Tracing the circumgalactic medium with optical emission lines

星系被巨大的气体储存库包围，这些储气库由星系间介质的流入和银河系风的流出提供。沿明亮和稀有背景类星体的视线测量表明，这种“环星系介质”(CGM)远远超出了星系的星体范围。我们知道CGM具有复杂的多相结构，从整体上讲，它可能既不处于热平衡状态，也不处于动态平衡状态。然而，由于缺乏有用的吸收视线，而且很难在发射中直接探测到表面亮度极低的CGM气体，CGM各相的空间分布和运动学仍然缺乏约束。在这里，我们建议通过探测和表征光学发射线上的CGM来研究星系中CGM的冷暖电离成分。我们将在ESO超大型望远镜上使用多单元光谱探测器(MUSE)仪器，特别是利用分配给我们团队的充足的保证时间观测(GTO)，作为建造和交付MUSE的回报。大视场提供的高吞吐量和大的多路复用系数，再加上我们创新的每个单点曝光数十小时的“缪斯深场”观测策略，将使灵敏度与现有测量相比至少提高一个数量级。我们的目标是氢复合线(H-α和H-β)以及碰撞激发的金属线，因此也可以测量气体诊断线比率。这些数据将限制热电离气体的几何形状、分层和聚集，阐明主要的激发机制，并通过追踪空间分辨气体运动学来帮助描述气体流动模式。这也将为与数值模拟的比较开辟一个新的维度。我们将通过两个途径来追求这一目标：我们搜索单个星系周围显著延伸到其星体之外的空间延伸的线辐射；我们还通过堆叠星系群的代表性(子)样本来测量平均CGM性质。这项实验建立在我们最近对高红移(Z&GT；3)星系周围扩展的莱曼-阿尔法光晕的突破性结果的基础上，我们证明了高红移(Z&GT；3)星系是普遍存在的，甚至在微弱和低质量系统周围也是如此。我们的莱曼-阿尔法工作也为我们提供了一个很好的理解，如何在控制系统学的同时向极微弱的表面亮度极限推进，以及必要的分析工具。这项研究将极大地推进这一领域的最先进观测技术，并为研究正常星系的环星系介质开辟一扇新的窗口。

Galaxies are surrounded by large reservoirs of gas that are fed by inflows from the intergalactic medium and by outflows from galactic winds. Absorption-line measurements along the lines of sight to bright and rare background quasars have shown that this “circumgalactic medium” (CGM) extends far beyond the stellar bodies of galaxies. We know that the CGM has a complex multiphase structure and is, globally speaking, probably neither in thermal nor in dynamic equilibrium. However, the spatial distribution and kinematics of the various CGM phases remain poorly constrained, due to the scarcity of useful absorption lines of sight and the difficulties of detecting the extremely low surface brightness CGM gas directly in emission. Here we propose to perform a novel investigation of the cool-warm ionized component of the CGM in galaxies by detecting and characterising the CGM in optical emission lines. We will use the Multi-Unit Spectroscopic Explorer (MUSE) instrument at the ESO Very Large Telescope, in particular taking advantage of the ample Guaranteed Time Observations (GTO) allocation to our team in return for constructing and delivering MUSE. The high throughput and the large multiplexing factor afforded by the large field of view, combined with our innovative “MUSE Deep Fields” observing strategy of exposing for several tens of hours per single pointing, will provide at least an order of magnitude improvement in sensitivity compared to existing measurements. We target hydrogen recombination lines (H-alpha and H-beta) as well as collisionally excited metal lines, so that also gas-diagnostic line ratios can be measured. Such data will constrain the geometry, layering, and clumping of the warm ionized gas, elucidate the principal excitation mechanisms, and help in the characterisation of the gas flow patterns by tracing spatially resolved gas kinematics. It will also open a new dimension for the comparison with numerical simulations. We will pursue this goal by two avenues: We search for spatially extended line emission around individual galaxies that stretches significantly beyond their stellar bodies; we also measure mean CGM properties by stacking representative (sub)samples of the galaxy population. This experiment builds on our recent breakthrough results on extended Lyman-alpha haloes around high-redshift (z>3) galaxies which we demonstrated to be ubiquitous, even around faint and low-mass systems. Our Lyman-alpha work also provides us with a good understanding of how to push towards extremely faint surface brightness limits while controling the systematics, as well as with the necessary analysis tools. Upon outcome this investigation will substantially advance the observational state-of-the-art in this field and open up a new window for studying the circumgalactic medium of normal galaxies.