[论文解读] The impact of cosmic rays on the interstellar medium and galactic outflows of Milky Way analogues

Francisco Rodríguez Montero, Sergio Martin-Alvarez+4 more|arXiv (Cornell University)|Jul 25, 2023

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy被引用 3

一句话总结

本研究通过宇宙学聚焦模拟研究了银河系类星系中宇宙射线（CR）反馈的作用，结果表明，CR通过抑制恒星形成并提高超新星效率，使高红移时星体质量减少约10倍，宇宙正午时减少约4倍。CR压力梯度驱动了以暖电离气体为主的多相星系外流，高红移时还存在显著的冷相和中性相，为詹姆斯·韦伯空间望远镜（JWST）观测提供了可检验的预测。

ABSTRACT

During the last decade, cosmological simulations have managed to reproduce realistic and morphologically diverse galaxies, spanning the Hubble sequence. Central to this success was a phenomenological calibration of the few included feedback processes, whilst glossing over higher complexity baryonic physics. This approach diminishes the predictive power of such simulations, preventing to further our understanding of galaxy formation. To tackle this fundamental issue, we investigate the impact of cosmic rays (CRs) and magnetic fields on the interstellar medium (ISM) and the launching of outflows in a cosmological zoom-in simulation of a Milky Way-like galaxy. We find that including CRs decreases the stellar mass of the galaxy by a factor of 10 at high redshift and $\sim 4$ at cosmic noon, leading to a stellar mass to halo mass ratio in good agreement with abundance matching models. Such decrease is caused by two effects: i) a reduction of cold, high-density, star-forming gas, and ii) a larger fraction of SN events exploding at lower densities, where they have a higher impact. SN-injected CRs produce enhanced, multi-phase galactic outflows, which are accelerated by CR pressure gradients in the circumgalactic medium of the galaxy. While the mass budget of these outflows is dominated by the warm ionised gas, warm neutral and cold gas phases contribute significantly at high redshifts. Importantly, our work shows that future JWST observations of galaxies and their multi-phase outflows across cosmic time have the ability to constrain the role of CRs in regulating star formation.

研究动机与目标

• 研究宇宙射线（CRs）和磁场在调控宇宙学尺度银河系类星系中恒星形成及驱动星系外流中的作用。
• 评估CR反馈相较于标准流体动力学（HD）和磁流体动力学（MHD）模拟，对星际介质（ISM）结构和热力学特性的影响。
• 确定CR驱动的反馈是否能自然再现观测到的星体质量-晕质量关系，而无需对反馈过程进行人为校准。
• 探讨星系外流的多相特性，以及CR压力梯度在星系晕介质（CGM）中加速外流的贡献。

提出的方法

• 利用RAMSES-自适应网格细化（AMR）代码，对一个银河系类星系进行了宇宙学聚焦模拟，实现了自洽的CR输运与能量沉积。
• 实施了一套包含扩散、漂移和能量损失过程的CR输运模型，并与热压和磁压动力学相耦合。
• 对比了三种模拟变体：流体动力学（HD）、磁流体动力学（MHD）和CR-MHD（CRMHD），以分离CR反馈效应。
• 追踪了星际介质（ISM）和外流中气体相（冷相、暖中性相、暖电离相、热相）的演化，分析其质量负载因子和热力学特性。
• 通过空间分辨分析超新星爆炸位点，量化了CR反馈对周围介质密度和冷却效率的影响。
• 评估了外流形态及其加速机制，区分CR压力梯度与热压支持的贡献。

Figure 1: Overview of the CRMHD model of the nut simulation at $z=1.5$ . Background: Colour composite of the zoom-in region centred on the nut galaxy, with gas density (silver blue), gas temperature (orange) and CR energy density (yellow), showing 600 kpc along the x-axis. Three cold filaments are f

实验结果

研究问题

• RQ1 宇宙射线如何影响银河系类星系在宇宙时空中星体质量的累积？
• RQ2 CR反馈对星际介质的多相结构和热力学特性有何影响？
• RQ3 CR压力梯度如何在星系晕介质中加速并塑造星系外流的形态？
• RQ4 CR在多大程度上通过改变爆炸环境和冷却损失，改变了超新星反馈的效率？
• RQ5 CR驱动的外流能否再现詹姆斯·韦伯空间望远镜（JWST）可探测到的多相外流特性？

主要发现

• 在高红移时，引入宇宙射线使模拟的银河系类星系星体质量减少约10倍，宇宙正午时减少约4倍，与丰度匹配模型一致。
• CR反馈抑制了冷、高密度、恒星形成性气体，并增加了超新星在低密度环境中爆炸的比例，从而提高了反馈效率。
• CRMHD模拟中的星系外流在质量上主要由暖电离相（9000–10⁵ K）主导，高红移时冷相和暖中性相应有显著贡献。
• CRMHD模拟中外流的质量负载因子高于HD和MHD模拟，表现为双锥形热外流，其中嵌入着丝状、团块状的冷相。
• CR压力梯度是暖相和冷相外流的主要加速机制，而热压支持可忽略不计。
• CRMHD模拟预测，CR驱动的外流将表现出多相结构，其中嵌入着丝状、团块状的冷组分，位于双锥形热外流之中——这一预测可通过即将开展的JWST观测进行检验。

Figure 2: Projections of the nut galaxy for the three main models explored in this work: HD (top 2 rows), MHD (middle 2 rows), and CRMHD (bottom 2 rows), generated at $z=1.5$ . For each model, the plot includes density-weighted projections of gas density (first column), gas temperature (second colum