Both observations and recent numerical simulations of the circumgalactic medium (CGM) support the hypothesis that a self-regulating feedback loop suspends the gas density of the ambient CGM close to the galaxy in a state with a ratio of cooling time to freefall time ≳10. This limiting ratio is thought to arise because circumgalactic gas becomes increasingly susceptible to multiphase condensation as the ratio declines. If the timescale ratio gets too small, then cold clouds precipitate out of the CGM, rain into the galaxy, and fuel energetic feedback that raises the ambient cooling time. The astrophysical origin of this so-called precipitation limit is not simple but is critical to understanding the CGM and its role in galaxy evolution. This paper therefore attempts to interpret its origin as simply as possible, relying mainly on conceptual reasoning and schematic diagrams. It illustrates how the precipitation limit can depend on both the global configuration of a galactic atmosphere and the degree to which dynamical disturbances drive CGM perturbations. It also frames some tests of the precipitation hypothesis that can be applied to both CGM observations and numerical simulations of galaxy evolution.

对环绕星系介质 (CGM) 的观测和最近的数值模拟都支持这样一个假设，即自调节反馈回路将靠近星系的环境 CGM 的气体密度悬浮在冷却时间与自由落体时间之比 ≳10 的状态下。这个限制比率被认为是因为随着比率的下降，环绕星系的气体变得越来越容易受到多相冷凝的影响。如果时间尺度比太小，冷云就会从 CGM 中沉淀出来，降雨进入星系，并为提高环境冷却时间的能量反馈提供燃料。这种所谓的降水极限的天体物理学起源并不简单，但对于理解 CGM 及其在星系演化中的作用至关重要。因此，本文试图尽可能简单地解释其起源，主要依靠概念推理和示意图。它说明了降水限制如何取决于银河系大气的全球配置和动力扰动驱动 CGM 扰动的程度。它还构建了一些降水假设的测试框架，这些测试可以应用于 CGM 观测和星系演化的数值模拟。