XXL Survey. X-ray emission in opt. select. group

Why are some galaxy groups pervaded by a hot X-ray emitting intracluster medium, whilst others have no detectable X-ray emission? Is the presence of hot gas a reliable indicator...为何部分星系群被炽热且发射X射线的星系团内介质（intracluster medium）所弥漫，而另一些星系群则未检测到任何X射线辐射？热气体的存在是否为可靠的……