Unraveling the observational mysteries of the CGM and IGM with non-equilibrium gas kinetics

我们都熟悉气体的温度：更高的温度意味着更多的能量原子，我们感觉到的是热。相同温度下的气体具有相同的能量分布：阿斯韦-玻尔兹曼分布。如果我们加热一些气体，碰撞会迅速重建麦克斯韦-玻尔兹曼分布。此外，电子和离子之间的碰撞可以激发电子状态（例如，在霓虹灯标志中）。激发率敏感地依赖于麦克斯韦-玻尔兹曼分布的尾部。天体物理学家利用这些想法来推断宇宙中气体的温度、密度和成分。对发射光子的强度和波长的观察提供了有关气体组成和能量的信息。然而，宇宙大部分地区的气体密度非常低。它是如此之低，以至于加热和冷却时间比麦克斯韦-玻尔兹曼分布的平衡时间更长。热分布的差异影响了辐射冷却速率和电离与复合之间的平衡。这改变了我们对观测光谱的解释。光谱是整个宇宙中气体性质的重要诊断。这项研究的结果将提高对宇宙气体的理解，供所有天文学家使用。建议的研究项目将为研究生提供高性能计算和动力系统方面的培训。这些技能将转化为各种各样的STEM职业。所提出的模拟将产生一些适合本科项目的小规模分析。我们将为五校联盟的所有机构提供学期和暑期实习机会。这将有助于招募和留住未来的STEM领导者。这些基本的物理问题激发了一种新的基于直接模拟蒙特卡罗（DSMC）的碰撞玻尔兹曼方程求解器。该代码的开发激发了计算在脆弱的宇宙等离子体中发现的多重电离态的新算法。新的计算方法再现了标准的LTEcooling曲线。但是，它也跟踪受多种加热、冷却和动态过程影响的气体的非热时间依赖性演变。这种新颖的方法有望对星系中奇异的气体状态提供新的见解。例如，考虑环银河系介质（CGM）。在热极限下解释非热电离部分可能会产生错误的元素丰度和意想不到的大量热不稳定气体。即使在流体静力晕的简单情况下，这种非热分布也可能通过多个CGM气相。这导致了能量和电离部分的整体非热混合，这影响了我们对星系中控制恒星形成的气体吸积和风过程的理解。对于较小规模的结构，如嵌入在热阱中的气体云或冲击，其影响甚至更为剧烈。总的来说，这些非热分布影响了光学、紫外和x射线的吸收和发射特征。它们改变了我们对类星体吸收线和x射线光谱的解释。简而言之，所提出的工作将提高我们对非流体气体动力学的核心理解。这可能会为突出的观测难题带来突破性的解决方案。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

We are all familiar with gas temperature: a higher temperature impliesmore energetic atoms which we feel as heat. Gasses at the sametemperature have the same distribution of energies: aMaxwell-Boltzmann distribution. If we heat some of the gas,collisions quickly re-establish the Maxwell-Boltzmann distribution.In addition, collisions between electrons and ions can exciteelectronic states (in a neon sign, for example). The excitation ratesdepend sensitively on the tails of the Maxwell-Boltzmann distribution.Astrophysicists use these ideas to infer the temperatures, densities,and compositions of gases in the cosmos. Observations of theintensity and wavelengths of the emitted photons provide informationabout the composition and energies of the gas. However, the gasdensity in much of the Universe is extraordinarily low. It is so lowthat the heating and cooling times are longer than the equilibrationtime for the Maxwell-Boltzmann distribution. The discrepancy from thethermal distribution affects radiative cooling rates and the balancebetween ionization and recombination. This changes our interpretationof observed spectra. Spectra are important diagnostic of gasproperties throughout the Universe. The results of this research willbe an improved understanding of cosmic gas for use by all astronomers.The proposed research projects will provide training for graduatestudents in high-performance computing and dynamical systems. Theseskills will translate to wide variety of STEM careers. The proposedsimulations will generate a number of small-scale analyses suitablefor undergraduate projects. We will provide both term-time and summerinternships for all institutions in the 5-College Consortium. Thiswill help recruit and retain future STEM leaders.These fundamental physical issues motivated a new collisionalBoltzmann equation solver based on Direct Simulation Monte Carlo(DSMC). Development of this code motivated new algorithms forcomputing the multiple ionization states found in tenuous cosmicplasma. The new computational method reproduces the standard LTEcooling curve. But, it also tracks the non-thermal time-dependentevolution of gas affected by multiple heating, cooling, and dynamicalprocesses. This novel approach promises new insight into the exoticstates of gas in galaxies. For example, consider the circumgalacticmedium (CGM). Interpreting non-thermal ionization fractions in thethermal limit may yield the wrong elemental abundances andunexpectedly large volumes of thermally unstable gas. Even in thesimple case of a hydrostatic halo, this non-thermal distribution maypass through multiple CGM gas phases. This results in an overallnon-thermal mixture of energies and ionization fractions, whichimpacts our understanding of gas accretion and wind processes thatgovern star formation in galaxies. The implications are even moredrastic for smaller scale structures like gas clouds embedded in a hothalo or for shocks. Overall, these non-thermal distributions affectabsorption and emission features in the optical, UV, and x-ray. Theyalter our interpretation of quasar absorption-line and x-ray spectra.In short, the proposed work will improve our core understanding ofnon-fluid gas dynamics. This may lead to ground-breaking solutions ofoutstanding observational dilemmas.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.