Galaxy cluster growth as a probe of dark matter

暗物质是我们宇宙中最神秘的成分之一，占宇宙物质含量的80%。大多数粒子物理理论预测，暗物质与标准模型粒子的相互作用非常微弱，因此在地面实验中基本上无法探测到它。如果是正确的，那么暗物质只能在它聚集的足够多的地方进行研究，因为它的引力会影响到周围我们能看到的东西。在这个博士项目中，我们将追踪星系团（宇宙中质量最大的结构）中暗物质的行为，以约束其粒子性质。与标准冷暗物质（CDM）相比，不同的暗物质模型对结构的组装速度做出了不同的可测试预测。例如，热暗物质抑制了早期宇宙中暗物质结构的初始播种，而自相互作用暗物质（SIDM）预测了暗物质粒子之间在后期的相互作用，阻止了密度最大的区域的增长。星系团是研究其性质的理想实验室，因为它们仍然是通过较小的星系团和星系群（通常称为子结构）的合并而形成的，每次合并都像一个巨大的粒子对撞机。在大质量子结构合并过程中重子（恒星和气体）与暗物质的解耦为研究作用于暗物质粒子的非引力提供了重要的约束条件。如果是SIDM，我们预计与CDM相比，子结构的恒星和气体密度会有几个百分点（在3%到5%之间）的变化，以及导致暗物质逐渐与恒星和气体分离的摩擦力。有了这个项目，人们将独立地“跟踪”暗物质、恒星和气体在大质量星团中的含量，通过绘制它们的分布，称重，并识别任何差异/相似之处（分布峰值，数量等）。这将利用哈勃太空望远镜在大型财政计划BUFFALO （https://buffalo.ipac.caltech.edu/）的背景下获得的观测结果，以及地球上最大的望远镜（VLT）的后续光谱。布法罗观测的目的是通过“引力透镜”的喷射来绘制星团的暗物质图，引力透镜会扭曲和放大星团后面的物体。获得的结果将在最先进的达勒姆宇宙模拟的理论框架内进行解释。

Dark matter is one of the most mysterious constituents of our Universe, and constitutes up to 80% of its matter content. Most particle physics theories predict dark matter to interact so weakly with standard model particles that it will remain fundamentally undetectable in terrestrial experiments. If correct, dark matter can only be studied where it gathers in sufficient quantities for its gravity to affect things around it we can see. With this PhD project, we will track the behaviour of dark matter in galaxy clusters (the most massive structures in the universe), to constrain its particle nature. Different dark matter models make different testable predictions for the rate at which structures, assemble compared to the standard cold dark matter (CDM). For example, warm dark matter inhibits the initial seeding of dark matter structures in the early Universe, while self-interacting dark matter (SIDM) predicts interactions between dark matter particles at late times that prevent the densest regions from growing. Galaxy clusters are ideal laboratories in which to study its properties because they are still forming through mergers of smaller clusters and galaxy groups, commonly called substructures, and every merger acts like a gigantic particle collider. The decoupling of baryons (stars and gas) and dark matter during mergers in massive substructures provides an important constraint on the non-gravitational forces acting on dark matter particle. If SIDM, we expect variation of a few percent (between 3 and 5%) of substructures' stellar and gas density compared to CDM, as well as frictional forces that cause dark matter to gradually separate from stars and gas. With this project, one will independently 'follow' the dark matter, stellar and gas contents in massive clusters, by mapping their distributions, weighing them, and identifying any differences/similarities (distribution peaks, quantities, etc). For this one will exploit observations obtained by the Hubble Space Telescope in the context of the large treasury programme, BUFFALO (https://buffalo.ipac.caltech.edu/), and follow-up spectroscopy from the largest telescope on Earth (VLT). BUFFALO observations were designed to map clusters' dark matter via the eject of 'gravitational lensing', which distorts and magnifies objects behind the cluster. The results obtained will be interpreted within the theoretical framework of state-of-the-art Durham's simulations of our universe.