[论文解读] Sharpening quasar absorption lines with ESPRESSO: Temperature of warm gas at $z\sim2$, constraints on the Mg isotopic ratio, and structure of cold gas at $z\sim0.5$

P. Noterdaeme, S. A. Balashev+8 more|arXiv (Cornell University)|May 3, 2021

Galaxies: Formation, Evolution, Phenomena参考文献 1被引用 1

一句话总结

本研究利用甚大望远镜上的ESPRESSO高分辨率光谱仪对类星体HE 0001−2340进行观测，测量了红移z ≈ 2.19的亚-damped Lyman-alpha吸收系统中的热展宽，测得其动能温度为16,000 ± 1,300 K，约为银河系暖中性介质典型值的两倍。研究还约束了z = 0.45和z = 1.65处的Mg同位素比值，结果与太阳值一致，并在z = 0.45的冷气体组分中发现了多普勒参数低至0.3 km s⁻¹的速度子结构，表明其动能温度约为100 K。

ABSTRACT

We present a high-resolution (R=140,000) spectrum of the bright quasar HE0001-2340 (z=2.26), obtained with ESPRESSO at the Very Large Telescope. We analyse three systems at z=0.45, z=1.65, and z=2.19 using multiple-component Voigt-profile fitting. We also compare our spectrum with those obtained with VLT/UVES, covering a total period of 17 years. We disentangle turbulent and thermal broadening in many components spread over about 400 km/s in the z~2.19 sub-DLA system. We derive an average temperature of 16000+/-1300 K, i.e., about twice the canonical value of the warm neutral medium in the Galactic interstellar medium. A comparison with other high-z, low-metallicity absorbers reveals an anti-correlation between gas temperature and total HI column density. Although requiring confirmation, this could be the first observational evidence of a thermal decrease with galacto-centric distance, i.e., we may be witnessing a thermal transition between the circum-galactic medium and the cooler ISM. We revisit the Mg isotopic ratios at z=0.45 and z=1.65 and constrain them to be xi = (26Mg+25Mg)/24Mg <0.6 and <1.4 in these two systems, respectively. These values are consistent with the standard Solar ratio, i.e., we do not confirm strong enhancement of heavy isotopes previously inferred from UVES data. Finally, we confirm the partial coverage of the quasar emission-line region by a FeI-bearing cloud in the z=0.45 system and present evidence for velocity sub-structure of the gas that has Doppler parameters of the order of only ~0.3 km/s. This work demonstrates the uniqueness of high-fidelity, high-resolution optical spectrographs on large telescopes as tools to investigate the thermal state of the gas in and around galaxies as well as its spatial and velocity structure on small scales, and to constrain the associated stellar nucleosynthetic history. [abridged]

研究动机与目标

• 以前所未有的精度测量高红移类星体吸收线中的热展宽与湍流展宽。
• 探究星系晕介质与星际介质中弥散气体的物理条件与热状态。
• 利用高分辨率数据，约束z ≈ 0.45和z ≈ 1.65低金属丰度系统中的Mg同位素比值。
• 通过部分覆盖与子结构分析，探测冷气体组分的空间与速度结构。

提出的方法

• 利用甚大望远镜上的ESPRESSO对类星体HE 0001−2340进行高分辨率（R ≈ 140,000）光谱观测，实现了优异的波长定标与信噪比。
• 对三个红移系统（z ≈ 0.45、z ≈ 1.65 和 z ≈ 2.19）的吸收线应用多组分Voigt轮廓拟合。
• 通过Voigt轮廓拟合分离热展宽与湍流展宽，从而直接测量动能温度。
• 通过建模Mg谱线的相对深度与宽度，并考虑同位素位移，约束同位素比值。
• 通过比较不同历元的流量比并建模覆盖因子（Cf），评估类星体发射线区域的部分覆盖情况。
• 利用UVES（2001–2017）与ESPRESSO（2018）的长期监测，通过轮廓形态变化检测到速度子结构。

实验结果

研究问题

• RQ1 z ≈ 2.19的亚-damped Lyman-alpha系统中暖气体的动能温度是多少？其与银河系星际介质中典型值相比如何？
• RQ2 是否存在星系晕介质与较冷星际介质之间热转变的证据，如温度与H i柱密度之间存在相关性？
• RQ3 先前报道的z ≈ 0.45与z ≈ 1.65低金属丰度系统中重Mg同位素的增强是否真实，还是由仪器系统误差所致？
• RQ4 z ≈ 0.45处冷气体组分的速度结构如何？其是否表现出低于光谱分辨率元的子结构？
• RQ5 高分辨率光谱能否分辨出多普勒参数低至~0.3 km s⁻¹的吸收系统中气体的物理条件？

主要发现

• z ≈ 2.19的亚-damped Lyman-alpha系统中暖气体的动能温度被测得为16,000 ± 1,300 K，约为银河系暖中性介质典型值的两倍。
• 观测到单个速度组分的动能温度与总H i柱密度之间存在负相关性，提示星系晕介质中可能存在热过渡。
• 在z = 0.45处，Mg同位素比值(26Mg + 25Mg)/24Mg被约束为< 0.6；在z = 1.65处为< 1.4，与太阳比值一致，与先前报道的强同位素增强结论不一致。
• 在z = 0.45处的Fe i吸收系统表现出类星体源的部分覆盖，覆盖因子Cf ≈ 0.61，表明约一半的类星体光子在Ly-α波长被遮挡。
• 在Fe i系统中检测到速度子结构，两个组分的多普勒参数仅为b ≈ 0.3 km s⁻¹，与物理建模推算出的约100 K低温一致。
• ESPRESSO的高光谱保真度使其能够分辨谱线的本征子结构，并打破热运动与湍流运动之间的混淆，充分展示了其在探测小尺度气体结构方面的独特能力。