Dwarf irregular (dIrr) galaxies are among the most common type of galaxy in the Universe. They typically have gas-rich, low-surface-brightness, metal-poor, and relatively thick disks. Here, we summarize the current state of our knowledge of the interstellar medium (ISM), including atomic, molecular, and ionized gas, along with their dust properties and metals. We also discuss star-formation feedback, gas accretion, and mergers with other dwarfs that connect the ISM to the circumgalactic and intergalactic media. We highlight one of the most persistent mysteries: the nature of pervasive gas that is yet undetected as either molecular or cold hydrogen, the “dark gas.” Some highlights include the following: ▪Significant quantities of Hi are in far-outer gas disks.▪Cold Hi in dIrrs would be molecular in the Milky Way, making the chemical properties of star-forming clouds significantly different.▪Stellar feedback has a much larger impact in dIrrs than in spiral galaxies.▪The escape fraction of ionizing photons is significant, making dIrrs a plausible source for reionization in the early Universe.▪Observations suggest a significantly higher abundance of hydrogen (H2 or cold Hi) associated with CO in star-forming regions than that traced by the CO alone.

中文翻译：

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矮不规则星系中的星际介质


矮不规则 （dIrr） 星系是宇宙中最常见的星系类型之一。它们通常具有富含气体、低表面亮度、贫金属且相对较厚的圆盘。在这里，我们总结了我们对星际介质 （ISM） 的了解现状，包括原子、分子和电离气体，以及它们的尘埃特性和金属。我们还讨论了恒星形成反馈、气体吸积以及与其他矮星的合并，这些矮星将 ISM 与星系周围和星系间介质连接起来。我们重点介绍了一个最持久的谜团：尚未检测到分子氢或冷氢（即“暗气体”）的普遍气体的性质。一些亮点包括：▪大量的Hi存在于遥远的外部气体盘中。▪dIrrs中的冷Hi在银河系中是分子，使恒星形成云的化学性质明显不同。▪恒星反馈对dIrrs的影响比在螺旋星系中大得多。▪电离光子的逃逸分数很大，这使得dirrs成为早期宇宙中再电离的合理来源。▪观测表明氢的丰度要高得多（H2 或冷 Hi）与恒星形成区域中的 CO 相关，而不是单独由 CO 追踪的 CO。