Dwarf Galaxies and the Nature of Dark Matter

在过去的几十年里，出现了一种关于宇宙内容和几何的范式。这种范式（通常简称为LCDM）设想，宇宙的物质-能量含量由辐射和两种奇异成分（冷暗物质和暗能量）之间的演化平衡所主导。无论这听起来多么不可能，LCDM理论成功地解释了大尺度上的大多数宇宙学观测，并被广泛认为是第一个成功的宇宙学理论。尽管LCDM在大尺度上取得了成功，但它在小尺度上的预测显然与对微弱星系的观测相冲突。随着对大部分天空的光度测量和红移测量的完成，这些问题已经浮出水面，这些测量彻底改变了我们对微弱星系的理解，将矮星系的清单扩展到如此微弱的亮度，以至于模糊了星系和星团之间的传统区别。这些观测结果强调了LCDM在小尺度上可能面临的致命挑战，包括在矮星系中心缺乏明显的暗物质“尖端”；低质量星系的稀缺性与预测的大量低质量暗物质晕相比，原则上能够承载它们；以及银河系和仙女座星系的矮卫星在运动和空间排列上的奇怪之处。所有这些问题都是积极研究的主题，特别是因为对LCDM范式的合理（但激进）修改，如“温暖”或“自相互作用”暗物质，提供了有吸引力的替代方案。因此，LCDM对已知最小星系的预测数据与LCDM预测的对抗有望在与调整其主要参数的尺度相距甚远的尺度上验证或挑战已建立的范式。现在时机似乎已经成熟，可以共同努力，评估大自然中最小的河外天体的宇宙学意义。我建议使用各种观测和理论工具来研究这个主题，从暗物质晕形成的详细n体模拟，到发光矮星系形成的气体动力学模拟，再到设计用于调查我们本地宇宙中最微弱星系的直接观测工作，并了解它们对银河系形成的影响。

The past few decades have seen the emergence of a paradigm for the content and geometry of the Universe. This paradigm (usually referred to by its acronym LCDM) envisions the matter-energy content of the Universe as dominated by the evolving balance between radiation and two exotic components: cold dark matter and dark energy. However improbable this may sound, LCDM successfully accounts for most cosmological observations on large scales, and is widely regarded as the first successful cosmological theory ever devised. Its success on large scales notwithstanding, LCDM makes predictions on small scales that apparently run into conflict with observations of faint galaxies. These issues have surfaced as the completion of photometric and redshift surveys of large portions of the sky has revolutionized our understanding of faint galaxies, extending the inventory of dwarf galaxies to luminosities so faint that they blur the traditional distinction between galaxies and star clusters. These observations have highlighted potentially fatal challenges to LCDM on small scales, including the lack of a conspicuous “cusp” of dark matter at the center of dwarf galaxies; the scarcity of low-mass galaxies compared with the huge predicted abundance of low-mass dark matter halos capable, in principle, of hosting them; as well as oddities in the kinematic and spatial arrangement of the dwarf satellites of the Milky Way and Andromeda galaxies. All of these problems are the subject of active investigation, especially since plausible (but radical) modifications to the LCDM paradigm, such as “warm” or “self-interacting” dark matter, provide attractive alternatives. The confrontation of data with LCDM predictions for the smallest galaxies known thus holds the promise to validate or challenge the established paradigm on scales far removed from those used to tune its main parameters. The time seems ripe for a concerted effort designed to assess the cosmological significance of Nature's smallest extragalactic objects. I propose to study this topic using a variety of observational and theoretical tools, from detailed N-body simulation of the formation of dark matter halos, to gas-dynamical simulation of the formation of luminous dwarf galaxies, to direct observational efforts designed to survey the faintest galaxies in our Local Universe and to understand their impact on the formation of our Milky Way galaxy.