)处朝向红移\(z = 4.7\)的类星体SDSS J080023.02 + 305101.22发现一个极端的居间分子吸收体，通过强\(H_2\)吸收揭示，该吸收在存档数据中约二十年未被注意到。我们用新的VLT/X - 射手观测分析该系统，其金属丰度约为太阳值的五分之一，与中等尘埃消光\(A_V\sim0.06\)星等相符。更惊人的是，它展现出最高的分子分数，\(f_{H_2}=2N(H_2)/(2N(H_2)+N(HI))\sim60\%\)，这是以直接测定原子和分子氢柱密度在\(z>0\)时测得的。此分数与本星系群中的最高值相当。推断的\(f_{H_2}\)仍是局部分子分数的保守下限，因为\(H_i\)吸收很可能包含与实际分子成分无关的原子气体。鉴于探测\(z>4\)的类星体光谱数量有限，在早期宇宙时代探测到这样一个系统很显著。这表明\(H_2\)发生率意外地高，对分子气体演化有重要影响。这种高发生率可能由那些红移处的高平均密度和增强的湍流驱动。同时，我们的结果也突出了在类星体选择和识别强\(H_2\)吸收方面可能存在的观测偏差，表明很大一部分分子气体可能仍未被探测到，特别是在恒星形成峰值附近。当前系统为开发高效探测算法提供了独特基准。未来进展将受益于与颜色无关的类星体巡天，而限制此类极端吸收体的物理条件和环境将需要未来极大望远镜的观测。
英文摘要
We report the discovery of an extreme intervening molecular absorber at zabs = 4.1 towards the z = 4.7 quasar SDSS J080023.02+305101.22 revealed through strong H2 absorption that had remained unnoticed in archival data for about two decades. The system, which we analysed with new VLT/X-shooter observations, has a metallicity of about one-fifth the solar value, in line with the moderate dust reddening AV~0.06 mag. More strikingly, it exhibits the highest molecular fraction, fH2 = 2N(H2)/(2N(H2) + N(HI))~=60%, measured at z>0 from direct determinations of both atomic and molecular hydrogen column densities. Notably, this fraction is comparable to the very highest values in the Local Group. The inferred fH2 still represents a conservative lower limit to the local molecular fraction since the H i absorption very likely includes atomic gas unrelated to the actual molecular component, as indicated by the wide (Delta v~500 km/s) multi-component low-ionisation metal profile. The detection of such a system at early cosmic times is remarkable given the limited number of quasar spectra probing z>4. It suggests an unexpectedly high incidence of H2, with important implications for the evolution of molecular gas. This high incidence may be driven by high average densities and enhanced turbulence at those redshifts. At the same time, our results also highlight possible observational biases, both in quasar selection and in recognising strong H2 absorption, suggesting that a significant fraction of molecular gas may remain undetected, in particular near the peak of star formation. The present system offers a unique benchmark for developing efficient detection algorithms. Further progress will benefit from colour-independent quasar surveys, while constraining the physical conditions and environments of such extreme absorbers will require observations on future extremely large telescopes.
CommentsAccepted for publication as Letter to the Editor in Astronomy and Astrophysics
