Galactic Cosmology: Dark Matter and the Faint End of Galaxy Formation

我们对宇宙学的理解依赖于两个基本的难题“暗物质”和“暗能量”，其解决方案将重新定义我们理解物理学的方式。特别是暗物质的性质，是扩展粒子物理学标准模型和理解宇宙结构形成的层次性的关键。对宇宙微波背景辐射和星系团的研究已经导致了一个真正的范例，即暗物质以弱相互作用的基本粒子的形式出现，这些粒子以可以忽略不计的热速度从早期宇宙中出现。这种冷暗物质（LCDM）的设想现在已经成熟为一个完整的、可证伪的理论，在这个理论中，详细的理论预测可以直接与观测和实验相对抗。 LCDM的一个基本预测是暗物质晕（星系形成的非线性结构）大量存在，质量范围很大。然而，LCDM晕的质量函数与星系质量函数明显不同：星系质量和晕质量必须由一个强非线性函数关联，该函数包含了支配星系形成的天体物理过程的特征。这种关系在暗星系（矮星系）中有特别有趣的结果。由于低质量晕的数量远远超过了矮星，使LCDM与观测相一致需要发光星系只能在一小部分低质量晕中形成。因此，大多数低质量的晕必须是“黑暗的”，矮星系的形成必须有效地切断低于109太阳质量的特征晕质量。这种“临界”晕质量应该在最微弱的矮星中留下清晰的观测印记，如果得到适当的解释，可以验证或证伪冷暗物质范式的一些基本原则。其中一个例子是刚好低于阈值的晕，虽然不能形成恒星，但仍然应该保留部分原始气体成分。探测这种充满气体、没有恒星的“迷你晕”将是一个重大发现，也是LCDM在小尺度上的巨大成功。 最微弱的矮星最好在银河系本星系群中研究，因为它们只能被探测到几个百万秒差距。此外，一些矮星可能在早期就融入了银河系，在银河系恒星的运动学、金属丰度和丰度模式中留下了它们起源的线索。我提出了一个雄心勃勃的计划，将银河系（“银河系”）变成一个强大的宇宙学探测器，结合超级计算机模拟，数据挖掘和直接观测调查，以破译隐藏在银河系及其近邻中的宇宙学线索。这个“银河宇宙学”的目的是学习如何使用最微弱，最令人费解的星系种群样本来撬开一个独特的天文学窗口，进入我们的暗物质的神秘本质的定义物理学挑战之一。

Our understanding of Cosmology rests on two fundamental puzzles“dark matter” and “dark energy” whose solution will redefine the way we understand Physics. The nature of dark matter, in particular, is key to extending the Standard Model of Particle Physics and to understanding the hierarchical nature of structure formation in the Universe. Studies of the Cosmic Microwave Background radiation and of galaxy clustering have led to a true paradigm, where dark matter takes the form of weakly-interacting elementary particles that emerge from the early Universe with negligible thermal velocities. This Lambda-Cold-Dark-Matter (LCDM) scenario has now matured into a full, falsifiable theory where detailed theoretical predictions may be directly confronted with observation and experiment. A cornerstone prediction of LCDM is that dark matter halos (non-linear structures where galaxies form) exist in vast numbers and span a very large range in mass. The mass function of LCDM halos, however, differs markedly from the galaxy mass function: galaxy masses and halo masses must thus be related by a strongly non-linear function that contains signatures of the astrophysical processes that govern galaxy formation. This relation has particularly intriguing consequences in the regime of faint (dwarf) galaxies. Since low-mass halos vastly outnumber dwarfs, reconciling LCDM with observation requires that luminous galaxies have only been able to form in a small fraction of low-mass halos. Most low-mass halos must thus be “dark”, and dwarf galaxy formation must effectively be cut off below a characteristic halo mass of order ~109 solar masses. This “threshold” halo mass should leave clear observational imprints in the faintest dwarfs which, if properly elucidated, could serve to validate or falsify some of the basic tenets of the cold dark matter paradigm. One example are halos just below the threshold, which, although unable to form stars, should still retain part of their primordial gas content. Detecting this population of gas-filled, star-less “mini-halos” filled would be a major discovery and a resounding success for LCDM on small scales. The faintest dwarfs are best studied in the Local Group of Galaxies, since they can only be detected out to a few Mpc. Some dwarfs, in addition, may have melded into the Galaxy at early times, leaving clues to their origin in the kinematics, metallicities, and abundance patterns of Galactic stars. I propose an ambitious plan to turn the Milky Way (the "Galaxy") into a powerful cosmological probe, combining supercomputer simulations, data mining, and direct observational surveys to decipher the cosmological clues buried in the Galaxy and its immediate neighbours. The aim of this “Galactic Cosmology” is to learn how to use the faintest, and most puzzling, specimens of the galaxy population to pry open a unique astronomical window into one of the defining Physics challenges of our erathe nature of dark matter.