Ultra faint dwarf galaxies for probing the cosmological framework

低质量暗物质晕的存在是和谐宇宙模型兰姆达冷暗物质(LCDM)的核心预言之一。事实证明，十多年来，将这些预测与位于这些光晕中心的弱(矮小)星系的观测结果相一致是具有挑战性的(Bullock&Boylan-Kolchin 2017)。因此，应对这些挑战并了解低质量星系的形成和演化，对于检验宇宙学框架的有效性至关重要。由于近年来宇宙学星系形成模拟的进步，它自我一致地跟踪星系的形成和演化，解决了与矮小星系有关的一些紧张局势(Sales等人，2022)。然而，这些星系大多与较亮的(或“经典的”)矮星系有关。理解超弱粒子的形成和演化仍然是宇宙学流体动力学模拟中的一个主要挑战。研究超暗矮星的重要性是多方面的。这些极端天体中的大多数是最近才被发现的，更多的极端天体可以在即将到来的新调查中发现，比如鲁宾天文台。这些天体的丰度对暗物质粒子的性质(即冷暗物质与暖暗物质)以及星系形成的物理学非常敏感。要排除或接受某些暗物质模型，就必须结合新的观测结果，对它们的丰度进行可靠的理论预测。此外，矮小星系是宇宙中暗物质最多的星系，是研究暗物质在星系中心部分的分布(又称核心-尖点问题)和约束各种暗物质模型的理想场所，例如自相互作用的暗物质。这篇博士论文旨在探索最暗星系在其低质量晕中的形成，目的是了解最暗端的星系与晕的联系，并限制这些天体的丰度。作为天琴座项目的一部分，该项目将利用并进一步发展LCDM框架内对矮星系形成和演化的一些最高分辨率的宇宙学模拟(Gutcke等人。2021,2022)。在第一阶段，该项目将分析目前可用的模拟，以调查在这些低质量系统中建立的星系和暗物质，特别是中央区域的暗物质分布。在第二阶段，该项目将通过加入自我屏蔽等额外的物理成分来进一步完善星系形成模型。然后，将对低质量晕的样本进行模拟和分析，以探索一些晕在其整个生命周期中保持黑暗的制度。这个过渡质量尺度，所有的晕都保持黑暗，直到它们都承载着一个暗淡的星系，虽然约束很差，但对预测的超暗矮星数量有很大影响，因此暗物质模型。

Existence of low mass dark matter halos is one of the core predictions of the concordance cosmological model, Lambda Cold Dark Matter (LCDM). Reconciling these predictions with the observations of faint (dwarf) galaxies, which inhabit the centre of these halos, has proven to be challenging for more than a decade (Bullock & Boylan-Kolchin 2017). Addressing these challenges and understanding the formation and evolution of the low mass galaxies are therefore critical for testing the validity of the cosmological framework. Thanks to the advancement of cosmological galaxy formation simulations in recent years, which follow the formation and evolution of galaxies self-consistently, some of the tensions concerning dwarf galaxies are addressed (Sales et al 2022). However these are mostly related to brighter (or "classical") dwarf galaxies. Understanding the formation and evolution of ultra faint ones still remain a major challenge in cosmological hydrodynamical simulations. The importance of studying ultra faint dwarfs is manifold. Most of these extreme objects have been discovered only recently and many more can be found with new upcoming surveys such as the Rubin Observatory. The abundance of these objects are very sensitive to the properties of dark matter particles (i.e. cold vs warm dark matter), as well as the physics of galaxy formation. Robust theoretical predictions for their abundance combined with the new observations are necessary to rule out or accept certain dark matter models. Moreover, dwarf galaxies are the most dark matter dominated galaxies in the universe and ideal places for studying the distribution of dark matter in the central parts of galaxies (aka core-cusp problem) and constrain various dark matter models, such as self-interacting dark matter. This PhD thesis aims to probe the formation of the faintest galaxies in their low mass halos with the aim of understanding the galaxy-halo connection at the faintest end and putting constraints on the abundance of these objects. The project will exploit, and develop further, some of the highest resolution cosmological simulations of dwarf galaxy formation and evolution in the LCDM framework, as part of the LYRA project (Gutcke et al. 2021,2022). In the first stage, the project will analyse the currently available simulations to investigate the galaxy and dark matter build up in these low mass systems, and in particular the dark matter distribution in the central regions. In the second stage, the project will further improve the galaxy formation model by including extra physical ingredients such as self-shielding. Then, a sample of lower mass halos will be simulated and analysed in order to probe the regime where some halos remain dark for their entire lifetime. This transition mass scale, where all halos remain dark to where all of them host a faint galaxy, is poorly constrained but has significant impact on the predicted number of ultra faint dwarfs, hence dark matter models.