Most of a galaxy’s mass is located beyond its stellar component, spread out to hundreds of kiloparsecs. This diffuse reservoir of gas, the circumgalactic medium, acts as the interface between a galaxy and the cosmic web that connects galaxies. We present kiloparsec-scale-resolution integral field spectroscopy of emission lines that trace cool ionized gas from the centre of a nearby galaxy to 30 kpc into its circumgalactic medium. We find a smooth surface brightness profile with a break in slope at twice the 90% stellar radius. The gas also changes from being photoionized by H ii star-forming regions in the disk to being ionized by shocks or the extragalactic UV background at greater distances. This transition represents the boundary between the interstellar medium and the circumgalactic medium, revealing how the dominant reservoir of baryonic matter directly connects to its galaxy.

星系的大部分质量位于其恒星成分之外，分布在数百千秒差距内。这种扩散的气体储层，即环绕星系的介质，充当星系和连接星系的宇宙网之间的界面。我们提出了千秒差距尺度分辨率的发射线积分场光谱，追踪冷电离气体从附近星系中心到 30 kpc 进入其环绕星系介质。我们发现了一个平滑的表面亮度分布，在 90% 恒星半径的两倍处有一个斜率中断。气体也从被盘中的 H ii恒星形成区域光电离转变为被更远距离的冲击或河外紫外线背景电离。这种转变代表了星际介质和环星系介质之间的边界，揭示了重子物质的主要储存库如何直接与其星系相连。