The study of galaxy protoclusters is beginning to fill in unknown details of the important phase of the assembly of clusters and cluster galaxies. This review describes the current status of this field and highlights promising recent findings related to galaxy formation in the densest regions of the early universe. We discuss the main search techniques and the characteristic properties of protoclusters in observations and simulations, and show that protoclusters will have present-day masses similar to galaxy clusters when fully collapsed. We discuss the physical properties of galaxies in protoclusters, including (proto-)brightest cluster galaxies, and the forming red sequence. We highlight the fact that the most massive halos at high redshift are found in protoclusters, making these objects uniquely suited for testing important recent models of galaxy formation. We show that galaxies in protoclusters should be among the first galaxies at high redshift making the transition from a gas cooling regime dominated by cold streams to a regime dominated by hot intracluster gas, which could be tested observationally. We also discuss the possible connections between protoclusters and radio galaxies, quasars, and $$\hbox {Ly}\alpha $$Lyα blobs. Because of their early formation, large spatial sizes and high total star-formation rates, protoclusters have also likely played a crucial role during the epoch of reionization, which can be tested with future experiments that will map the neutral and ionized cosmic web. Lastly, we review a number of promising observational projects that are expected to make significant impact in this growing, exciting field.

中文翻译：

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星系原星系团的领域

星系原星系团的研究开始填补星系团和星系团聚集重要阶段的未知细节。这篇综述描述了该领域的现状，并强调了与早期宇宙最密集区域中星系形成相关的有希望的最新发现。我们在观测和模拟中讨论了主要的搜索技术和原始星团的特征，并表明原始星团在完全坍缩时将具有与星系团相似的现代质量。我们讨论了原星系团中星系的物理特性，包括（原）最亮的星系团，以及形成的红色序列。我们强调了这样一个事实，即在原星团中发现了高红移的最大光晕，使这些物体特别适合测试重要的近期星系形成模型。我们表明，原星系团中的星系应该是第一批处于高红移的星系，从以冷流为主的气体冷却状态过渡到以热星系团内气体为主的状态，这可以通过观测进行测试。我们还讨论了原星团与射电星系、类星体和 $$\hbox {Ly}\alpha $$Lyα 斑点之间可能的联系。由于它们的早期形成、大空间尺寸和高总恒星形成率，原星团在再电离时期也可能发挥了关键作用，这可以通过未来的实验进行测试，这些实验将绘制中性和电离的宇宙网。最后，