Tracing the circumgalactic medium with optical emission lines

星系周围有大量的气体储存，这些气体来自星系际介质的流入和星系风的流出。沿着视线对明亮而稀有的背景类星体进行的吸收线测量表明，这种“环星系介质”（CGM）远远超出了星系的恒星体。我们知道，CGM具有复杂的多相结构，并且从全球来看，可能既不处于热平衡也不处于动态平衡。然而，各种CGM阶段的空间分布和运动学仍然约束不良，由于有用的吸收视线的稀缺性和检测极低的表面亮度CGM气体直接发射的困难。在这里，我们提出了一个新的调查的冷-暖电离成分的CGM在星系中的检测和特征的CGM在光学发射线。我们将在ESO甚大望远镜上使用多单元光谱探测器（MUSE）仪器，特别是利用充足的保证时间观测（GTO）分配给我们的团队，以换取建造和交付MUSE。大视场提供的高吞吐量和大复用因子，结合我们创新的“MUSE Deep Fields”观察策略（每次指向暴露数十小时），与现有测量相比，将提供至少一个数量级的灵敏度提高。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。这样的数据将约束的几何形状，分层，和结块的温暖的电离气体，阐明的主要激发机制，并帮助在表征的气体流动模式，通过跟踪空间分辨气体运动学。这也将为与数值模拟的比较开辟一个新的维度。我们将通过两种途径来实现这一目标：我们搜索单个星系周围的空间扩展线发射，这些星系明显超出了它们的恒星体;我们还通过堆叠星系群的代表性（子）样本来测量平均CGM特性。这项实验建立在我们最近关于高红移（z>3）星系周围扩展莱曼α晕的突破性结果的基础上，我们证明了这些星系是普遍存在的，即使在微弱和低质量的系统周围也是如此。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推动极其微弱的表面亮度极限，以及必要的分析工具。这项研究的成果将大大推进该领域的观测技术，并为研究正常星系的环星系介质开辟了一个新的窗口。

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.