[论文解读] Interstellar and Circumgalactic Properties of an Unseen $z=6.84$ Galaxy: Abundances, Ionization, and Heating in the Earliest Known Quasar Absorber

Robert A. Simcoe, Masafusa Onoue+6 more|arXiv (Cornell University)|Nov 20, 2020

Astronomy and Astrophysical Research被引用 5

一句话总结

本研究通过z=7.54类星体ULAS J1342的光谱中吸收线，分析z=6.84星系的星际介质与星系晕介质，利用高分辨率FIRE与XShooter光谱仪。结果揭示了一种金属丰度较低、温度适中的中性气体，且未检测到C IV或Si IV，表明电离物质的富集程度极低；同时检测到C II*精细结构跃迁线，暗示存在超出宇宙微波背景辐射光致激发的局部加热源。研究结果表明，该星系处于早期化学演化阶段，金属元素在电离相中混合程度有限。

ABSTRACT

We analyze relative abundances and ionization conditions in a strong absorption system at z=6.84, seen in the spectrum of the z=7.54 background quasar ULAS J134208.10+092838.61. Singly ionized C, Si, Fe, Mg, and Al measurements are consistent with a warm neutral medium that is metal-poor but not chemically pristine. Firm non-detections of C IV and Si IV imply that any warm ionized phase of the IGM or CGM has not yet been enriched past the ultra-metal-poor regime (<0.001Z_{solar}), unlike lower redshift DLAs where these lines are nearly ubiquitous. Relative abundances of the heavy elements 794 Myr after the Big Bang resemble those of metal-poor damped Lyman Alpha systems at intermediate redshift and Milky Way halo stars, and show no evidence of enhanced [alpha/Fe], [C/Fe] or other signatures of yields dominated by massive stars. A detection of the CII* fine structure line reveals local sources of excitation from heating, beyond the level of photo-excitation supplied by the CMB. We estimate the total and [CII] cooling rates, balancing against ISM heating sources to develop an heuristic two-phase model of the neutral medium. The implied heating requires a surface density of star formation slightly exceeding that of the Milky Way but not at the level of a strong starburst. For a typical (assumed) NHI=10^{20.6}, an abundance of [Fe/H]=-2.2 matches the columns of species in the neutral phase. To remain undetected in C IV, a warm ionized phase would either need much lower [C/H]<-4.2 over an absorption path of 1 kpc, or else a very small absorption path (a few pc). While still speculative, these results suggest a significant reduction in heavy element enrichment outside of neutral star forming regions of the ISM, as would be expected in early stages of galactic chemical evolution.

研究动机与目标

• 利用类星体吸收线，确定z=6.84星系的星际与星系晕介质特性。
• 测量中性气体相中C、Fe、Si、Al和Mg的相对丰度。
• 研究此早期宇宙时期星际介质的电离状态与加热机制。
• 在C IV与Si IV未检测到的条件下，评估任何暖电离相的存在性及其金属丰度。
• 将星际介质建模为双相介质，并约束恒星形成率与冷却率。

提出的方法

• 利用Magellan与VLT望远镜上的FIRE与XShooter光谱仪进行高分辨率光谱观测。
• 采用马尔可夫链蒙特卡洛方法拟合，从观测吸收线中推导柱密度与丰度比。
• 利用C II* 1334.53 Å精细结构跃迁线推断超出宇宙微波背景辐射光致激发的局部加热源。
• 应用Cloudy光致电离模型，探索电离条件及C IV与Si IV的柱密度预测。
• 采用启发式双相星际介质模型，平衡冷却（通过[C II]与总冷却率）与恒星形成加热。
• 假设典型的HI柱密度为10^20.6 cm⁻²，以锚定丰度测定。

实验结果

研究问题

• RQ1 该z=6.84吸收体中性气体相的C、Fe、Si、Al与Mg的相对丰度是多少？
• RQ2 尽管存在强低电离态谱线，为何C IV与Si IV谱线未被检测到？
• RQ3 C II*精细结构跃迁线激发的来源是什么？其对局部加热意味着什么？
• RQ4 观测到的电离与丰度模式与低红移DLAs及银河系晕恒星相比如何？
• RQ5 C IV与Si IV的未检测结果对任何暖电离相的金属丰度与路径长度有何约束？

主要发现

• 中性气体的金属丰度为[Fe/H] = -2.2，与金属丰度较低但非原始环境一致。
• 相对丰度与金属贫乏DLAs及银河系晕恒星相似，未发现[α/Fe]或[C/Fe]增强的证据。
• C II*谱线的检测表明存在超出宇宙微波背景辐射光致激发的局部加热源，要求恒星形成面密度略高于银河系水平。
• C IV与Si IV的缺失表明，任何暖电离相的金属丰度必须满足[C/H] < -4.2，或路径长度极短（≤8 pc），暗示金属混合程度有限。
• Cloudy模型显示，若存在金属丰度与银河系相当、路径长度为1 kpc的暖电离相，应能检测到C IV，与观测结果矛盾。
• 该系统与质量约10⁸–10⁹ M☉、恒星形成率为~1 M☉ yr⁻¹、[C II]光度适中的SMC类星系一致。